GPUFrameBuffer: GL backend isolation

This is related to the Vulkan port T68990.

This is a full cleanup of the Framebuffer module and a separation
of OpenGL related functions.

There is some changes with how the default framebuffers are handled.
Now the default framebuffers are individually wrapped inside special
GLFrameBuffers. This make it easier to keep track of the currently bound
framebuffer state and have some specificity for operations on these
framebuffers.

Another change is dropping the optimisation of only configuring the
changed attachements during framebuffers update. This does not give
any benefits and add some complexity to the code. This might be brought
back if it has a performance impact on some systems.

This also adds support for naming framebuffers but it is currently not
used.

source/blender/draw/intern/draw_manager.c

@@ -1462,7 +1462,7 @@ void DRW_draw_render_loop_ex(struct Depsgraph *depsgraph,
   DRW_hair_init();
 
   /* No framebuffer allowed before drawing. */
-  BLI_assert(GPU_framebuffer_active_get() == NULL);
+  BLI_assert(GPU_framebuffer_active_get() == GPU_framebuffer_back_get());
 
   /* Init engines */
   drw_engines_init();

source/blender/draw/intern/draw_manager_exec.c

@@ -970,19 +970,14 @@ static void draw_shgroup(DRWShadingGroup *shgroup, DRWState pass_state)
 
       switch (cmd_type) {
         case DRW_CMD_CLEAR:
-          GPU_framebuffer_clear(
-#ifndef NDEBUG
-              GPU_framebuffer_active_get(),
-#else
-              NULL,
-#endif
-              cmd->clear.clear_channels,
-              (float[4]){cmd->clear.r / 255.0f,
-                         cmd->clear.g / 255.0f,
-                         cmd->clear.b / 255.0f,
-                         cmd->clear.a / 255.0f},
-              cmd->clear.depth,
-              cmd->clear.stencil);
+          GPU_framebuffer_clear(GPU_framebuffer_active_get(),
+                                cmd->clear.clear_channels,
+                                (float[4]){cmd->clear.r / 255.0f,
+                                           cmd->clear.g / 255.0f,
+                                           cmd->clear.b / 255.0f,
+                                           cmd->clear.a / 255.0f},
+                                cmd->clear.depth,
+                                cmd->clear.stencil);
           break;
         case DRW_CMD_DRWSTATE:
           state.drw_state_enabled |= cmd->state.enable;

source/blender/gpu/CMakeLists.txt

@@ -92,6 +92,7 @@ set(SRC
   opengl/gl_batch.cc
   opengl/gl_context.cc
   opengl/gl_drawlist.cc
+  opengl/gl_framebuffer.cc
   opengl/gl_shader.cc
   opengl/gl_shader_interface.cc
   opengl/gl_state.cc
@@ -151,9 +152,11 @@ set(SRC
   opengl/gl_batch.hh
   opengl/gl_context.hh
   opengl/gl_drawlist.hh
+  opengl/gl_framebuffer.hh
   opengl/gl_shader.hh
   opengl/gl_shader_interface.hh
   opengl/gl_state.hh
+  opengl/gl_texture.hh
   opengl/gl_uniform_buffer.hh
   opengl/gl_vertex_array.hh
 )

source/blender/gpu/GPU_framebuffer.h

@@ -19,6 +19,13 @@
 
 /** \file
  * \ingroup gpu
+ *
+ * GPU Framebuffer
+ * - this is a wrapper for an OpenGL framebuffer object (FBO). in practice
+ *   multiple FBO's may be created.
+ * - actual FBO creation & config is deferred until GPU_framebuffer_bind or
+ *   GPU_framebuffer_check_valid to allow creation & config while another
+ *   opengl context is bound (since FBOs are not shared between ogl contexts).
  */
 
 #pragma once
@@ -41,9 +48,8 @@ typedef enum eGPUFrameBufferBits {
 } eGPUFrameBufferBits;
 
 typedef enum eGPUBackBuffer {
-  GPU_BACKBUFFER = 0,
+  GPU_BACKBUFFER_LEFT = 0,
   GPU_BACKBUFFER_RIGHT,
-  GPU_BACKBUFFER_LEFT,
 } eGPUBackBuffer;
 
 /** Opaque pointer hiding blender::gpu::FrameBuffer. */
@@ -53,15 +59,6 @@ typedef struct GPUFrameBuffer {
 
 typedef struct GPUOffScreen GPUOffScreen;
 
-/* GPU Framebuffer
- * - this is a wrapper for an OpenGL framebuffer object (FBO). in practice
- *   multiple FBO's may be created, to get around limitations on the number
- *   of attached textures and the dimension requirements.
- * - actual FBO creation & config is deferred until GPU_framebuffer_bind or
- *   GPU_framebuffer_check_valid to allow creation & config while another
- *   opengl context is bound (since FBOs are not shared between ogl contexts).
- */
-
 GPUFrameBuffer *GPU_framebuffer_create(void);
 void GPU_framebuffer_free(GPUFrameBuffer *fb);
 void GPU_framebuffer_bind(GPUFrameBuffer *fb);
@@ -72,6 +69,7 @@ bool GPU_framebuffer_bound(GPUFrameBuffer *fb);
 bool GPU_framebuffer_check_valid(GPUFrameBuffer *fb, char err_out[256]);
 
 GPUFrameBuffer *GPU_framebuffer_active_get(void);
+GPUFrameBuffer *GPU_framebuffer_back_get(void);
 
 #define GPU_FRAMEBUFFER_FREE_SAFE(fb) \
   do { \
@@ -84,13 +82,10 @@ GPUFrameBuffer *GPU_framebuffer_active_get(void);
 /* Framebuffer setup : You need to call GPU_framebuffer_bind for these
  * to be effective. */
 
-void GPU_framebuffer_texture_attach(GPUFrameBuffer *fb, struct GPUTexture *tex, int slot, int mip);
-void GPU_framebuffer_texture_layer_attach(
-    GPUFrameBuffer *fb, struct GPUTexture *tex, int slot, int layer, int mip);
-void GPU_framebuffer_texture_cubeface_attach(
-    GPUFrameBuffer *fb, struct GPUTexture *tex, int slot, int face, int mip);
+void GPU_framebuffer_texture_attach_ex(GPUFrameBuffer *gpu_fb,
+                                       GPUAttachment attachement,
+                                       int slot);
 void GPU_framebuffer_texture_detach(GPUFrameBuffer *fb, struct GPUTexture *tex);
-void GPU_framebuffer_texture_detach_slot(GPUFrameBuffer *fb, struct GPUTexture *tex, int type);
 
 /**
  * How to use #GPU_framebuffer_ensure_config().
@@ -155,6 +150,16 @@ void GPU_framebuffer_config_array(GPUFrameBuffer *fb, const GPUAttachment *confi
     _tex, _face, _mip, \
   }
 
+#define GPU_framebuffer_texture_attach(_fb, _texture, _slot, _mip) \
+  GPU_framebuffer_texture_attach_ex( \
+      _fb, (GPUAttachment)GPU_ATTACHMENT_TEXTURE_MIP(_texture, _mip), _slot)
+#define GPU_framebuffer_texture_layer_attach(_fb, _texture, _slot, layer, _mip) \
+  GPU_framebuffer_texture_attach_ex( \
+      _fb, (GPUAttachment)GPU_ATTACHMENT_TEXTURE_LAYER_MIP(_texture, layer, _mip), _slot)
+#define GPU_framebuffer_texture_cubeface_attach(_fb, _texture, _slot, face, _mip) \
+  GPU_framebuffer_texture_attach_ex( \
+      _fb, (GPUAttachment)GPU_ATTACHMENT_TEXTURE_CUBEFACE_MIP(_texture, face, _mip), _slot)
+
 /* Framebuffer operations */
 
 void GPU_framebuffer_viewport_set(GPUFrameBuffer *fb, int x, int y, int w, int h);

source/blender/gpu/GPU_texture.h

@@ -275,8 +275,10 @@ void GPU_texture_mipmap_mode(GPUTexture *tex, bool use_mipmap, bool use_filter);
 void GPU_texture_wrap_mode(GPUTexture *tex, bool use_repeat, bool use_clamp);
 void GPU_texture_swizzle_set(GPUTexture *tex, const char swizzle[4]);
 
+/* TODO should be private internal functions. */
 void GPU_texture_attach_framebuffer(GPUTexture *tex, struct GPUFrameBuffer *fb, int attachment);
-int GPU_texture_detach_framebuffer(GPUTexture *tex, struct GPUFrameBuffer *fb);
+void GPU_texture_detach_framebuffer(GPUTexture *tex, struct GPUFrameBuffer *fb);
+int GPU_texture_framebuffer_attachement_get(GPUTexture *tex, struct GPUFrameBuffer *fb);
 
 int GPU_texture_target(const GPUTexture *tex);
 int GPU_texture_width(const GPUTexture *tex);

source/blender/gpu/intern/gpu_backend.hh

@@ -32,6 +32,7 @@ namespace gpu {
 
 class Batch;
 class DrawList;
+class FrameBuffer;
 class Shader;
 class UniformBuf;
 
@@ -45,7 +46,7 @@ class GPUBackend {
 
   virtual Batch *batch_alloc(void) = 0;
   virtual DrawList *drawlist_alloc(int list_length) = 0;
-  // virtual FrameBuffer *framebuffer_alloc(void) = 0;
+  virtual FrameBuffer *framebuffer_alloc(const char *name) = 0;
   virtual Shader *shader_alloc(const char *name) = 0;
   // virtual Texture *texture_alloc(void) = 0;
   virtual UniformBuf *uniformbuf_alloc(int size, const char *name) = 0;

source/blender/gpu/intern/gpu_context_private.hh

@@ -49,6 +49,18 @@ struct GPUContext {
   GPUMatrixState *matrix_state = NULL;
   blender::gpu::GPUStateManager *state_manager = NULL;
 
+  /**
+   * All 4 window framebuffers.
+   * None of them are valid in an offscreen context.
+   * Right framebuffers are only available if using stereo rendering.
+   * Front framebuffers contains (in principle, but not always) the last frame color.
+   * Default framebuffer is back_left.
+   */
+  blender::gpu::FrameBuffer *back_left = NULL;
+  blender::gpu::FrameBuffer *front_left = NULL;
+  blender::gpu::FrameBuffer *back_right = NULL;
+  blender::gpu::FrameBuffer *front_right = NULL;
+
  protected:
   /** Thread on which this context is active. */
   pthread_t thread_;

source/blender/gpu/intern/gpu_framebuffer_private.hh

@@ -19,19 +19,26 @@
 
 /** \file
  * \ingroup gpu
+ *
+ * GPU Framebuffer
+ * - this is a wrapper for an OpenGL framebuffer object (FBO). in practice
+ *   multiple FBO's may be created.
+ * - actual FBO creation & config is deferred until GPU_framebuffer_bind or
+ *   GPU_framebuffer_check_valid to allow creation & config while another
+ *   opengl context is bound (since FBOs are not shared between ogl contexts).
  */
 
 #pragma once
 
+#include "BLI_span.hh"
+
 #include "MEM_guardedalloc.h"
 
 #include "GPU_framebuffer.h"
 
-#include "glew-mx.h" /* For GLuint. To remove. */
-
 struct GPUTexture;
 
-typedef enum {
+typedef enum GPUAttachmentType : int {
   GPU_FB_DEPTH_ATTACHMENT = 0,
   GPU_FB_DEPTH_STENCIL_ATTACHMENT,
   GPU_FB_COLOR_ATTACHMENT0,
@@ -45,50 +52,99 @@ typedef enum {
   /* Keep in mind that GL max is GL_MAX_DRAW_BUFFERS and is at least 8, corresponding to
    * the maximum number of COLOR attachments specified by glDrawBuffers. */
   GPU_FB_MAX_ATTACHEMENT,
+
+  GPU_FB_MAX_COLOR_ATTACHMENT = (GPU_FB_MAX_ATTACHEMENT - GPU_FB_COLOR_ATTACHMENT0),
 } GPUAttachmentType;
 
+inline constexpr GPUAttachmentType operator-(GPUAttachmentType a, int b)
+{
+  return static_cast<GPUAttachmentType>(static_cast<int>(a) - b);
+}
+
+inline constexpr GPUAttachmentType operator+(GPUAttachmentType a, int b)
+{
+  return static_cast<GPUAttachmentType>(static_cast<int>(a) + b);
+}
+
+inline GPUAttachmentType &operator++(GPUAttachmentType &a)
+{
+  a = a + 1;
+  return a;
+}
+
+inline GPUAttachmentType &operator--(GPUAttachmentType &a)
+{
+  a = a - 1;
+  return a;
+}
+
 namespace blender {
 namespace gpu {
 
-#define FOREACH_ATTACHMENT_RANGE(att, _start, _end) \
-  for (GPUAttachmentType att = static_cast<GPUAttachmentType>(_start); att < _end; \
-       att = static_cast<GPUAttachmentType>(att + 1))
-
-#define GPU_FB_MAX_COLOR_ATTACHMENT (GPU_FB_MAX_ATTACHEMENT - GPU_FB_COLOR_ATTACHMENT0)
-
-#define GPU_FB_DIRTY_DRAWBUFFER (1 << 15)
-
-#define GPU_FB_ATTACHEMENT_IS_DIRTY(flag, type) ((flag & (1 << type)) != 0)
-#define GPU_FB_ATTACHEMENT_SET_DIRTY(flag, type) (flag |= (1 << type))
+#ifdef DEBUG
+#  define DEBUG_NAME_LEN 64
+#else
+#  define DEBUG_NAME_LEN 16
+#endif
 
 class FrameBuffer {
- public:
-  GPUContext *ctx;
-  GLuint object;
-  GPUAttachment attachments[GPU_FB_MAX_ATTACHEMENT];
-  uint16_t dirty_flag;
-  int width, height;
-  bool multisample;
-  /* TODO Check that we always use the right context when binding
-   * (FBOs are not shared across ogl contexts). */
-  // void *ctx;
+ protected:
+  /** Set of texture attachements to render to. DEPTH and DEPTH_STENCIL are mutualy exclusive. */
+  GPUAttachment attachments_[GPU_FB_MAX_ATTACHEMENT];
+  /** Is true if internal representation need to be updated. */
+  bool dirty_attachments_;
+  /** Size of attachement textures. */
+  int width_, height_;
+  /** Debug name. */
+  char name_[DEBUG_NAME_LEN];
 
  public:
-  GPUTexture *depth_tex(void) const
+  FrameBuffer(const char *name);
+  virtual ~FrameBuffer();
+
+  virtual void bind(bool enabled_srgb) = 0;
+  virtual bool check(char err_out[256]) = 0;
+  virtual void clear(eGPUFrameBufferBits buffers,
+                     const float clear_col[4],
+                     float clear_depth,
+                     uint clear_stencil) = 0;
+  virtual void clear_multi(const float (*clear_col)[4]) = 0;
+
+  virtual void read(eGPUFrameBufferBits planes,
+                    eGPUDataFormat format,
+                    const int area[4],
+                    int channel_len,
+                    int slot,
+                    void *r_data) = 0;
+
+  virtual void blit_to(eGPUFrameBufferBits planes,
+                       int src_slot,
+                       FrameBuffer *dst,
+                       int dst_slot,
+                       int dst_offset_x,
+                       int dst_offset_y) = 0;
+
+  void attachment_set(GPUAttachmentType type, const GPUAttachment &new_attachment);
+
+  void recursive_downsample(int max_lvl,
+                            void (*callback)(void *userData, int level),
+                            void *userData);
+
+  inline GPUTexture *depth_tex(void) const
   {
-    if (attachments[GPU_FB_DEPTH_ATTACHMENT].tex) {
-      return attachments[GPU_FB_DEPTH_ATTACHMENT].tex;
+    if (attachments_[GPU_FB_DEPTH_ATTACHMENT].tex) {
+      return attachments_[GPU_FB_DEPTH_ATTACHMENT].tex;
     }
-    return attachments[GPU_FB_DEPTH_STENCIL_ATTACHMENT].tex;
+    return attachments_[GPU_FB_DEPTH_STENCIL_ATTACHMENT].tex;
   };
 
-  GPUTexture *color_tex(int slot) const
+  inline GPUTexture *color_tex(int slot) const
   {
-    return attachments[GPU_FB_COLOR_ATTACHMENT0 + slot].tex;
+    return attachments_[GPU_FB_COLOR_ATTACHMENT0 + slot].tex;
   };
-
-  MEM_CXX_CLASS_ALLOC_FUNCS("FrameBuffer")
 };
 
+#undef DEBUG_NAME_LEN
+
 }  // namespace gpu
 }  // namespace blender

source/blender/gpu/intern/gpu_texture.cc

@@ -44,6 +44,7 @@
 #include "GPU_texture.h"
 
 #include "gpu_context_private.hh"
+#include "gpu_framebuffer_private.hh"
 
 #define WARN_NOT_BOUND(_tex) \
   { \
@@ -109,6 +110,9 @@ struct GPUTexture {
   GPUContext *copy_fb_ctx;
 };
 
+using namespace blender;
+using namespace blender::gpu;
+
 static uint gpu_get_bytesize(eGPUTextureFormat data_type);
 static void gpu_texture_framebuffer_ensure(GPUTexture *tex);
 
@@ -2020,7 +2024,8 @@ void GPU_texture_free(GPUTexture *tex)
   if (tex->refcount == 0) {
     for (int i = 0; i < GPU_TEX_MAX_FBO_ATTACHED; i++) {
       if (tex->fb[i] != NULL) {
-        GPU_framebuffer_texture_detach_slot(tex->fb[i], tex, tex->fb_attachment[i]);
+        FrameBuffer *framebuffer = reinterpret_cast<FrameBuffer *>(tex->fb[i]);
+        framebuffer->attachment_set((GPUAttachmentType)tex->fb_attachment[i], GPU_ATTACHMENT_NONE);
       }
     }
 
@@ -2132,17 +2137,26 @@ void GPU_texture_attach_framebuffer(GPUTexture *tex, GPUFrameBuffer *fb, int att
 }
 
 /* Return previous attachment point */
-int GPU_texture_detach_framebuffer(GPUTexture *tex, GPUFrameBuffer *fb)
+void GPU_texture_detach_framebuffer(GPUTexture *tex, GPUFrameBuffer *fb)
 {
   for (int i = 0; i < GPU_TEX_MAX_FBO_ATTACHED; i++) {
     if (tex->fb[i] == fb) {
       tex->fb[i] = NULL;
+      return;
+    }
+  }
+  BLI_assert(!"Error: Texture: Framebuffer is not attached");
+}
+
+/* Return attachment type for the given framebuffer or -1 if not attached. */
+int GPU_texture_framebuffer_attachement_get(GPUTexture *tex, GPUFrameBuffer *fb)
+{
+  for (int i = 0; i < GPU_TEX_MAX_FBO_ATTACHED; i++) {
+    if (tex->fb[i] == fb) {
       return tex->fb_attachment[i];
     }
   }
-
-  BLI_assert(!"Error: Texture: Framebuffer is not attached");
-  return 0;
+  return -1;
 }
 
 void GPU_texture_get_mipmap_size(GPUTexture *tex, int lvl, int *size)

source/blender/gpu/intern/gpu_texture_private.hh

@@ -0,0 +1,53 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2020 Blender Foundation.
+ * All rights reserved.
+ */
+
+/** \file
+ * \ingroup gpu
+ */
+
+#pragma once
+
+#include "BLI_assert.h"
+
+namespace blender {
+namespace gpu {
+
+class Texture {
+ public:
+  /** TODO(fclem): make it a non-static function. */
+  static GPUAttachmentType attachment_type(GPUTexture *tex, int slot)
+  {
+    switch (GPU_texture_format(tex)) {
+      case GPU_DEPTH_COMPONENT32F:
+      case GPU_DEPTH_COMPONENT24:
+      case GPU_DEPTH_COMPONENT16:
+        BLI_assert(slot == 0);
+        return GPU_FB_DEPTH_ATTACHMENT;
+      case GPU_DEPTH24_STENCIL8:
+      case GPU_DEPTH32F_STENCIL8:
+        BLI_assert(slot == 0);
+        return GPU_FB_DEPTH_STENCIL_ATTACHMENT;
+      default:
+        return static_cast<GPUAttachmentType>(GPU_FB_COLOR_ATTACHMENT0 + slot);
+    }
+  }
+};
+
+}  // namespace gpu
+}  // namespace blender

source/blender/gpu/opengl/gl_backend.hh

@@ -30,6 +30,7 @@
 #include "gl_batch.hh"
 #include "gl_context.hh"
 #include "gl_drawlist.hh"
+#include "gl_framebuffer.hh"
 #include "gl_shader.hh"
 #include "gl_uniform_buffer.hh"
 
@@ -61,6 +62,11 @@ class GLBackend : public GPUBackend {
     return new GLDrawList(list_length);
   };
 
+  FrameBuffer *framebuffer_alloc(const char *name)
+  {
+    return new GLFrameBuffer(name);
+  };
+
   Shader *shader_alloc(const char *name)
   {
     return new GLShader(name);

source/blender/gpu/opengl/gl_context.cc

@@ -59,6 +59,35 @@ GLContext::GLContext(void *ghost_window, GLSharedOrphanLists &shared_orphan_list
   glBindBuffer(GL_ARRAY_BUFFER, 0);
 
   state_manager = new GLStateManager();
+
+  if (ghost_window) {
+    GLuint default_fbo = GHOST_GetDefaultOpenGLFramebuffer((GHOST_WindowHandle)ghost_window);
+    GHOST_RectangleHandle bounds = GHOST_GetClientBounds((GHOST_WindowHandle)ghost_window);
+    int w = GHOST_GetWidthRectangle(bounds);
+    int h = GHOST_GetHeightRectangle(bounds);
+    GHOST_DisposeRectangle(bounds);
+
+    if (default_fbo != 0) {
+      front_left = new GLFrameBuffer("front_left", this, GL_COLOR_ATTACHMENT0, default_fbo, w, h);
+      back_left = new GLFrameBuffer("back_left", this, GL_COLOR_ATTACHMENT0, default_fbo, w, h);
+    }
+    else {
+      front_left = new GLFrameBuffer("front_left", this, GL_FRONT_LEFT, 0, w, h);
+      back_left = new GLFrameBuffer("back_left", this, GL_BACK_LEFT, 0, w, h);
+    }
+    /* TODO(fclem) enable is supported. */
+    const bool supports_stereo_quad_buffer = false;
+    if (supports_stereo_quad_buffer) {
+      front_right = new GLFrameBuffer("front_right", this, GL_FRONT_RIGHT, 0, w, h);
+      back_right = new GLFrameBuffer("back_right", this, GL_BACK_RIGHT, 0, w, h);
+    }
+  }
+  else {
+    /* For offscreen contexts. Default framebuffer is NULL. */
+    back_left = new GLFrameBuffer("back_left", this, GL_NONE, 0, 0, 0);
+  }
+
+  active_fb = back_left;
 }
 
 GLContext::~GLContext()
@@ -73,6 +102,19 @@ GLContext::~GLContext()
   }
   glDeleteVertexArrays(1, &default_vao_);
   glDeleteBuffers(1, &default_attr_vbo_);
+
+  if (front_left) {
+    delete front_left;
+  }
+  if (back_left) {
+    delete back_left;
+  }
+  if (front_right) {
+    delete front_right;
+  }
+  if (back_right) {
+    delete back_right;
+  }
 }
 
 /** \} */

source/blender/gpu/opengl/gl_framebuffer.cc

@@ -0,0 +1,420 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2020 Blender Foundation.
+ * All rights reserved.
+ */
+
+/** \file
+ * \ingroup gpu
+ */
+
+#include "BKE_global.h"
+
+#include "GPU_extensions.h"
+
+#include "gl_backend.hh"
+#include "gl_framebuffer.hh"
+#include "gl_texture.hh"
+
+namespace blender::gpu {
+
+/* -------------------------------------------------------------------- */
+/** \name Creation & Deletion
+ * \{ */
+
+GLFrameBuffer::GLFrameBuffer(const char *name) : FrameBuffer(name)
+{
+  /* Just-In-Time init. See GLFrameBuffer::init(). */
+  immutable_ = false;
+  fbo_id_ = 0;
+}
+
+GLFrameBuffer::GLFrameBuffer(
+    const char *name, GLContext *ctx, GLenum target, GLuint fbo, int w, int h)
+    : FrameBuffer(name)
+{
+  context_ = ctx;
+  immutable_ = true;
+  fbo_id_ = fbo;
+  gl_attachments_[0] = target;
+  /* Never update an internal framebuffer. */
+  dirty_attachments_ = false;
+  width_ = w;
+  height_ = h;
+  srgb_ = false;
+
+#ifndef __APPLE__
+  if (fbo_id_ && (G.debug & G_DEBUG_GPU) && (GLEW_VERSION_4_3 || GLEW_KHR_debug)) {
+    char sh_name[32];
+    SNPRINTF(sh_name, "FrameBuffer-%s", name);
+    glObjectLabel(GL_FRAMEBUFFER, fbo_id_, -1, sh_name);
+  }
+#endif
+}
+
+GLFrameBuffer::~GLFrameBuffer()
+{
+  if (context_ != NULL) {
+    context_->fbo_free(fbo_id_);
+    /* Restore default framebuffer if this framebuffer was bound. */
+    if (context_->active_fb == this && context_->back_left != this) {
+      /* If this assert triggers it means the framebuffer is being freed while in use by another
+       * context which, by the way, is TOTALLY UNSAFE!!!  */
+      BLI_assert(context_ == GPU_context_active_get());
+      GPU_framebuffer_restore();
+    }
+  }
+}
+
+void GLFrameBuffer::init(void)
+{
+  context_ = static_cast<GLContext *>(GPU_context_active_get());
+  glGenFramebuffers(1, &fbo_id_);
+
+#ifndef __APPLE__
+  if ((G.debug & G_DEBUG_GPU) && (GLEW_VERSION_4_3 || GLEW_KHR_debug)) {
+    char sh_name[64];
+    SNPRINTF(sh_name, "FrameBuffer-%s", name_);
+    /* Binding before setting the label is needed on some drivers. */
+    glBindFramebuffer(GL_FRAMEBUFFER, fbo_id_);
+    glObjectLabel(GL_FRAMEBUFFER, fbo_id_, -1, sh_name);
+  }
+#endif
+}
+
+/** \} */
+
+/* -------------------------------------------------------------------- */
+/** \name Config
+ * \{ */
+
+/* This is a rather slow operation. Don't check in normal cases. */
+bool GLFrameBuffer::check(char err_out[256])
+{
+  this->bind(true);
+
+  GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
+
+#define FORMAT_STATUS(X) \
+  case X: { \
+    err = #X; \
+    break; \
+  }
+
+  const char *err;
+  switch (status) {
+    FORMAT_STATUS(GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT);
+    FORMAT_STATUS(GL_FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT);
+    FORMAT_STATUS(GL_FRAMEBUFFER_UNSUPPORTED);
+    FORMAT_STATUS(GL_FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER);
+    FORMAT_STATUS(GL_FRAMEBUFFER_INCOMPLETE_READ_BUFFER);
+    FORMAT_STATUS(GL_FRAMEBUFFER_INCOMPLETE_MULTISAMPLE);
+    FORMAT_STATUS(GL_FRAMEBUFFER_INCOMPLETE_LAYER_TARGETS);
+    FORMAT_STATUS(GL_FRAMEBUFFER_UNDEFINED);
+    case GL_FRAMEBUFFER_COMPLETE:
+      return true;
+    default:
+      err = "unknown";
+      break;
+  }
+
+#undef FORMAT_STATUS
+
+  const char *format = "GPUFrameBuffer: framebuffer status %s\n";
+
+  if (err_out) {
+    BLI_snprintf(err_out, 256, format, err);
+  }
+  else {
+    fprintf(stderr, format, err);
+  }
+
+  return false;
+}
+
+void GLFrameBuffer::update_attachments(void)
+{
+  /* Default framebuffers cannot have attachements. */
+  BLI_assert(immutable_ == false);
+
+  /* First color texture OR the depth texture if no color is attached.
+   * Used to determine framebuffer colorspace and dimensions. */
+  GPUAttachmentType first_attachment = GPU_FB_MAX_ATTACHEMENT;
+  /* NOTE: Inverse iteration to get the first color texture. */
+  for (GPUAttachmentType type = GPU_FB_MAX_ATTACHEMENT - 1; type >= 0; --type) {
+    GPUAttachment &attach = attachments_[type];
+    GLenum gl_attachment = to_gl(type);
+
+    if (type >= GPU_FB_COLOR_ATTACHMENT0) {
+      gl_attachments_[type - GPU_FB_COLOR_ATTACHMENT0] = (attach.tex) ? gl_attachment : GL_NONE;
+      first_attachment = (attach.tex) ? type : first_attachment;
+    }
+    else if (first_attachment == GPU_FB_MAX_ATTACHEMENT) {
+      /* Only use depth texture to get infos if there is no color attachment. */
+      first_attachment = (attach.tex) ? type : first_attachment;
+    }
+
+    if (attach.tex == NULL) {
+      glFramebufferTexture(GL_FRAMEBUFFER, gl_attachment, 0, 0);
+      continue;
+    }
+    GLuint gl_tex = GPU_texture_opengl_bindcode(attach.tex);
+    if (attach.layer > -1 && GPU_texture_cube(attach.tex) && !GPU_texture_array(attach.tex)) {
+      /* Could be avoided if ARB_direct_state_access is required. In this case
+       * glFramebufferTextureLayer would bind the correct face. */
+      GLenum gl_target = GL_TEXTURE_CUBE_MAP_POSITIVE_X + attach.layer;
+      glFramebufferTexture2D(GL_FRAMEBUFFER, gl_attachment, gl_target, gl_tex, attach.mip);
+    }
+    else if (attach.layer > -1) {
+      glFramebufferTextureLayer(GL_FRAMEBUFFER, gl_attachment, gl_tex, attach.mip, attach.layer);
+    }
+    else {
+      /* The whole texture level is attached. The framebuffer is potentially layered. */
+      glFramebufferTexture(GL_FRAMEBUFFER, gl_attachment, gl_tex, attach.mip);
+    }
+    /* We found one depth buffer type. Stop here, otherwise we would
+     * override it by setting GPU_FB_DEPTH_ATTACHMENT */
+    if (type == GPU_FB_DEPTH_STENCIL_ATTACHMENT) {
+      break;
+    }
+  }
+
+  if (GPU_unused_fb_slot_workaround()) {
+    /* Fill normally un-occupied slots to avoid rendering artifacts on some hardware. */
+    GLuint gl_tex = 0;
+    /* NOTE: Inverse iteration to get the first color texture. */
+    for (int i = ARRAY_SIZE(gl_attachments_) - 1; i >= 0; --i) {
+      GPUAttachmentType type = GPU_FB_COLOR_ATTACHMENT0 + i;
+      GPUAttachment &attach = attachments_[type];
+      if (attach.tex != NULL) {
+        gl_tex = GPU_texture_opengl_bindcode(attach.tex);
+      }
+      else if (gl_tex != 0) {
+        GLenum gl_attachment = to_gl(type);
+        gl_attachments_[i] = gl_attachment;
+        glFramebufferTexture(GL_FRAMEBUFFER, gl_attachment, gl_tex, 0);
+      }
+    }
+  }
+
+  if (first_attachment != GPU_FB_MAX_ATTACHEMENT) {
+    GPUAttachment &attach = attachments_[first_attachment];
+    int size[3];
+    GPU_texture_get_mipmap_size(attach.tex, attach.mip, size);
+    width_ = size[0];
+    height_ = size[1];
+    srgb_ = (GPU_texture_format(attach.tex) == GPU_SRGB8_A8);
+  }
+
+  dirty_attachments_ = false;
+
+  glDrawBuffers(ARRAY_SIZE(gl_attachments_), gl_attachments_);
+
+  if (G.debug & G_DEBUG_GPU) {
+    BLI_assert(this->check(NULL));
+  }
+}
+
+/** \} */
+
+/* -------------------------------------------------------------------- */
+/** \name Binding
+ * \{ */
+
+void GLFrameBuffer::bind(bool enabled_srgb)
+{
+  GPUContext *ctx = GPU_context_active_get();
+  BLI_assert(ctx);
+
+  if (context_ != NULL && context_ != ctx) {
+    BLI_assert(!"Trying to use the same framebuffer in multiple context");
+  }
+
+  if (!immutable_ && fbo_id_ == 0) {
+    this->init();
+  }
+
+  if (ctx->active_fb != this) {
+    glBindFramebuffer(GL_FRAMEBUFFER, fbo_id_);
+    /* Internal framebuffers have only one color output and needs to be set everytime. */
+    if (immutable_ && fbo_id_ == 0) {
+      glDrawBuffer(gl_attachments_[0]);
+    }
+  }
+
+  if (dirty_attachments_) {
+    this->update_attachments();
+  }
+
+  if (ctx->active_fb != this) {
+    ctx->active_fb = this;
+
+    if (enabled_srgb) {
+      glEnable(GL_FRAMEBUFFER_SRGB);
+    }
+    else {
+      glDisable(GL_FRAMEBUFFER_SRGB);
+    }
+
+    GPU_shader_set_framebuffer_srgb_target(enabled_srgb && srgb_);
+  }
+
+  GPU_viewport(0, 0, width_, height_);
+}
+
+/** \} */
+
+/* -------------------------------------------------------------------- */
+/** \name Operations.
+ * \{ */
+
+void GLFrameBuffer::clear(eGPUFrameBufferBits buffers,
+                          const float clear_col[4],
+                          float clear_depth,
+                          uint clear_stencil)
+{
+  /* Save and restore the state. */
+  eGPUWriteMask write_mask = GPU_write_mask_get();
+  uint stencil_mask = GPU_stencil_mask_get();
+  eGPUStencilTest stencil_test = GPU_stencil_test_get();
+
+  if (buffers & GPU_COLOR_BIT) {
+    GPU_color_mask(true, true, true, true);
+    glClearColor(clear_col[0], clear_col[1], clear_col[2], clear_col[3]);
+  }
+  if (buffers & GPU_DEPTH_BIT) {
+    GPU_depth_mask(true);
+    glClearDepth(clear_depth);
+  }
+  if (buffers & GPU_STENCIL_BIT) {
+    GPU_stencil_write_mask_set(0xFFu);
+    GPU_stencil_test(GPU_STENCIL_ALWAYS);
+    glClearStencil(clear_stencil);
+  }
+
+  context_->state_manager->apply_state();
+
+  GLbitfield mask = to_gl(buffers);
+  glClear(mask);
+
+  if (buffers & (GPU_COLOR_BIT | GPU_DEPTH_BIT)) {
+    GPU_write_mask(write_mask);
+  }
+  if (buffers & GPU_STENCIL_BIT) {
+    GPU_stencil_write_mask_set(stencil_mask);
+    GPU_stencil_test(stencil_test);
+  }
+}
+
+void GLFrameBuffer::clear_multi(const float (*clear_cols)[4])
+{
+  /* Save and restore the state. */
+  eGPUWriteMask write_mask = GPU_write_mask_get();
+  GPU_color_mask(true, true, true, true);
+
+  context_->state_manager->apply_state();
+
+  /* WATCH: This can easilly access clear_cols out of bounds it clear_cols is not big enough for
+   * all attachments.
+   * TODO(fclem) fix this insecurity? */
+  int type = GPU_FB_COLOR_ATTACHMENT0;
+  for (int i = 0; type < GPU_FB_MAX_ATTACHEMENT; i++, type++) {
+    if (attachments_[type].tex != NULL) {
+      glClearBufferfv(GL_COLOR, i, clear_cols[i]);
+    }
+  }
+
+  GPU_write_mask(write_mask);
+}
+
+void GLFrameBuffer::read(eGPUFrameBufferBits plane,
+                         eGPUDataFormat data_format,
+                         const int area[4],
+                         int channel_len,
+                         int slot,
+                         void *r_data)
+{
+  GLenum format, type, mode;
+  mode = gl_attachments_[slot];
+  type = to_gl(data_format);
+
+  switch (plane) {
+    case GPU_DEPTH_BIT:
+      format = GL_DEPTH_COMPONENT;
+      break;
+    case GPU_COLOR_BIT:
+      format = channel_len_to_gl(channel_len);
+      /* TODO: needed for selection buffers to work properly, this should be handled better. */
+      if (format == GL_RED && type == GL_UNSIGNED_INT) {
+        format = GL_RED_INTEGER;
+      }
+      break;
+    case GPU_STENCIL_BIT:
+      fprintf(stderr, "GPUFramebuffer: Error: Trying to read stencil bit. Unsupported.");
+      return;
+    default:
+      fprintf(stderr, "GPUFramebuffer: Error: Trying to read more than one framebuffer plane.");
+      return;
+  }
+
+  glBindFramebuffer(GL_READ_FRAMEBUFFER, fbo_id_);
+  glReadBuffer(mode);
+  glReadPixels(UNPACK4(area), format, type, r_data);
+}
+
+/* Copy src at the give offset inside dst. */
+void GLFrameBuffer::blit_to(
+    eGPUFrameBufferBits planes, int src_slot, FrameBuffer *dst_, int dst_slot, int x, int y)
+{
+  GLFrameBuffer *src = this;
+  GLFrameBuffer *dst = static_cast<GLFrameBuffer *>(dst_);
+
+  /* Framebuffers must be up to date. This simplify this function. */
+  if (src->dirty_attachments_) {
+    src->bind(true);
+  }
+  if (dst->dirty_attachments_) {
+    dst->bind(true);
+  }
+
+  glBindFramebuffer(GL_READ_FRAMEBUFFER, src->fbo_id_);
+  glBindFramebuffer(GL_DRAW_FRAMEBUFFER, dst->fbo_id_);
+
+  if (planes & GPU_COLOR_BIT) {
+    BLI_assert(src->immutable_ == false || src_slot == 0);
+    BLI_assert(dst->immutable_ == false || dst_slot == 0);
+    BLI_assert(src->gl_attachments_[src_slot] != GL_NONE);
+    BLI_assert(dst->gl_attachments_[dst_slot] != GL_NONE);
+    glReadBuffer(src->gl_attachments_[src_slot]);
+    glDrawBuffer(dst->gl_attachments_[dst_slot]);
+  }
+
+  GPU_context_active_get()->state_manager->apply_state();
+
+  int w = src->width_;
+  int h = src->height_;
+  GLbitfield mask = to_gl(planes);
+  glBlitFramebuffer(0, 0, w, h, x, y, x + w, y + h, mask, GL_NEAREST);
+
+  if (!dst->immutable_) {
+    /* Restore the draw buffers. */
+    glDrawBuffers(ARRAY_SIZE(dst->gl_attachments_), dst->gl_attachments_);
+  }
+}
+
+/** \} */
+
+}  // namespace blender::gpu

source/blender/gpu/opengl/gl_framebuffer.hh

@@ -0,0 +1,142 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2020 Blender Foundation.
+ * All rights reserved.
+ */
+
+/** \file
+ * \ingroup gpu
+ *
+ * Encapsulation of Framebuffer states (attached textures, viewport, scissors).
+ */
+
+#pragma once
+
+#include "MEM_guardedalloc.h"
+
+#include "glew-mx.h"
+
+#include "gpu_framebuffer_private.hh"
+
+namespace blender::gpu {
+
+/**
+ * Implementation of FrameBuffer object using OpenGL.
+ **/
+class GLFrameBuffer : public FrameBuffer {
+ private:
+  /** OpenGL handle. */
+  GLuint fbo_id_ = 0;
+  /** Context the handle is from. Framebuffers are not shared accros contexts. */
+  GLContext *context_ = NULL;
+  /** Copy of the GL state. Contains ONLY color attachments enums for slot binding. */
+  GLenum gl_attachments_[GPU_FB_MAX_COLOR_ATTACHMENT];
+  /** Internal framebuffers are immutable. */
+  bool immutable_;
+  /** True is the framebuffer has it's first color target using the GPU_SRGB8_A8 format. */
+  bool srgb_;
+
+ public:
+  /**
+   * Create a conventional framebuffer to attach texture to.
+   **/
+  GLFrameBuffer(const char *name);
+
+  /**
+   * Special Framebuffer encapsulating internal window framebuffer.
+   *  (i.e.: GL_FRONT_LEFT, GL_BACK_RIGHT, ...)
+   * @param ctx     context the handle is from.
+   * @param target  the internal GL name (i.e: GL_BACK_LEFT).
+   * @param fbo     the (optional) already created object for some implementation. Default is 0.
+   * @param w       buffer width.
+   * @param h       buffer height.
+   **/
+  GLFrameBuffer(const char *name, GLContext *ctx, GLenum target, GLuint fbo, int w, int h);
+
+  ~GLFrameBuffer();
+
+  void bind(bool enabled_srgb) override;
+
+  bool check(char err_out[256]) override;
+
+  void clear(eGPUFrameBufferBits buffers,
+             const float clear_col[4],
+             float clear_depth,
+             uint clear_stencil) override;
+  void clear_multi(const float (*clear_cols)[4]) override;
+
+  void read(eGPUFrameBufferBits planes,
+            eGPUDataFormat format,
+            const int area[4],
+            int channel_len,
+            int slot,
+            void *r_data) override;
+
+  void blit_to(eGPUFrameBufferBits planes,
+               int src_slot,
+               FrameBuffer *dst,
+               int dst_slot,
+               int dst_offset_x,
+               int dst_offset_y) override;
+
+ private:
+  void init(void);
+  void update_attachments(void);
+  void update_drawbuffers(void);
+
+  MEM_CXX_CLASS_ALLOC_FUNCS("GLFrameBuffer");
+};
+
+/* -------------------------------------------------------------------- */
+/** \name Enums Conversion
+ * \{ */
+
+static inline GLenum to_gl(const GPUAttachmentType type)
+{
+#define ATTACHMENT(X) \
+  case GPU_FB_##X: { \
+    return GL_##X; \
+  } \
+    ((void)0)
+
+  switch (type) {
+    ATTACHMENT(DEPTH_ATTACHMENT);
+    ATTACHMENT(DEPTH_STENCIL_ATTACHMENT);
+    ATTACHMENT(COLOR_ATTACHMENT0);
+    ATTACHMENT(COLOR_ATTACHMENT1);
+    ATTACHMENT(COLOR_ATTACHMENT2);
+    ATTACHMENT(COLOR_ATTACHMENT3);
+    ATTACHMENT(COLOR_ATTACHMENT4);
+    ATTACHMENT(COLOR_ATTACHMENT5);
+    default:
+      BLI_assert(0);
+      return GL_COLOR_ATTACHMENT0;
+  }
+#undef ATTACHMENT
+}
+
+static inline GLbitfield to_gl(const eGPUFrameBufferBits bits)
+{
+  GLbitfield mask = 0;
+  mask |= (bits & GPU_DEPTH_BIT) ? GL_DEPTH_BUFFER_BIT : 0;
+  mask |= (bits & GPU_STENCIL_BIT) ? GL_STENCIL_BUFFER_BIT : 0;
+  mask |= (bits & GPU_COLOR_BIT) ? GL_COLOR_BUFFER_BIT : 0;
+  return mask;
+}
+
+/** \} */
+
+}  // namespace blender::gpu

source/blender/gpu/opengl/gl_texture.hh

@@ -0,0 +1,81 @@
+
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2020 Blender Foundation.
+ * All rights reserved.
+ */
+
+/** \file
+ * \ingroup gpu
+ *
+ * GPU Framebuffer
+ * - this is a wrapper for an OpenGL framebuffer object (FBO). in practice
+ *   multiple FBO's may be created.
+ * - actual FBO creation & config is deferred until GPU_framebuffer_bind or
+ *   GPU_framebuffer_check_valid to allow creation & config while another
+ *   opengl context is bound (since FBOs are not shared between ogl contexts).
+ */
+
+#pragma once
+
+#include "BLI_assert.h"
+
+#include "glew-mx.h"
+
+namespace blender {
+namespace gpu {
+
+static GLenum to_gl(eGPUDataFormat format)
+{
+  switch (format) {
+    case GPU_DATA_FLOAT:
+      return GL_FLOAT;
+    case GPU_DATA_INT:
+      return GL_INT;
+    case GPU_DATA_UNSIGNED_INT:
+      return GL_UNSIGNED_INT;
+    case GPU_DATA_UNSIGNED_BYTE:
+      return GL_UNSIGNED_BYTE;
+    case GPU_DATA_UNSIGNED_INT_24_8:
+      return GL_UNSIGNED_INT_24_8;
+    case GPU_DATA_10_11_11_REV:
+      return GL_UNSIGNED_INT_10F_11F_11F_REV;
+    default:
+      BLI_assert(!"Unhandled data format");
+      return GL_FLOAT;
+  }
+}
+
+/* Assume Unorm / Float target. Used with glReadPixels. */
+static GLenum channel_len_to_gl(int channel_len)
+{
+  switch (channel_len) {
+    case 1:
+      return GL_RED;
+    case 2:
+      return GL_RG;
+    case 3:
+      return GL_RGB;
+    case 4:
+      return GL_RGBA;
+    default:
+      BLI_assert(!"Wrong number of texture channels");
+      return GL_RED;
+  }
+}
+
+}  // namespace gpu
+}  // namespace blender

source/blender/windowmanager/intern/wm_draw.c

@@ -826,13 +826,11 @@ static void wm_draw_window(bContext *C, wmWindow *win)
   }
   else if (win->stereo3d_format->display_mode == S3D_DISPLAY_PAGEFLIP) {
     /* For pageflip we simply draw to both back buffers. */
-    GPU_backbuffer_bind(GPU_BACKBUFFER_LEFT);
-    wm_draw_window_onscreen(C, win, 0);
-
     GPU_backbuffer_bind(GPU_BACKBUFFER_RIGHT);
     wm_draw_window_onscreen(C, win, 1);
 
-    GPU_backbuffer_bind(GPU_BACKBUFFER);
+    GPU_backbuffer_bind(GPU_BACKBUFFER_LEFT);
+    wm_draw_window_onscreen(C, win, 0);
   }
   else if (ELEM(win->stereo3d_format->display_mode, S3D_DISPLAY_ANAGLYPH, S3D_DISPLAY_INTERLACE)) {
     /* For anaglyph and interlace, we draw individual regions with

source/blender/windowmanager/intern/wm_surface.c

@@ -84,7 +84,7 @@ void wm_surface_set_drawable(wmSurface *surface, bool activate)
 
 void wm_surface_make_drawable(wmSurface *surface)
 {
-  BLI_assert(GPU_framebuffer_active_get() == NULL);
+  BLI_assert(GPU_framebuffer_active_get() == GPU_framebuffer_back_get());
 
   if (surface != g_drawable) {
     wm_surface_clear_drawable();
@@ -95,7 +95,7 @@ void wm_surface_make_drawable(wmSurface *surface)
 void wm_surface_reset_drawable(void)
 {
   BLI_assert(BLI_thread_is_main());
-  BLI_assert(GPU_framebuffer_active_get() == NULL);
+  BLI_assert(GPU_framebuffer_active_get() == GPU_framebuffer_back_get());
 
   if (g_drawable) {
     wm_surface_clear_drawable();

source/blender/windowmanager/intern/wm_window.c

@@ -1115,7 +1115,7 @@ void wm_window_clear_drawable(wmWindowManager *wm)
 
 void wm_window_make_drawable(wmWindowManager *wm, wmWindow *win)
 {
-  BLI_assert(GPU_framebuffer_active_get() == NULL);
+  BLI_assert(GPU_framebuffer_active_get() == GPU_framebuffer_back_get());
 
   if (win != wm->windrawable && win->ghostwin) {
     //      win->lmbut = 0; /* keeps hanging when mousepressed while other window opened */
@@ -1136,7 +1136,7 @@ void wm_window_make_drawable(wmWindowManager *wm, wmWindow *win)
 void wm_window_reset_drawable(void)
 {
   BLI_assert(BLI_thread_is_main());
-  BLI_assert(GPU_framebuffer_active_get() == NULL);
+  BLI_assert(GPU_framebuffer_active_get() == GPU_framebuffer_back_get());
   wmWindowManager *wm = G_MAIN->wm.first;
 
   if (wm == NULL) {
@@ -2487,25 +2487,25 @@ void *WM_opengl_context_create(void)
    * So we should call this function only on the main thread.
    */
   BLI_assert(BLI_thread_is_main());
-  BLI_assert(GPU_framebuffer_active_get() == NULL);
+  BLI_assert(GPU_framebuffer_active_get() == GPU_framebuffer_back_get());
   return GHOST_CreateOpenGLContext(g_system);
 }
 
 void WM_opengl_context_dispose(void *context)
 {
-  BLI_assert(GPU_framebuffer_active_get() == NULL);
+  BLI_assert(GPU_framebuffer_active_get() == GPU_framebuffer_back_get());
   GHOST_DisposeOpenGLContext(g_system, (GHOST_ContextHandle)context);
 }
 
 void WM_opengl_context_activate(void *context)
 {
-  BLI_assert(GPU_framebuffer_active_get() == NULL);
+  BLI_assert(GPU_framebuffer_active_get() == GPU_framebuffer_back_get());
   GHOST_ActivateOpenGLContext((GHOST_ContextHandle)context);
 }
 
 void WM_opengl_context_release(void *context)
 {
-  BLI_assert(GPU_framebuffer_active_get() == NULL);
+  BLI_assert(GPU_framebuffer_active_get() == GPU_framebuffer_back_get());
   GHOST_ReleaseOpenGLContext((GHOST_ContextHandle)context);
 }