Draw Manager: OpenGL render support

Works for clay-engine but doesn't draw objects with eevee.

source/blender/draw/DRW_engine.h

@@ -41,6 +41,7 @@ struct ViewContext;
 struct ViewportEngineData;
 struct View3D;
 struct rcti;
+struct GPUOffScreen;
 
 #include "BLI_sys_types.h"  /* for bool */
 
@@ -67,6 +68,10 @@ void DRW_draw_view(const struct bContext *C);
 void DRW_draw_render_loop(
         struct Depsgraph *graph,
         struct View3D *v3d, struct ARegion *ar);
+void DRW_draw_render_loop_offscreen(
+        struct Depsgraph *graph,
+        struct View3D *v3d, struct ARegion *ar,
+        struct GPUOffScreen *ofs);
 void DRW_draw_select_loop(
         struct Depsgraph *graph,
         struct View3D *v3d, struct ARegion *ar,

source/blender/draw/intern/draw_manager.c

@@ -2188,6 +2188,11 @@ void DRW_draw_view(const bContext *C)
 	DRW_draw_render_loop(graph, v3d, ar);
 }
 
+/**
+ * Used for both regular drawing and off-screen drawing.
+ *
+ * \param ofs: When not NULL, use this data to create the viewport.
+ */
 void DRW_draw_render_loop(
         struct Depsgraph *graph,
         View3D *v3d, ARegion *ar)
@@ -2273,6 +2278,37 @@ void DRW_draw_render_loop(
 	memset(&DST, 0x0, sizeof(DST));
 }
 
+void DRW_draw_render_loop_offscreen(
+        struct Depsgraph *graph,
+        View3D *v3d, ARegion *ar, GPUOffScreen *ofs)
+{
+	RegionView3D *rv3d = ar->regiondata;
+
+	/* backup */
+	void *backup_viewport = rv3d->viewport;
+	{
+		/* backup (_never_ use rv3d->viewport) */
+		rv3d->viewport = GPU_viewport_create_from_offscreen(ofs);
+	}
+
+	DST.draw_ctx.evil_C = NULL;
+
+	DRW_draw_render_loop(graph, v3d, ar);
+
+	/* restore */
+	{
+		/* don't free data owned by 'ofs' */
+		GPU_viewport_clear_from_offscreen(rv3d->viewport);
+		GPU_viewport_free(rv3d->viewport);
+		MEM_freeN(rv3d->viewport);
+
+		rv3d->viewport = backup_viewport;
+	}
+
+	/* we need to re-bind (annoying!) */
+	GPU_offscreen_bind(ofs, false);
+}
+
 /**
  * object mode select-loop, see: ED_view3d_draw_select_loop (legacy drawing).
  */

source/blender/editors/space_view3d/view3d_draw_legacy.c

@@ -107,6 +107,10 @@
 #include "GPU_select.h"
 #include "GPU_matrix.h"
 
+#include "RE_engine.h"
+
+#include "DRW_engine.h"
+
 #include "view3d_intern.h"  /* own include */
 
 /* prototypes */
@@ -1811,7 +1815,14 @@ void ED_view3d_draw_offscreen(
 	}
 
 	/* main drawing call */
-	view3d_draw_objects(NULL, scene, v3d, ar, NULL, do_bgpic, true, do_compositing ? fx : NULL);
+	RenderEngineType *type = RE_engines_find(scene->r.engine);
+	if (IS_VIEWPORT_LEGACY(v3d) && ((type->flag & RE_USE_LEGACY_PIPELINE) != 0)) {
+		view3d_draw_objects(NULL, scene, v3d, ar, NULL, do_bgpic, true, do_compositing ? fx : NULL);
+	}
+	else {
+		/* XXX, should take depsgraph as arg */
+		DRW_draw_render_loop_offscreen(scene->depsgraph, v3d, ar, ofs);
+	}
 
 	/* post process */
 	if (do_compositing) {

source/blender/gpu/GPU_framebuffer.h

@@ -84,6 +84,10 @@ int GPU_offscreen_width(const GPUOffScreen *ofs);
 int GPU_offscreen_height(const GPUOffScreen *ofs);
 int GPU_offscreen_color_texture(const GPUOffScreen *ofs);
 
+void GPU_offscreen_viewport_data_get(
+        GPUOffScreen *ofs,
+        GPUFrameBuffer **r_fb, struct GPUTexture **r_color, struct GPUTexture **r_depth);
+
 #ifdef __cplusplus
 }
 #endif

source/blender/gpu/GPU_viewport.h

@@ -88,6 +88,9 @@ void GPU_viewport_bind(GPUViewport *viewport, const rcti *rect);
 void GPU_viewport_unbind(GPUViewport *viewport);
 void GPU_viewport_free(GPUViewport *viewport);
 
+GPUViewport *GPU_viewport_create_from_offscreen(struct GPUOffScreen *ofs);
+void GPU_viewport_clear_from_offscreen(GPUViewport *viewport);
+
 void *GPU_viewport_engine_data_create(GPUViewport *viewport, void *engine_type);
 void *GPU_viewport_engine_data_get(GPUViewport *viewport, void *engine_type);
 void *GPU_viewport_framebuffer_list_get(GPUViewport *viewport);

source/blender/gpu/intern/gpu_framebuffer.c

@@ -682,3 +682,12 @@ int GPU_offscreen_color_texture(const GPUOffScreen *ofs)
 	return GPU_texture_opengl_bindcode(ofs->color);
 }
 
+/* only to be used by viewport code! */
+void GPU_offscreen_viewport_data_get(
+        GPUOffScreen *ofs,
+        GPUFrameBuffer **r_fb, GPUTexture **r_color, GPUTexture **r_depth)
+{
+	*r_fb = ofs->fb;
+	*r_color = ofs->color;
+	*r_depth = ofs->depth;
+}

source/blender/gpu/intern/gpu_viewport.c

@@ -83,6 +83,24 @@ GPUViewport *GPU_viewport_create(void)
 	return viewport;
 }
 
+GPUViewport *GPU_viewport_create_from_offscreen(struct GPUOffScreen *ofs)
+{
+	GPUViewport *viewport = GPU_viewport_create();
+	GPU_offscreen_viewport_data_get(ofs, &viewport->fbl->default_fb, &viewport->txl->color, &viewport->txl->depth);
+	viewport->size[0] = GPU_offscreen_width(ofs);
+	viewport->size[1] = GPU_offscreen_height(ofs);
+	return viewport;
+}
+/**
+ * Clear vars assigned from offscreen, so we don't free data owned by `GPUOffScreen`.
+ */
+void GPU_viewport_clear_from_offscreen(GPUViewport *viewport)
+{
+	viewport->fbl->default_fb = NULL;
+	viewport->txl->color = NULL;
+	viewport->txl->depth = NULL;
+}
+
 void *GPU_viewport_engine_data_create(GPUViewport *viewport, void *engine_type)
 {
 	LinkData *ld = MEM_callocN(sizeof(LinkData), "LinkData");