Cycles: apply basic gamma correction in stadalone OpenImageIO output driver

For file formats like PNG, JPEG and TIFF. Eventually this should use
the OpenColorIO view transform, but this at least makes the image
closer to what it should be in most cases.

Differential Revision: https://developer.blender.org/D16482

intern/cycles/app/oiio_output_driver.cpp

@@ -3,6 +3,11 @@
 
 #include "app/oiio_output_driver.h"
 
+#include "scene/colorspace.h"
+
+#include <OpenImageIO/imagebuf.h>
+#include <OpenImageIO/imagebufalgo.h>
+
 CCL_NAMESPACE_BEGIN
 
 OIIOOutputDriver::OIIOOutputDriver(const string_view filepath,
@@ -47,11 +52,23 @@ void OIIOOutputDriver::write_render_tile(const Tile &tile)
   }
 
   /* Manipulate offset and stride to convert from bottom-up to top-down convention. */
-  image_output->write_image(TypeDesc::FLOAT,
-                            pixels.data() + (height - 1) * width * 4,
-                            AutoStride,
-                            -width * 4 * sizeof(float),
-                            AutoStride);
+  ImageBuf image_buffer(spec,
+                        pixels.data() + (height - 1) * width * 4,
+                        AutoStride,
+                        -width * 4 * sizeof(float),
+                        AutoStride);
+
+  /* Apply gamma correction for (some) non-linear file formats.
+   * TODO: use OpenColorIO view transform if available. */
+  if (ColorSpaceManager::detect_known_colorspace(
+          u_colorspace_auto, "", image_output->format_name(), true) == u_colorspace_srgb) {
+    const float g = 1.0f / 2.2f;
+    ImageBufAlgo::pow(image_buffer, image_buffer, {g, g, g, 1.0f});
+  }
+
+  /* Write to disk and close */
+  image_buffer.set_write_format(TypeDesc::FLOAT);
+  image_buffer.write(image_output.get());
   image_output->close();
 }