Cycles/Eevee: unified and improved texture image color space handling

Cycles now uses the color space on the image datablock, and uses OpenColorIO
to convert to scene linear as needed. Byte images do not take extra memory,
they are compressed in scene linear + sRGB transfer function which in common
cases is a no-op.

Eevee and workbench were changed to work similar. Float images are stored as
scene linear. Byte images are compressed as scene linear + sRGB and stored in
a GL_SRGB8_ALPHA8 texture. From the GLSL shader side this means they are read
as scene linear, simplifying the code and taking advantage of hardware support.

Further, OpenGL image textures are now all stored with premultiplied alpha.
Eevee texture sampling looks a little different now because interpolation
happens premultiplied and in scene linear space.

Overlays and grease pencil work in sRGB space so those now have an extra
conversion to sRGB after reading from image textures. This is not particularly
elegant but as long as engines use different conventions, one or the other
needs to do conversion.

This change breaks compatibility for cases where multiple image texture nodes
were using the same image with different color space node settings. However it
gives more predictable behavior for baking and texture painting if save, load
and image editing operations have a single color space to handle.

Differential Revision: https://developer.blender.org/D4807

3 files changed, 111 insertions, 0 deletions

diff --git a/source/blender/imbuf/IMB_colormanagement.h b/source/blender/imbuf/IMB_colormanagement.h
index 620f8984d9f..e683d38a0aa 100644
--- a/source/blender/imbuf/IMB_colormanagement.h
+++ b/source/blender/imbuf/IMB_colormanagement.h
@@ -58,6 +58,10 @@ void IMB_colormanagement_assign_rect_colorspace(struct ImBuf *ibuf, const char *
 const char *IMB_colormanagement_get_float_colorspace(struct ImBuf *ibuf);
 const char *IMB_colormanagement_get_rect_colorspace(struct ImBuf *ibuf);
 
+bool IMB_colormanagement_space_is_data(struct ColorSpace *colorspace);
+bool IMB_colormanagement_space_is_scene_linear(struct ColorSpace *colorspace);
+bool IMB_colormanagement_space_is_srgb(struct ColorSpace *colorspace);
+
 BLI_INLINE float IMB_colormanagement_get_luminance(const float rgb[3]);
 BLI_INLINE unsigned char IMB_colormanagement_get_luminance_byte(const unsigned char[3]);
 BLI_INLINE void IMB_colormangement_xyz_to_rgb(float rgb[3], const float xyz[3]);
@@ -124,6 +128,14 @@ void IMB_colormanagement_colorspace_to_scene_linear(float *buffer,
                                                     struct ColorSpace *colorspace,
                                                     bool predivide);
 
+void IMB_colormanagement_imbuf_to_srgb_texture(unsigned char *rect,
+                                               const int x,
+                                               const int y,
+                                               const int width,
+                                               const int height,
+                                               const struct ImBuf *ibuf,
+                                               const bool compress_as_srgb);
+
 void IMB_colormanagement_scene_linear_to_color_picking_v3(float pixel[3]);
 void IMB_colormanagement_color_picking_to_scene_linear_v3(float pixel[3]);
 
@@ -340,6 +352,7 @@ enum {
   COLOR_ROLE_DEFAULT_SEQUENCER,
   COLOR_ROLE_DEFAULT_BYTE,
   COLOR_ROLE_DEFAULT_FLOAT,
+  COLOR_ROLE_DATA,
 };
 
 #include "intern/colormanagement_inline.c"
diff --git a/source/blender/imbuf/intern/IMB_colormanagement_intern.h b/source/blender/imbuf/intern/IMB_colormanagement_intern.h
index 2566016ffdd..a83f2d60b8c 100644
--- a/source/blender/imbuf/intern/IMB_colormanagement_intern.h
+++ b/source/blender/imbuf/intern/IMB_colormanagement_intern.h
@@ -48,6 +48,13 @@ typedef struct ColorSpace {
 
   bool is_invertible;
   bool is_data;
+
+  /* Additional info computed only when needed since it's not cheap. */
+  struct {
+    bool cached;
+    bool is_srgb;
+    bool is_scene_linear;
+  } info;
 } ColorSpace;
 
 typedef struct ColorManagedDisplay {
diff --git a/source/blender/imbuf/intern/colormanagement.c b/source/blender/imbuf/intern/colormanagement.c
index f31d4ede693..b460d268d38 100644
--- a/source/blender/imbuf/intern/colormanagement.c
+++ b/source/blender/imbuf/intern/colormanagement.c
@@ -65,6 +65,7 @@
 #define DISPLAY_BUFFER_CHANNELS 4
 
 /* ** list of all supported color spaces, displays and views */
+static char global_role_data[MAX_COLORSPACE_NAME];
 static char global_role_scene_linear[MAX_COLORSPACE_NAME];
 static char global_role_color_picking[MAX_COLORSPACE_NAME];
 static char global_role_texture_painting[MAX_COLORSPACE_NAME];
@@ -488,6 +489,7 @@ static void colormanage_load_config(OCIO_ConstConfigRcPtr *config)
   const char *name;
 
   /* get roles */
+  colormanage_role_color_space_name_get(config, global_role_data, OCIO_ROLE_DATA, NULL);
   colormanage_role_color_space_name_get(
       config, global_role_scene_linear, OCIO_ROLE_SCENE_LINEAR, NULL);
   colormanage_role_color_space_name_get(
@@ -1260,6 +1262,8 @@ void IMB_colormanagement_validate_settings(const ColorManagedDisplaySettings *di
 const char *IMB_colormanagement_role_colorspace_name_get(int role)
 {
   switch (role) {
+    case COLOR_ROLE_DATA:
+      return global_role_data;
     case COLOR_ROLE_SCENE_LINEAR:
       return global_role_scene_linear;
     case COLOR_ROLE_COLOR_PICKING:
@@ -1341,6 +1345,42 @@ const char *IMB_colormanagement_get_rect_colorspace(ImBuf *ibuf)
   }
 }
 
+bool IMB_colormanagement_space_is_data(ColorSpace *colorspace)
+{
+  return (colorspace && colorspace->is_data);
+}
+
+static void colormanage_ensure_srgb_scene_linear_info(ColorSpace *colorspace)
+{
+  if (!colorspace->info.cached) {
+    OCIO_ConstConfigRcPtr *config = OCIO_getCurrentConfig();
+    OCIO_ConstColorSpaceRcPtr *ocio_colorspace = OCIO_configGetColorSpace(config,
+                                                                          colorspace->name);
+
+    bool is_scene_linear, is_srgb;
+    OCIO_colorSpaceIsBuiltin(config, ocio_colorspace, &is_scene_linear, &is_srgb);
+
+    OCIO_colorSpaceRelease(ocio_colorspace);
+    OCIO_configRelease(config);
+
+    colorspace->info.is_scene_linear = is_scene_linear;
+    colorspace->info.is_srgb = is_srgb;
+    colorspace->info.cached = true;
+  }
+}
+
+bool IMB_colormanagement_space_is_scene_linear(ColorSpace *colorspace)
+{
+  colormanage_ensure_srgb_scene_linear_info(colorspace);
+  return (colorspace && colorspace->info.is_scene_linear);
+}
+
+bool IMB_colormanagement_space_is_srgb(ColorSpace *colorspace)
+{
+  colormanage_ensure_srgb_scene_linear_info(colorspace);
+  return (colorspace && colorspace->info.is_srgb);
+}
+
 /*********************** Threaded display buffer transform routines *************************/
 
 typedef struct DisplayBufferThread {
@@ -2111,6 +2151,57 @@ void IMB_colormanagement_colorspace_to_scene_linear(float *buffer,
   }
 }
 
+void IMB_colormanagement_imbuf_to_srgb_texture(unsigned char *out_buffer,
+                                               const int offset_x,
+                                               const int offset_y,
+                                               const int width,
+                                               const int height,
+                                               const struct ImBuf *ibuf,
+                                               const bool compress_as_srgb)
+{
+  /* Convert byte buffer for texture storage on the GPU. These have builtin
+   * support for converting sRGB to linear, which allows us to store textures
+   * without precision or performance loss at minimal memory usage. */
+  BLI_assert(ibuf->rect && ibuf->rect_float == NULL);
+
+  OCIO_ConstProcessorRcPtr *processor = NULL;
+  if (compress_as_srgb && ibuf->rect_colorspace &&
+      !IMB_colormanagement_space_is_srgb(ibuf->rect_colorspace)) {
+    processor = colorspace_to_scene_linear_processor(ibuf->rect_colorspace);
+  }
+
+  /* TODO(brecht): make this multithreaded, or at least process in batches. */
+  const unsigned char *in_buffer = (unsigned char *)ibuf->rect;
+
+  for (int y = 0; y < height; y++) {
+    const size_t in_offset = (offset_y + y) * ibuf->x + offset_x;
+    const size_t out_offset = y * width;
+    const unsigned char *in = in_buffer + in_offset * 4;
+    unsigned char *out = out_buffer + out_offset * 4;
+
+    if (processor) {
+      /* Convert to scene linear, to sRGB and premultiply. */
+      for (int x = 0; x < width; x++, in += 4, out += 4) {
+        float pixel[4];
+        rgba_uchar_to_float(pixel, in);
+        OCIO_processorApplyRGB(processor, pixel);
+        linearrgb_to_srgb_v3_v3(pixel, pixel);
+        mul_v3_fl(pixel, pixel[3]);
+        rgba_float_to_uchar(out, pixel);
+      }
+    }
+    else {
+      /* Premultiply only. */
+      for (int x = 0; x < width; x++, in += 4, out += 4) {
+        out[0] = (in[0] * in[3]) >> 8;
+        out[1] = (in[1] * in[3]) >> 8;
+        out[2] = (in[2] * in[3]) >> 8;
+        out[3] = in[3];
+      }
+    }
+  }
+}
+
 /* Conversion between color picking role. Typically we would expect such a
  * requirements:
  * - It is approximately perceptually linear, so that the HSV numbers and