[Mesa-dev] [PATCH - Mesa 10/10] st/mesa: Make Gallium's BlitFramebuffer follow the GL 4.4 sRGB rules.

[Kenneth Graunke]

OpenGL 4.4 specifies that BlitFramebuffer should perform sRGB encode
and decode like ES 3.x does, but only when GL_FRAMEBUFFER_SRGB is
enabled.  This is technically incompatible in certain cases, but is
more consistent across GL, ES, and WebGL, and more flexible.

The NVIDIA 367.35 drivers appear to follow this behavior.

For the awful spec analysis, please read Piglit's
tests/spec/arb_framebuffer_srgb/blit.c, which explains the differences
between GL 4.1, 4.2, 4.3 (2012), 4.3 (2013), and 4.4, and why this
is the right rule to implement.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/mesa/state_tracker/st_cb_blit.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

Whoever designed this interface did a very nice job.  Making this change
was really simple.  I wrote the obvious code, tested it with llvmpipe,
and it appears to be working.

diff --git a/src/mesa/state_tracker/st_cb_blit.c b/src/mesa/state_tracker/st_cb_blit.c
index 5e7c34c..cfcf3f7 100644
--- a/src/mesa/state_tracker/st_cb_blit.c
+++ b/src/mesa/state_tracker/st_cb_blit.c
@@ -44,6 +44,14 @@
 
 #include "util/u_format.h"
 
+static void
+st_adjust_blit_for_srgb(struct pipe_blit_info *blit, bool framebuffer_srgb)
+{
+   if (!framebuffer_srgb) {
+      blit->dst.format = util_format_linear(blit->dst.format);
+      blit->src.format = util_format_linear(blit->src.format);
+   }
+}
 
 static void
 st_BlitFramebuffer(struct gl_context *ctx,
@@ -197,12 +205,14 @@ st_BlitFramebuffer(struct gl_context *ctx,
                   blit.dst.resource = dstSurf->texture;
                   blit.dst.level = dstSurf->u.tex.level;
                   blit.dst.box.z = dstSurf->u.tex.first_layer;
-                  blit.dst.format = util_format_linear(dstSurf->format);
+                  blit.dst.format = dstSurf->format;
 
                   blit.src.resource = srcObj->pt;
                   blit.src.level = srcAtt->TextureLevel;
                   blit.src.box.z = srcAtt->Zoffset + srcAtt->CubeMapFace;
-                  blit.src.format = util_format_linear(srcObj->pt->format);
+                  blit.src.format = srcObj->pt->format;
+
+                  st_adjust_blit_for_srgb(&blit, ctx->Color.sRGBEnabled);
 
                   st->pipe->blit(st->pipe, &blit);
                   dstRb->defined = true; /* front buffer tracking */
@@ -233,12 +243,14 @@ st_BlitFramebuffer(struct gl_context *ctx,
                   blit.dst.resource = dstSurf->texture;
                   blit.dst.level = dstSurf->u.tex.level;
                   blit.dst.box.z = dstSurf->u.tex.first_layer;
-                  blit.dst.format = util_format_linear(dstSurf->format);
+                  blit.dst.format = dstSurf->format;
 
                   blit.src.resource = srcSurf->texture;
                   blit.src.level = srcSurf->u.tex.level;
                   blit.src.box.z = srcSurf->u.tex.first_layer;
-                  blit.src.format = util_format_linear(srcSurf->format);
+                  blit.src.format = srcSurf->format;
+
+                  st_adjust_blit_for_srgb(&blit, ctx->Color.sRGBEnabled);
 
                   st->pipe->blit(st->pipe, &blit);
                   dstRb->defined = true; /* front buffer tracking */
-- 
2.9.2