[Mesa-dev] [PATCH V2 4/4] i965: Enable ext_framebuffer_multisample_blit_scaled on intel h/w

[Anuj Phogat]

This patch enables ext_framebuffer_multisample_blit_scaled extension
on intel h/w >= gen6.

Note: Patches for piglit tests to verify this functionality are out
for review on piglit mailing list. Tests pass for all of the scaling
factors from 0.1 to 2.4.

Comment from Paul Berry:
I have some concerns about the image quality of the method you've
implemented.  As I understand it, the primary use case of this extension
is to allow the client to do multisampled rendering at slightly less
than screen resolution (e.g. 720p instead of 1080p), and then blit the
result to the screen in one step while keeping most of the quality
benefits of multisampling.  Since your implementation is effectively
equivalent to downsampling and then blitting using GL_NEAREST filtering,
my fear is that it will lead to blocky artifacts that are severe enough
to negate the benefit of multisampling in the first place.

Before we turn this extension on in the Intel driver, I'd like to look
at a comparison of:

(1) your technique
(2) downsampling followed by scaling with GL_LINEAR filtering
(3) The nVidia implementation, in GL_SCALED_RESOLVE_FASTEST_EXT mode
(4) The nVidia implementation, in GL_SCALED_RESOLVE_NICEST_EXT mode
(5) Just rendering the image directly to the single-sampled destination
buffer

Observation: Image quality is better in cases 2, 3, 4 and 5 as
compared to case 1. Although extension's implementation meets the
specification's requirements, using it leads to  blocky artifacts
due to nearest filtering.

I'll work on implementing a better filtering technique in blorp.

Signed-off-by: Anuj Phogat <anuj.phogat at gmail.com>
---
 src/mesa/drivers/dri/intel/intel_extensions.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/mesa/drivers/dri/intel/intel_extensions.c b/src/mesa/drivers/dri/intel/intel_extensions.c
index 8d8e325..de12ec3 100644
--- a/src/mesa/drivers/dri/intel/intel_extensions.c
+++ b/src/mesa/drivers/dri/intel/intel_extensions.c
@@ -97,6 +97,7 @@ intelInitExtensions(struct gl_context *ctx)
 
    if (intel->gen >= 6) {
       ctx->Extensions.EXT_framebuffer_multisample = true;
+      ctx->Extensions.EXT_framebuffer_multisample_blit_scaled = true;
       ctx->Extensions.ARB_blend_func_extended = !driQueryOptionb(&intel->optionCache, "disable_blend_func_extended");
       ctx->Extensions.ARB_draw_buffers_blend = true;
       ctx->Extensions.ARB_ES3_compatibility = true;
-- 
1.8.1.4