[Piglit] [PATCH] getteximage-formats: Disable dithering during glDrawPixels

[Chad Versace]

The test generates a mipmap by "copying" a client-array of pixels to
miplevel 0 with glDrawPixels, then calling glGenerateMipmap. The test
expects the rendered level 0 to match the client-array source. Dithering
during glDrawPixels inteferes with that expectation.

Fixes on Intel Skylake:
  spec.ext_framebuffer_object.getteximage-formats init-by-clear-and-render
  spec.ext_framebuffer_object.getteximage-formats init-by-render

Regressed by Mesa:
  commit c4b87f129eb036c9615df3adcc1cebd9df10fc84
  Author: Chad Versace <chadversary at chromium.org>
  Subject: meta: Disable dithering during glGenerateMipmap

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99266
Cc: Mark Janes <mark.a.janes at intel.com>
Cc: Kenneth Graunke <kenneth at whitecape.org>
---
 tests/texturing/getteximage-formats.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/tests/texturing/getteximage-formats.c b/tests/texturing/getteximage-formats.c
index 6f4fb029e..02d78d441 100644
--- a/tests/texturing/getteximage-formats.c
+++ b/tests/texturing/getteximage-formats.c
@@ -110,8 +110,33 @@ make_texture_image(GLenum intFormat, GLubyte upperRightTexel[4])
 
 		glViewport(0, 0, TEX_SIZE, TEX_SIZE);
 
+		/* Disable dithering during glDrawPixels to prevent
+		 * disagreement with glGenerateMipmap.  The spec requires
+		 * glDrawPixels to respect the dither state, but the spec
+		 * strongly implies that glGenerateMipmap should not dither.
+		 *
+		 * On dithering and glDrawPixels, see the OpenGL 4.5
+		 * Compatibility Specification, Section 18.1 Drawing Pixels,
+		 * p620:
+		 *
+		 *   Once pixels are transferred, DrawPixels performs final
+		 *   conversion on pixel values [...] which are processed in
+		 *   the same fashion as fragments generated by rasterization
+		 *   (see chapters 15 and 16).
+		 *
+		 * On dithering and glGenerateMipmap, see the Mesa commit
+		 * message in:
+		 *
+		 *   commit c4b87f129eb036c9615df3adcc1cebd9df10fc84
+		 *   Author: Chad Versace <chadversary at chromium.org>
+		 *   Date:   Thu Dec 29 13:05:27 2016 -0800
+		 *   Subject: meta: Disable dithering during glGenerateMipmap
+		 */
+		glDisable(GL_DITHER);
 		glWindowPos2iARB(0, 0);
 		glDrawPixels(TEX_SIZE, TEX_SIZE, GL_RGBA, GL_UNSIGNED_BYTE, tex);
+		glEnable(GL_DITHER);
+
 		glGenerateMipmap(GL_TEXTURE_2D);
 
 		glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, piglit_winsys_fbo);
-- 
2.11.0