commit fc4f768e5aaf8efdd112f38295a35de83a0f9a55
author Jim Van Verth <jvanverth@google.com> Thu Jan 25 16:26:25 2018 -0500
committer Skia Commit-Bot <skia-commit-bot@chromium.org> Fri Jan 26 13:54:54 2018 +0000
tree 64571d9e826dd917299fd4973111732dd76ddcb1
parent fc807c885b3cce6252cc1d057098d96f8e14eadc

Use int when possible to calculate atlas indices in shaders.

On certain iOS devices half has a mantissa of only 10 bits, which is not
enough to perform the floating point trickery to get the lower bits
out of the "texture coordinates". Instead we use int if available, and
float if not available.

Also re-enables multitexturing for iOS and adds a sample which
stresses the issue, and a version of fontcache that tests multitexturing.

Bug: skia:7285
Change-Id: Ia541b6a418c1860c941071750ceb26459eb846ea
Reviewed-on: https://skia-review.googlesource.com/99800
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>

src/gpu/GrContext.cpp

diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index f6c66db..d106023 100644
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -277,20 +277,12 @@
             new GrDrawingManager(this, prcOptions, atlasTextContextOptions, &fSingleOwner));
 
     GrDrawOpAtlas::AllowMultitexturing allowMultitexturing;
-    switch (options.fAllowMultipleGlyphCacheTextures) {
-        case GrContextOptions::Enable::kDefault:
-#ifdef SK_BUILD_FOR_IOS
-            allowMultitexturing = GrDrawOpAtlas::AllowMultitexturing::kNo;
-#else
-            allowMultitexturing = GrDrawOpAtlas::AllowMultitexturing::kYes;
-#endif
-            break;
-        case GrContextOptions::Enable::kNo:
-            allowMultitexturing = GrDrawOpAtlas::AllowMultitexturing::kNo;
-            break;
-        case GrContextOptions::Enable::kYes:
-            allowMultitexturing = GrDrawOpAtlas::AllowMultitexturing::kYes;
-            break;
+    if (GrContextOptions::Enable::kNo == options.fAllowMultipleGlyphCacheTextures ||
+        // multitexturing supported only if range can represent the index + texcoords fully
+        !(fCaps->shaderCaps()->floatIs32Bits() || fCaps->shaderCaps()->integerSupport())) {
+        allowMultitexturing = GrDrawOpAtlas::AllowMultitexturing::kNo;
+    } else {
+        allowMultitexturing = GrDrawOpAtlas::AllowMultitexturing::kYes;
     }
     fAtlasGlyphCache = new GrAtlasGlyphCache(this, options.fGlyphCacheTextureMaximumBytes,
                                              allowMultitexturing);