Revert "Revert "clamp to premul when reading premul sRGB""

This reverts commit 2e018f548d76b0688f9873c683cffc681fec40ec.

Reason for revert: doesn't appear to have been the roll problem.

Original change's description:
> Revert "clamp to premul when reading premul sRGB"
> 
> This reverts commit 04e10da8362a0dcabd795a4ad53f617719ca0d20.
> 
> Reason for revert: roll?
> 
> Change-Id: Id0a8dcd62763bd6eddde120c513ca97e098a4268
> Reviewed-on: https://skia-review.googlesource.com/6022
> Commit-Queue: Mike Klein <mtklein@chromium.org>
> Reviewed-by: Mike Klein <mtklein@chromium.org>
> 

TBR=mtklein@chromium.org,reviews@skia.org,brianosman@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I399ca5e728ce6766c6707682c4c6b685681ffdeb
Reviewed-on: https://skia-review.googlesource.com/6025
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>

1 files changed, 34 insertions, 0 deletions

diff --git a/src/core/SkRasterPipeline.cpp b/src/core/SkRasterPipeline.cpp
index 4c1dbc762e..00ba81e6f3 100644
--- a/src/core/SkRasterPipeline.cpp
+++ b/src/core/SkRasterPipeline.cpp
@@ -11,6 +11,14 @@
 SkRasterPipeline::SkRasterPipeline() {}
 
 void SkRasterPipeline::append(StockStage stage, void* ctx) {
+#ifdef SK_DEBUG
+    switch (stage) {
+        case from_srgb:
+        case from_srgb_d:
+            SkDEBUGFAIL("Please use append_srgb[_d]() instead.");
+        default: break;
+    }
+#endif
     fStages.push_back({stage, ctx});
 }
 
@@ -42,3 +50,29 @@ void SkRasterPipeline::dump() const {
     }
     SkDebugf("\n");
 }
+
+// It's pretty easy to start with sound premultiplied linear floats, pack those
+// to sRGB encoded bytes, then read them back to linear floats and find them not
+// quite premultiplied, with a color channel just a smidge greater than the alpha
+// channel.  This can happen basically any time we have different transfer
+// functions for alpha and colors... sRGB being the only one we draw into.
+
+// This is an annoying problem with no known good solution.  So apply the clamp hammer.
+
+void SkRasterPipeline::append_from_srgb(SkAlphaType at) {
+    //this->append(from_srgb);
+    fStages.push_back({from_srgb, nullptr});
+
+    if (at == kPremul_SkAlphaType) {
+        this->append(SkRasterPipeline::clamp_a);
+    }
+}
+
+void SkRasterPipeline::append_from_srgb_d(SkAlphaType at) {
+    //this->append(from_srgb_d);
+    fStages.push_back({from_srgb_d, nullptr});
+
+    if (at == kPremul_SkAlphaType) {
+        this->append(SkRasterPipeline::clamp_a_d);
+    }
+}