Issue #2040 - Pre-multiply the alpha values in our JXL decode buffer.

Using selective calculation (only if not opaque) and fast integer
math here should optimize well in compilers.

1 files changed, 6 insertions, 0 deletions

diff --git a/image/decoders/nsJXLDecoder.cpp b/image/decoders/nsJXLDecoder.cpp
index fa4f6e77de..64a192877d 100644
--- a/image/decoders/nsJXLDecoder.cpp
+++ b/image/decoders/nsJXLDecoder.cpp
@@ -145,6 +145,12 @@ nsJXLDecoder::ReadJXLData(const char* aData, size_t aLength)
           // We currently have a channel ordering mis-match here.
           for (uint8_t* pixPtr = rowPtr; pixPtr < rowPtr + mInfo.xsize * 4; pixPtr+=4){
             std::swap(pixPtr[0], pixPtr[2]);
+            // Pre-multiply, too
+            if (pixPtr[3] < 255) {
+              pixPtr[0]=((uint16_t)pixPtr[0]*(uint16_t)pixPtr[3]) >> 8;
+              pixPtr[1]=((uint16_t)pixPtr[1]*(uint16_t)pixPtr[3]) >> 8;
+              pixPtr[2]=((uint16_t)pixPtr[2]*(uint16_t)pixPtr[3]) >> 8;
+            }
           }
           pipe->WriteBuffer(reinterpret_cast<uint32_t*>(rowPtr));
         }