Bug 869125 - Use the preallocated first frame in the PNG and GIF decoders when possible so we don't flash unnecessarily. r=seth

--HG--
extra : rebase_source : b2b914fd86c7946392588726082b885b09a97b37

Author: joedrew

image/decoders/nsGIFDecoder2.cpp

@@ -165,18 +165,31 @@ void nsGIFDecoder2::BeginImageFrame(uint16_t aDepth)
   else
     format = gfxASurface::ImageFormatRGB24;
 
+  MOZ_ASSERT(HasSize());
+
   // Use correct format, RGB for first frame, PAL for following frames
   // and include transparency to allow for optimization of opaque images
   if (mGIFStruct.images_decoded) {
     // Image data is stored with original depth and palette
     NeedNewFrame(mGIFStruct.images_decoded, mGIFStruct.x_offset,
                  mGIFStruct.y_offset, mGIFStruct.width, mGIFStruct.height,
                  format, aDepth);
-  } else {
+  }
+
+  // Our first full frame is automatically created by the image decoding
+  // infrastructure. Just use it as long as we're not creating a subframe.
+  else if (mGIFStruct.x_offset != 0 || mGIFStruct.y_offset != 0 ||
+           int32_t(mGIFStruct.width) != mImageMetadata.GetWidth() ||
+           int32_t(mGIFStruct.height) != mImageMetadata.GetHeight()) {
     // Regardless of depth of input, image is decoded into 24bit RGB
     NeedNewFrame(mGIFStruct.images_decoded, mGIFStruct.x_offset,
                  mGIFStruct.y_offset, mGIFStruct.width, mGIFStruct.height,
                  format);
+  } else {
+    // Our preallocated frame matches up, with the possible exception of alpha.
+    if (format == gfxASurface::ImageFormatRGB24) {
+      GetCurrentFrame()->SetHasNoAlpha();
+    }
   }
 
   mCurrentFrame = mGIFStruct.images_decoded;
@@ -893,19 +906,23 @@ nsGIFDecoder2::WriteInternal(const char *aBuffer, uint32_t aCount)
       mColorMask = 0xFF >> (8 - realDepth);
       BeginImageFrame(realDepth);
 
-      // We now need a new frame from the decoder framework. We leave all our
-      // data in the buffer as if it wasn't consumed, copy to our hold and return
-      // to the decoder framework.
-      uint32_t size = len + mGIFStruct.bytes_to_consume + mGIFStruct.bytes_in_hold;
-      if (size) {
-        if (SetHold(q, mGIFStruct.bytes_to_consume + mGIFStruct.bytes_in_hold, buf, len)) {
-          // Back into the decoder infrastructure so we can get called again.
-          GETN(9, gif_image_header_continue);
-          return;
+      if (NeedsNewFrame()) {
+        // We now need a new frame from the decoder framework. We leave all our
+        // data in the buffer as if it wasn't consumed, copy to our hold and return
+        // to the decoder framework.
+        uint32_t size = len + mGIFStruct.bytes_to_consume + mGIFStruct.bytes_in_hold;
+        if (size) {
+          if (SetHold(q, mGIFStruct.bytes_to_consume + mGIFStruct.bytes_in_hold, buf, len)) {
+            // Back into the decoder infrastructure so we can get called again.
+            GETN(9, gif_image_header_continue);
+            return;
+          }
         }
+        break;
+      } else {
+        // FALL THROUGH
       }
     }
-    break;
 
     case gif_image_header_continue:
     {

image/decoders/nsPNGDecoder.cpp

@@ -140,7 +140,19 @@ void nsPNGDecoder::CreateFrame(png_uint_32 x_offset, png_uint_32 y_offset,
                                int32_t width, int32_t height,
                                gfxASurface::gfxImageFormat format)
 {
-  NeedNewFrame(mNumFrames, x_offset, y_offset, width, height, format);
+  // Our first full frame is automatically created by the image decoding
+  // infrastructure. Just use it as long as we're not creating a subframe.
+  MOZ_ASSERT(HasSize());
+  if (mNumFrames != 0 ||
+      x_offset != 0 || y_offset != 0 ||
+      width != mImageMetadata.GetWidth() || height != mImageMetadata.GetHeight()) {
+    NeedNewFrame(mNumFrames, x_offset, y_offset, width, height, format);
+  } else if (mNumFrames == 0) {
+    // Our preallocated frame matches up, with the possible exception of alpha.
+    if (format == gfxASurface::ImageFormatRGB24) {
+      GetCurrentFrame()->SetHasNoAlpha();
+    }
+  }
 
   mFrameRect.x = x_offset;
   mFrameRect.y = y_offset;
@@ -651,7 +663,7 @@ nsPNGDecoder::info_callback(png_structp png_ptr, png_infop info_ptr)
     }
   }
 
-  if (!decoder->mFrameIsHidden) {
+  if (!decoder->mFrameIsHidden && decoder->NeedsNewFrame()) {
     /* We know that we need a new frame, so pause input so the decoder
      * infrastructure can give it to us.
      */