Bug 75077 - Interpolate interlaced PNG images instead of libpng blocky display. r=seth

image/decoders/nsPNGDecoder.cpp

@@ -679,13 +679,89 @@ nsPNGDecoder::PostFullInvalidation()
   }
 }
 
+static void
+InterpolateInterlacedPNG(const int aPass, const bool aHasAlpha,
+                         const uint32_t aWidth, const uint32_t aHeight,
+                         uint8_t* aImageData)
+{
+  // At this point we have a completed pass of an interlaced image in
+  // imageData as an array of uint8_t ARGB or XRGB pixels, optionally
+  // premultiplied, 4 bytes per pixel. If there are leftover partial
+  // blocks at the right edge or bottom of the image, we just use the
+  // uninterpolated pixels that libpng gave us.
+  //
+  // See Bug #75077, Interpolation of interlaced PNG
+  // See https://en.wikipedia.org/wiki/Bilinear_interpolation
+  //
+  // Note: this doesn't work when downscaling so we simply show
+  // the uninterpolated blocks that libpng gives us.
+  //
+  // Don't try to interpolate images that are less than 8 columns wide
+  // or 8 rows high; do only square passes (0, 2, 4)
+  if ((aPass != 0 && aPass != 2 && aPass != 4) || aWidth < 8 || aHeight < 8) {
+    return;
+  }
+
+  /* Block dimensions are defined by the PNG specification */
+  uint32_t block_width[]  = { 8, 4, 4, 2, 2 };
+  uint32_t bw = block_width[aPass];
+  uint32_t bh = bw;
+
+  bool first_component = aHasAlpha ? 0: 1;
+
+  // Reduced version of the PNG_PASS_ROW_SHIFT(pass) macro in libpng/png.h
+  // Only works with square passes 0, 2, and 4
+  uint32_t divisor_shift = 3 - (aPass >> 1);
+
+  // Loop over blocks
+  for (uint32_t y = 0; y < aHeight - bh; y += bh) {
+    for (uint32_t x = 0; x < aWidth - bw; x += bw) {
+      // (x,y) is the top left corner of the block
+      // topleft is the first component of the top left pixel of the block
+      uint8_t* topleft = aImageData + 4 * (x + aWidth * y);
+
+      // Loop over component=[A,]R,G,B
+      for (uint32_t component = first_component; component < 4; component++) {
+        if (x == 0) {
+          // Interpolate ARGB along the left side of the block
+          uint32_t top = *(topleft + component);
+          uint32_t bottom = *(topleft + component + (bh * 4 * aWidth));
+          for (uint32_t j = 1; j < bh; j++) {
+            *(topleft + component + j * 4 * aWidth) =
+              ((top * (bh - j) + bottom * j) >> divisor_shift) & 0xff;
+          }
+        }
+
+        // Interpolate ARGB along the right side of the block
+        uint32_t top = *(topleft + component + 4 * bw);
+        uint32_t bottom = *(topleft + component + 4 * (bw + (bh * aWidth)));
+        for (uint32_t j = 1; j < bh; j++) {
+          *(topleft + component + 4 * (bw + j * aWidth)) =
+          ((top * (bh - j) + bottom * j) >> divisor_shift) & 0xff;
+        }
+
+        // Interpolate ARGB in the X-direction along the top edge
+        // and within the block
+        for (uint32_t j = 0; j < bh; j++) {
+          uint32_t left = *(topleft + component + 4 * j * aWidth);
+          uint32_t right = *(topleft + component + 4 * (bw + j * aWidth));
+          for (uint32_t i = 1; i < bw; i++) {
+            *(topleft + component + 4 * (i + j * aWidth)) =
+            ((left * (bw - i) + right * i) >> divisor_shift) & 0xff;
+          } // i
+        } // j
+      } // component
+    } // x
+  } // y
+}
+
 void
 nsPNGDecoder::row_callback(png_structp png_ptr, png_bytep new_row,
                            png_uint_32 row_num, int pass)
 {
   /* libpng comments:
    *
-   * this function is called for every row in the image.  If the
+   * This function is called for every row in the image.  If the
    * image is interlacing, and you turned on the interlace handler,
    * this function will be called for every row in every pass.
    * Some of these rows will not be changed from the previous pass.
@@ -722,12 +798,19 @@ nsPNGDecoder::row_callback(png_structp png_ptr, png_bytep new_row,
     return;
   }
 
-  // If |new_row| is null, that indicates that this is an interlaced image and
-  // |row_callback| is being called for a row that hasn't changed.  Ordinarily
-  // we don't need to do anything in this case, but if we're downscaling, the
-  // downscaler doesn't store the rows from previous passes, so we still need to
-  // process the row.
-  if (new_row || decoder->mDownscaler) {
+  bool lastRow =
+    row_num == static_cast<png_uint_32>(decoder->mFrameRect.height) - 1;
+
+  if (!new_row && !decoder->mDownscaler && !lastRow) {
+    // If |new_row| is null, that indicates that this is an interlaced image
+    // and |row_callback| is being called for a row that hasn't changed.
+    // Ordinarily we don't need to do anything in this case, but if we're
+    // downscaling, the downscaler doesn't store the rows from previous passes,
+    // so we still need to process the row.  If |lastRow| is true we need
+    // to finish the interlace pass.
+    return;
+  }
+
   int32_t width = decoder->mFrameRect.width;
   uint32_t iwidth = decoder->mFrameRect.width;
 
@@ -811,9 +894,17 @@ nsPNGDecoder::row_callback(png_structp png_ptr, png_bytep new_row,
   if (!decoder->interlacebuf) {
     // Do line-by-line partial invalidations for non-interlaced images.
     decoder->PostPartialInvalidation(IntRect(0, row_num, width, 1));
-    } else if (row_num ==
-               static_cast<png_uint_32>(decoder->mFrameRect.height - 1)) {
-      // Do only one full image invalidation for each pass. (Bug 1187569)
+  } else if (lastRow) {
+    // Do only one full image invalidation for each even pass. (Bug 1187569)
+    if (decoder->mDownscaler) {
+      decoder->PostFullInvalidation();
+    } else if (pass % 2 == 0) {
+
+      const bool hasAlpha = decoder->format != SurfaceFormat::B8G8R8X8;
+      InterpolateInterlacedPNG(pass, hasAlpha,
+                               static_cast<uint32_t>(width),
+                               decoder->mFrameRect.height,
+                               decoder->mImageData);
       decoder->PostFullInvalidation();
     }
   }