Transparency

skia/ext/bitmap_platform_device_win.cc

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "skia/ext/bitmap_platform_device_win.h"
 
 #include "SkMatrix.h"
 #include "SkRefCnt.h"
 #include "SkRegion.h"
 #include "SkUtils.h"
 
 namespace skia {
 
-// When Windows draws text, is sets the fourth byte (which Skia uses for alpha)
-// to zero. This means that if we try compositing with text that Windows has
-// drawn, we get invalid color values (if the alpha is 0, the other channels
-// should be 0 since Skia uses premultiplied colors) and strange results.
-//
-// HTML rendering only requires one bit of transparency. When you ask for a
-// semitransparent div, the div itself is drawn in another layer as completely
-// opaque, and then composited onto the lower layer with a transfer function.
-// The only place an alpha channel is needed is to track what has been drawn
-// and what has not been drawn.
-//
-// Therefore, when we allocate a new device, we fill it with this special
-// color. Because Skia uses premultiplied colors, any color where the alpha
-// channel is smaller than any component is impossible, so we know that no
-// legitimate drawing will produce this color. We use 1 as the alpha value
-// because 0 is produced when Windows draws text (even though it should be
-// opaque).
-//
-// When a layer is done and we want to render it to a lower layer, we use
-// fixupAlphaBeforeCompositing. This replaces all 0 alpha channels with
-// opaque (to fix the text problem), and replaces this magic color value
-// with transparency. The result is something that can be correctly
-// composited. However, once this has been done, no more can be drawn to
-// the layer because fixing the alphas *again* will result in incorrect
-// values.
-static const uint32_t kMagicTransparencyColor = 0x01FFFEFD;
-
 namespace {
 
 // Constrains position and size to fit within available_size. If |size| is -1,
 // all the available_size is used. Returns false if the position is out of
 // available_size.
 bool Constrain(int available_size, int* position, int *size) {
   if (*size < -2)
     return false;
 
   if (*position < 0) {
     if (*size != -1)
       *size += *position;
     *position = 0;
   }
   if (*size == 0 || *position >= available_size)
     return false;
 
   if (*size > 0) {
     int overflow = (*position + *size) - available_size;
     if (overflow > 0) {
       *size -= overflow;
     }
   } else {
     // Fill up available size.
     *size = available_size - *position;
   }
   return true;
 }
 
-// If the pixel value is 0, it gets set to kMagicTransparencyColor.
-void PrepareAlphaForGDI(uint32_t* pixel) {
-  if (*pixel == 0) {
-    *pixel = kMagicTransparencyColor;
-  }
-}
-
-// If the pixel value is kMagicTransparencyColor, it gets set to 0. Otherwise
-// if the alpha is 0, the alpha is set to 255.
-void PostProcessAlphaForGDI(uint32_t* pixel) {
-  if (*pixel == kMagicTransparencyColor) {
-    *pixel = 0;
-  } else if ((*pixel & 0xFF000000) == 0) {
-    *pixel |= 0xFF000000;
-  }
-}
-
-// Sets the opacity of the specified value to 0xFF.
-void MakeOpaqueAlphaAdjuster(uint32_t* pixel) {
-  *pixel |= 0xFF000000;
-}
-
-// See the declaration of kMagicTransparencyColor at the top of the file.
-void FixupAlphaBeforeCompositing(uint32_t* pixel) {
-  if (*pixel == kMagicTransparencyColor)
-    *pixel = 0;
-  else
-    *pixel |= 0xFF000000;
-}
-
 }  // namespace
 
 class BitmapPlatformDeviceWin::BitmapPlatformDeviceWinData : public SkRefCnt {
  public:
   explicit BitmapPlatformDeviceWinData(HBITMAP hbitmap);
 
   // Create/destroy hdc_, which is the memory DC for our bitmap data.
   HDC GetBitmapDC();
   void ReleaseBitmapDC();
   bool IsBitmapDCCreated() const;
@@ skipping 163 matching lines @@
   bitmap.setPixels(data);
   bitmap.setIsOpaque(is_opaque);
 
   if (is_opaque) {
 #ifndef NDEBUG
     // To aid in finding bugs, we set the background color to something
     // obviously wrong so it will be noticable when it is not cleared
     bitmap.eraseARGB(255, 0, 255, 128);  // bright bluish green
 #endif
   } else {
-    // A transparent layer is requested: fill with our magic "transparent"
-    // color, see the declaration of kMagicTransparencyColor above
-    sk_memset32(static_cast<uint32_t*>(data), kMagicTransparencyColor,
-                width * height);
+    bitmap.eraseARGB(0, 0, 0, 0);
   }
 
   // The device object will take ownership of the HBITMAP. The initial refcount
   // of the data object will be 1, which is what the constructor expects.
   return new BitmapPlatformDeviceWin(new BitmapPlatformDeviceWinData(hbitmap),
                                      bitmap);
 }
 
 // The device will own the HBITMAP, which corresponds to also owning the pixel
 // data. Therefore, we do not transfer ownership to the SkDevice's bitmap.
@@ skipping 83 matching lines @@
                   copy_width,
                   copy_height,
                   blend_function);
   }
   LoadTransformToDC(source_dc, data_->transform());
 
   if (created_dc)
     data_->ReleaseBitmapDC();
 }
 
-void BitmapPlatformDeviceWin::prepareForGDI(int x, int y, int width,
-                                            int height) {
-  processPixels<PrepareAlphaForGDI>(x, y, width, height);
-}
-
-void BitmapPlatformDeviceWin::postProcessGDI(int x, int y, int width,
-                                             int height) {
-  processPixels<PostProcessAlphaForGDI>(x, y, width, height);
-}
-
 void BitmapPlatformDeviceWin::makeOpaque(int x, int y, int width, int height) {
-  processPixels<MakeOpaqueAlphaAdjuster>(x, y, width, height);
-}
-
-void BitmapPlatformDeviceWin::fixupAlphaBeforeCompositing() {
-  const SkBitmap& bitmap = accessBitmap(true);
-  SkAutoLockPixels lock(bitmap);
-  uint32_t* data = bitmap.getAddr32(0, 0);
-
-  size_t words = bitmap.rowBytes() / sizeof(uint32_t) * bitmap.height();
-  for (size_t i = 0; i < words; i++) {
-    if (data[i] == kMagicTransparencyColor)
-      data[i] = 0;
-    else
-      data[i] |= 0xFF000000;
-  }
-}
-
-// Returns the color value at the specified location.
-SkColor BitmapPlatformDeviceWin::getColorAt(int x, int y) {
-  const SkBitmap& bitmap = accessBitmap(false);
-  SkAutoLockPixels lock(bitmap);
-  uint32_t* data = bitmap.getAddr32(0, 0);
-  return static_cast<SkColor>(data[x + y * width()]);
-}
-
-void BitmapPlatformDeviceWin::onAccessBitmap(SkBitmap* bitmap) {
-  // FIXME(brettw) OPTIMIZATION: We should only flush if we know a GDI
-  // operation has occurred on our DC.
-  if (data_->IsBitmapDCCreated())
-    GdiFlush();
-}
-
-template<BitmapPlatformDeviceWin::adjustAlpha adjustor>
-void BitmapPlatformDeviceWin::processPixels(int x,
-                                            int y,
-                                            int width,
-                                            int height) {
   const SkBitmap& bitmap = accessBitmap(true);
   SkASSERT(bitmap.config() == SkBitmap::kARGB_8888_Config);
+
+  // FIXME(brettw): This is kind of lame, we shouldn't be dealing with
+  // transforms at this level. Probably there should be a PlatformCanvas
+  // function that does the transform (using the actual transform not just the
+  // translation) and calls us with the transformed rect.
   const SkMatrix& matrix = data_->transform();
   int bitmap_start_x = SkScalarRound(matrix.getTranslateX()) + x;
   int bitmap_start_y = SkScalarRound(matrix.getTranslateY()) + y;
 
   if (Constrain(bitmap.width(), &bitmap_start_x, &width) &&
       Constrain(bitmap.height(), &bitmap_start_y, &height)) {
     SkAutoLockPixels lock(bitmap);
     SkASSERT(bitmap.rowBytes() % sizeof(uint32_t) == 0u);
     size_t row_words = bitmap.rowBytes() / sizeof(uint32_t);
     // Set data to the first pixel to be modified.
     uint32_t* data = bitmap.getAddr32(0, 0) + (bitmap_start_y * row_words) +
                      bitmap_start_x;
     for (int i = 0; i < height; i++) {
-      for (int j = 0; j < width; j++) {
-        adjustor(data + j);
-      }
+      for (int j = 0; j < width; j++)
+        data[j] |= (0xFF << SK_A32_SHIFT);
       data += row_words;
     }
   }
 }
 
+// Returns the color value at the specified location.
+SkColor BitmapPlatformDeviceWin::getColorAt(int x, int y) {
+  const SkBitmap& bitmap = accessBitmap(false);
+  SkAutoLockPixels lock(bitmap);
+  uint32_t* data = bitmap.getAddr32(0, 0);
+  return static_cast<SkColor>(data[x + y * width()]);
+}
+
+void BitmapPlatformDeviceWin::onAccessBitmap(SkBitmap* bitmap) {
+  // FIXME(brettw) OPTIMIZATION: We should only flush if we know a GDI
+  // operation has occurred on our DC.
+  if (data_->IsBitmapDCCreated())
+    GdiFlush();
+}
+
 }  // namespace skia