LibWeb: Use bitmap's alpha type instead of assuming unpremultiplied

When converting a `Gfx::Bitmap` to a Skia bitmap, we cannot assume the
color data is unpremultiplied. For example, everything canvas-related
uses premultiplied color data:

  https://html.spec.whatwg.org/multipage/canvas.html#premultiplied-alpha-and-the-2d-rendering-context

We were probably assuming unpremultiplied since that is what the PNG
decoder gives us. Since we now make `Gfx::Bitmap` identify what alpha
type is being used, we can instruct Skia a bit better :^)

Update our `EdgeFlagPathRasterizer` to use premultiplied alpha instead
of unpremultiplied so we can apply alpha correctly for path masks.

This fixes the dark borders sometimes visible when SVGs are blended
with a colored background.

This also exposed an issue with our `CanvasRenderingContext2D`, which is
supposed to hold a bitmap with premultiplied alpha internally but expose
a bitmap with unpremultiplied alpha in `CanvasImageData`. Expand our C2D
test to include the alpha channel as well.

Finally, this also exposed an off-by-one issue in
`EdgeFlagPathRasterizer` which caused the last scanlines for edges to
render incorrectly. We had some reference images which included these
corruptions (they were almost unnoticeable), so update them as well.

Tests/LibWeb/Text/input/HTML/CanvasRenderingContext2D-get-image-data-correctness.html

@@ -12,7 +12,7 @@
 
         let imageData = areaCtx.getImageData(0, 0, area.width, area.height);
 
-        if (imageData.data[0] == 0xff && imageData.data[1] == 0x80 && imageData.data[2] == 0x40)
+        if (imageData.data[0] == 0xff && imageData.data[1] == 0x80 && imageData.data[2] == 0x40 && imageData.data[3] == 0xff)
             println("PASS");
         else
             println("FAIL");

Userland/Libraries/LibGfx/Color.h

@@ -281,6 +281,17 @@ public:
         return color_with_alpha;
     }
 
+    constexpr Color to_unpremultiplied() const
+    {
+        if (alpha() == 0 || alpha() == 255)
+            return *this;
+        return Color(
+            red() * 255 / alpha(),
+            green() * 255 / alpha(),
+            blue() * 255 / alpha(),
+            alpha());
+    }
+
     constexpr Color blend(Color source) const
     {
         if (alpha() == 0 || source.alpha() == 255)

Userland/Libraries/LibGfx/EdgeFlagPathRasterizer.cpp

@@ -29,7 +29,7 @@ static Vector<Detail::Edge> prepare_edges(ReadonlySpan<FloatLine> lines, unsigne
     // The first visible y value.
     auto top_clip = top_clip_scanline * int(samples_per_pixel);
     // The last visible y value.
-    auto bottom_clip = (bottom_clip_scanline + 1) * int(samples_per_pixel) - 1;
+    auto bottom_clip = (bottom_clip_scanline + 1) * int(samples_per_pixel);
     min_edge_y = bottom_clip;
     max_edge_y = top_clip;
 
@@ -75,9 +75,8 @@ static Vector<Detail::Edge> prepare_edges(ReadonlySpan<FloatLine> lines, unsigne
             start_x += dxdy * (top_clip - min_y);
             min_y = top_clip;
         }
-        if (max_y > bottom_clip) {
+        if (max_y > bottom_clip)
             max_y = bottom_clip;
-        }
 
         min_edge_y = min(min_y, min_edge_y);
         max_edge_y = max(max_y, max_edge_y);
@@ -240,8 +239,8 @@ Color EdgeFlagPathRasterizer<SamplesPerPixel>::scanline_color(int scanline, int
             return function({ offset, scanline });
         });
     if (color.alpha() == 255)
-        return color.with_alpha(alpha);
-    return color.with_alpha(color.alpha() * alpha / 255);
+        return color.with_alpha(alpha, AlphaType::Premultiplied);
+    return color.with_alpha(color.alpha() * alpha / 255, AlphaType::Premultiplied);
 }
 
 template<unsigned SamplesPerPixel>
@@ -309,7 +308,7 @@ auto EdgeFlagPathRasterizer<SamplesPerPixel>::accumulate_even_odd_scanline(EdgeE
     SampleType sample = init;
     VERIFY(edge_extent.min_x >= 0);
     VERIFY(edge_extent.max_x < static_cast<int>(m_scanline.size()));
-    for (int x = edge_extent.min_x; x <= edge_extent.max_x; x += 1) {
+    for (int x = edge_extent.min_x; x <= edge_extent.max_x; x++) {
         sample ^= m_scanline.data()[x];
         sample_callback(x, sample);
         m_scanline.data()[x] = 0;
@@ -323,7 +322,7 @@ auto EdgeFlagPathRasterizer<SamplesPerPixel>::accumulate_non_zero_scanline(EdgeE
     NonZeroAcc acc = init;
     VERIFY(edge_extent.min_x >= 0);
     VERIFY(edge_extent.max_x < static_cast<int>(m_scanline.size()));
-    for (int x = edge_extent.min_x; x <= edge_extent.max_x; x += 1) {
+    for (int x = edge_extent.min_x; x <= edge_extent.max_x; x++) {
         if (auto edges = m_scanline.data()[x]) {
             // We only need to process the windings when we hit some edges.
             for (auto y_sub = 0u; y_sub < SamplesPerPixel; y_sub++) {

Userland/Libraries/LibWeb/HTML/CanvasRenderingContext2D.cpp

@@ -359,10 +359,15 @@ WebIDL::ExceptionOr<JS::GCPtr<ImageData>> CanvasRenderingContext2D::get_image_da
 
     // 6. Set the pixel values of imageData to be the pixels of this's output bitmap in the area specified by the source rectangle in the bitmap's coordinate space units, converted from this's color space to imageData's colorSpace using 'relative-colorimetric' rendering intent.
     // FIXME: Can't use a Gfx::Painter + blit() here as it doesn't support ImageData bitmap's RGBA8888 format.
+    // NOTE: Internally we must use premultiplied alpha, but ImageData should hold unpremultiplied alpha. This conversion
+    //       might result in a loss of precision, but is according to spec.
+    //       See: https://html.spec.whatwg.org/multipage/canvas.html#premultiplied-alpha-and-the-2d-rendering-context
+    ASSERT(bitmap.alpha_type() == Gfx::AlphaType::Premultiplied);
+    ASSERT(image_data->bitmap().alpha_type() == Gfx::AlphaType::Unpremultiplied);
     for (int target_y = 0; target_y < source_rect_intersected.height(); ++target_y) {
         for (int target_x = 0; target_x < source_rect_intersected.width(); ++target_x) {
             auto pixel = bitmap.get_pixel(target_x + x, target_y + y);
-            image_data->bitmap().set_pixel(target_x, target_y, pixel);
+            image_data->bitmap().set_pixel(target_x, target_y, pixel.to_unpremultiplied());
         }
     }
 

Userland/Libraries/LibWeb/HTML/HTMLCanvasElement.cpp

@@ -212,7 +212,7 @@ bool HTMLCanvasElement::create_bitmap(size_t minimum_width, size_t minimum_heigh
         return false;
     }
     if (!m_bitmap || m_bitmap->size() != size) {
-        auto bitmap_or_error = Gfx::Bitmap::create(Gfx::BitmapFormat::BGRA8888, size);
+        auto bitmap_or_error = Gfx::Bitmap::create(Gfx::BitmapFormat::BGRA8888, Gfx::AlphaType::Premultiplied, size);
         if (bitmap_or_error.is_error())
             return false;
         m_bitmap = bitmap_or_error.release_value_but_fixme_should_propagate_errors();

Userland/Libraries/LibWeb/HTML/ImageData.cpp

@@ -30,7 +30,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<ImageData>> ImageData::create(JS::Realm& re
     // 2. Initialize this given sw, sh, and settings set to settings.
     // 3. Initialize the image data of this to transparent black.
     auto data = TRY(JS::Uint8ClampedArray::create(realm, sw * sh * 4));
-    auto bitmap = TRY_OR_THROW_OOM(vm, Gfx::Bitmap::create_wrapper(Gfx::BitmapFormat::RGBA8888, Gfx::AlphaType::Premultiplied, Gfx::IntSize(sw, sh), sw * sizeof(u32), data->data().data()));
+    auto bitmap = TRY_OR_THROW_OOM(vm, Gfx::Bitmap::create_wrapper(Gfx::BitmapFormat::RGBA8888, Gfx::AlphaType::Unpremultiplied, Gfx::IntSize(sw, sh), sw * sizeof(u32), data->data().data()));
 
     return realm.heap().allocate<ImageData>(realm, realm, bitmap, data);
 }

Userland/Libraries/LibWeb/Painting/DisplayListPlayerSkia.cpp

@@ -331,7 +331,8 @@ static SkColorType to_skia_color_type(Gfx::BitmapFormat format)
 static SkBitmap to_skia_bitmap(Gfx::Bitmap const& bitmap)
 {
     SkColorType color_type = to_skia_color_type(bitmap.format());
-    SkImageInfo image_info = SkImageInfo::Make(bitmap.width(), bitmap.height(), color_type, kUnpremul_SkAlphaType);
+    SkAlphaType alpha_type = bitmap.alpha_type() == Gfx::AlphaType::Premultiplied ? kPremul_SkAlphaType : kUnpremul_SkAlphaType;
+    SkImageInfo image_info = SkImageInfo::Make(bitmap.width(), bitmap.height(), color_type, alpha_type);
     SkBitmap sk_bitmap;
     sk_bitmap.setInfo(image_info);
 

Userland/Libraries/LibWeb/SVG/SVGDecodedImageData.cpp

@@ -87,7 +87,7 @@ void SVGDecodedImageData::visit_edges(Cell::Visitor& visitor)
 
 RefPtr<Gfx::Bitmap> SVGDecodedImageData::render(Gfx::IntSize size) const
 {
-    auto bitmap = Gfx::Bitmap::create(Gfx::BitmapFormat::BGRA8888, size).release_value_but_fixme_should_propagate_errors();
+    auto bitmap = Gfx::Bitmap::create(Gfx::BitmapFormat::BGRA8888, Gfx::AlphaType::Premultiplied, size).release_value_but_fixme_should_propagate_errors();
     VERIFY(m_document->navigable());
     m_document->navigable()->set_viewport_size(size.to_type<CSSPixels>());
     m_document->update_layout();