LibGfx: Correctly decode webp lossless with small palette and odd width

WebP lossless files that use a color indexing transform with <= 16
colors use pixel bundling to pack 2, 4, or 8 pixels into a single pixel.

If the image's width doesn't happen to be an exact multiple of the
bundling factor, we need to:

1. Use ceil_div() instead of just dividing the width by the bundling
   factor

2. Remember the original width and use it instead of computing
   reduced width times bundling factor

This does these changes, and adds a simple test for it -- it at least
checks that the decoded images have the right size.

(I created these images myself in Photoshop, and used the same
technique as for Tests/LibGfx/test-inputs/catdog-alert-*.webp
to create images with a certain number of colors.)

1 files changed, 10 insertions, 7 deletions

diff --git a/Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp b/Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
index 87083ec6f1..d4c8a28748 100644
--- a/Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
+++ b/Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
@@ -1083,7 +1083,7 @@ ErrorOr<NonnullRefPtr<Bitmap>> SubtractGreenTransform::transform(NonnullRefPtr<B
 // https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#44_color_indexing_transform
 class ColorIndexingTransform : public Transform {
 public:
-    static ErrorOr<NonnullOwnPtr<ColorIndexingTransform>> read(WebPLoadingContext&, LittleEndianInputBitStream&);
+    static ErrorOr<NonnullOwnPtr<ColorIndexingTransform>> read(WebPLoadingContext&, LittleEndianInputBitStream&, int original_width);
     virtual ErrorOr<NonnullRefPtr<Bitmap>> transform(NonnullRefPtr<Bitmap>) override;
 
     // For a color indexing transform, the green channel of the source image is used as the index into a palette to produce an output color.
@@ -1102,17 +1102,19 @@ public:
     int pixels_per_pixel() const { return m_pixels_per_pixel; }
 
 private:
-    ColorIndexingTransform(unsigned pixels_per_pixel, NonnullRefPtr<Bitmap> palette_bitmap)
+    ColorIndexingTransform(int pixels_per_pixel, int original_width, NonnullRefPtr<Bitmap> palette_bitmap)
         : m_pixels_per_pixel(pixels_per_pixel)
+        , m_original_width(original_width)
         , m_palette_bitmap(palette_bitmap)
     {
     }
 
     int m_pixels_per_pixel;
+    int m_original_width;
     NonnullRefPtr<Bitmap> m_palette_bitmap;
 };
 
-ErrorOr<NonnullOwnPtr<ColorIndexingTransform>> ColorIndexingTransform::read(WebPLoadingContext& context, LittleEndianInputBitStream& bit_stream)
+ErrorOr<NonnullOwnPtr<ColorIndexingTransform>> ColorIndexingTransform::read(WebPLoadingContext& context, LittleEndianInputBitStream& bit_stream, int original_width)
 {
     // color-indexing-image =  8BIT ; color count
     //                         entropy-coded-image
@@ -1137,7 +1139,7 @@ ErrorOr<NonnullOwnPtr<ColorIndexingTransform>> ColorIndexingTransform::read(WebP
     for (ARGB32* pixel = palette_bitmap->begin() + 1; pixel != palette_bitmap->end(); ++pixel)
         *pixel = add_argb32(*pixel, pixel[-1]);
 
-    return adopt_nonnull_own_or_enomem(new (nothrow) ColorIndexingTransform(pixels_per_pixel, move(palette_bitmap)));
+    return adopt_nonnull_own_or_enomem(new (nothrow) ColorIndexingTransform(pixels_per_pixel, original_width, move(palette_bitmap)));
 }
 
 ErrorOr<NonnullRefPtr<Bitmap>> ColorIndexingTransform::transform(NonnullRefPtr<Bitmap> bitmap)
@@ -1156,7 +1158,8 @@ ErrorOr<NonnullRefPtr<Bitmap>> ColorIndexingTransform::transform(NonnullRefPtr<B
     }
 
     // Pixel bundling case.
-    IntSize unbundled_size = { bitmap->size().width() * pixels_per_pixel(), bitmap->size().height() };
+    VERIFY(ceil_div(m_original_width, pixels_per_pixel()) == bitmap->size().width());
+    IntSize unbundled_size = { m_original_width, bitmap->size().height() };
     auto new_bitmap = TRY(Bitmap::create(BitmapFormat::BGRA8888, unbundled_size));
 
     unsigned bits_per_pixel = 8 / pixels_per_pixel();
@@ -1252,8 +1255,8 @@ static ErrorOr<void> decode_webp_chunk_VP8L(WebPLoadingContext& context, Chunk c
             TRY(transforms.try_append(TRY(try_make<SubtractGreenTransform>())));
             break;
         case COLOR_INDEXING_TRANSFORM: {
-            auto color_indexing_transform = TRY(ColorIndexingTransform::read(context, bit_stream));
-            stored_size.set_width(stored_size.width() / color_indexing_transform->pixels_per_pixel());
+            auto color_indexing_transform = TRY(ColorIndexingTransform::read(context, bit_stream, stored_size.width()));
+            stored_size.set_width(ceil_div(stored_size.width(), color_indexing_transform->pixels_per_pixel()));
             TRY(transforms.try_append(move(color_indexing_transform)));
             break;
         }