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LibGfx: Store alpha type information in Gfx::Bitmap

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We use instances of `Gfx::Bitmap` to move pixel data all the way from
raw image bytes up to the Skia renderer. A vital piece of information
for correct blending of bitmaps is the alpha type, i.e. are we dealing
with premultiplied or unpremultiplied color values?

Premultiplied means that the RGB colors have been multiplied with the
associated alpha value, i.e. RGB(255, 255, 255) with an alpha of 2% is
stored as RGBA(5, 5, 5, 2%).

Unpremultiplied means that the original RGB colors are stored,
regardless of the alpha value. I.e. RGB(255, 255, 255) with an alpha of
2% is stored as RGBA(255, 255, 255, 2%).

It is important to know how the color data is stored in a
`Gfx::Bitmap`, because correct blending depends on knowing the alpha
type: premultiplied blending uses `S + (1 - A) * D`, while
unpremultiplied blending uses `A * S + (1 - A) * D`.

This adds the alpha type information to `Gfx::Bitmap` across the board.
It isn't used anywhere yet.
This commit is contained in:
Jelle Raaijmakers
2024-08-02 12:52:14 +02:00
committed by Alexander Kalenik
parent 99dd063c58
commit b193fe658d
9 changed files with 71 additions and 33 deletions
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2
Ladybird/Android/src/main/cpp/WebViewImplementationNative.cpp
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@@ -69,7 +69,7 @@ void WebViewImplementationNative::paint_into_bitmap(void* android_bitmap_raw, An
// Software bitmaps only for now!
VERIFY((info.flags & ANDROID_BITMAP_FLAGS_IS_HARDWARE) == 0);
auto android_bitmap = MUST(Gfx::Bitmap::create_wrapper(to_gfx_bitmap_format(info.format), { info.width, info.height }, info.stride, android_bitmap_raw));
auto android_bitmap = MUST(Gfx::Bitmap::create_wrapper(to_gfx_bitmap_format(info.format), Gfx::AlphaType::Premultiplied, { info.width, info.height }, info.stride, android_bitmap_raw));
Gfx::Painter painter(android_bitmap);
if (auto* bitmap = m_client_state.has_usable_bitmap ? m_client_state.front_bitmap.bitmap.ptr() : m_backup_bitmap.ptr())
painter.blit({ 0, 0 }, *bitmap, bitmap->rect());