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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
Notes: github-actions[bot] 2024-08-07 18:17:06 +00:00
9 changed files with 71 additions and 33 deletions
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4
Userland/Libraries/LibWebView/ViewImplementation.cpp
View file

@ -431,8 +431,8 @@ void ViewImplementation::did_allocate_iosurface_backing_stores(i32 front_id, Cor
auto bytes_per_row = front_iosurface.bytes_per_row();
auto front_bitmap = Gfx::Bitmap::create_wrapper(Gfx::BitmapFormat::BGRA8888, front_size, bytes_per_row, front_iosurface.data(), [handle = move(front_iosurface)] {});
auto back_bitmap = Gfx::Bitmap::create_wrapper(Gfx::BitmapFormat::BGRA8888, back_size, bytes_per_row, back_iosurface.data(), [handle = move(back_iosurface)] {});
auto front_bitmap = Gfx::Bitmap::create_wrapper(Gfx::BitmapFormat::BGRA8888, Gfx::AlphaType::Premultiplied, front_size, bytes_per_row, front_iosurface.data(), [handle = move(front_iosurface)] {});
auto back_bitmap = Gfx::Bitmap::create_wrapper(Gfx::BitmapFormat::BGRA8888, Gfx::AlphaType::Premultiplied, back_size, bytes_per_row, back_iosurface.data(), [handle = move(back_iosurface)] {});
m_client_state.front_bitmap.bitmap = front_bitmap.release_value_but_fixme_should_propagate_errors();
m_client_state.front_bitmap.id = front_id;