LibGfx+Tests: Load bmp files with unpremultiplied alpha

InvalidUsernameException · gmta · commit bc44203744cf · 2025-11-28T17:00:29.000+01:00
From what I can tell BMP files with an alpha channel always store
unpremultiplied alpha. So let's load them as such to avoid rendering
artifacts from using the wrong alpha type.
diff --git a/Libraries/LibGfx/ImageFormats/BMPLoader.cpp b/Libraries/LibGfx/ImageFormats/BMPLoader.cpp
@@ -1298,7 +1298,7 @@ static ErrorOr<void> decode_bmp_pixel_data(BMPLoadingContext& context)
     u32 const width = abs(context.dib.core.width);
     u32 const height = !context.is_included_in_ico ? abs(context.dib.core.height) : (abs(context.dib.core.height) / 2);
 
-    context.bitmap = TRY(Bitmap::create(format, { static_cast<int>(width), static_cast<int>(height) }));
+    context.bitmap = TRY(Bitmap::create(format, Gfx::AlphaType::Unpremultiplied, { static_cast<int>(width), static_cast<int>(height) }));
 
     ByteBuffer rle_buffer;
     ReadonlyBytes bytes { context.file_bytes + context.data_offset, context.file_size - context.data_offset };
diff --git a/Tests/LibWeb/Ref/expected/bmp-load-as-unpremultiplied-alpha-ref.html b/Tests/LibWeb/Ref/expected/bmp-load-as-unpremultiplied-alpha-ref.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<style>
+body {
+  color: white;
+  background-color: black;
+}
+</style>
+<p>All three images should render the same</p>
+<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAMAAAAoLQ9TAAAAS1BMVEUAAAAAgAAAqgAA+QAA/QAA/QAA/QAA/QAA/QAA/QAA/QAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/wDNmTtpAAAAGHRSTlMAAQIkXF11d3h6foCBiImXnZ7Iy87S3d5CsE4PAAAAZ0lEQVQY02VPwRaAIAijSMvSsrLa/39pil6KPQ5s8MYgEvDzMDV0w9CRA1ztiAwQ+wVY+giYKiB6wGcuQhkgSeVVMdnQsBfOdpyOys9ptEw3Pri1wHMIqZJrDTNrU31WBVPR1XO/918rbA2ED1iblwAAAABJRU5ErkJggg==">
+<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAMAAAAoLQ9TAAAAS1BMVEUAAAAAgAAAqgAA+QAA/QAA/QAA/QAA/QAA/QAA/QAA/QAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/wDNmTtpAAAAGHRSTlMAAQIkXF11d3h6foCBiImXnZ7Iy87S3d5CsE4PAAAAZ0lEQVQY02VPwRaAIAijSMvSsrLa/39pil6KPQ5s8MYgEvDzMDV0w9CRA1ztiAwQ+wVY+giYKiB6wGcuQhkgSeVVMdnQsBfOdpyOys9ptEw3Pri1wHMIqZJrDTNrU31WBVPR1XO/918rbA2ED1iblwAAAABJRU5ErkJggg==">
+<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAMAAAAoLQ9TAAAAS1BMVEUAAAAAgAAAqgAA+QAA/QAA/QAA/QAA/QAA/QAA/QAA/QAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/wDNmTtpAAAAGHRSTlMAAQIkXF11d3h6foCBiImXnZ7Iy87S3d5CsE4PAAAAZ0lEQVQY02VPwRaAIAijSMvSsrLa/39pil6KPQ5s8MYgEvDzMDV0w9CRA1ztiAwQ+wVY+giYKiB6wGcuQhkgSeVVMdnQsBfOdpyOys9ptEw3Pri1wHMIqZJrDTNrU31WBVPR1XO/918rbA2ED1iblwAAAABJRU5ErkJggg==">
diff --git a/Tests/LibWeb/Ref/input/bmp-load-as-unpremultiplied-alpha.html b/Tests/LibWeb/Ref/input/bmp-load-as-unpremultiplied-alpha.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<link rel="match" href="../expected/bmp-load-as-unpremultiplied-alpha-ref.html" />
+<style>
+body {
+  color: white;
+  background-color: black;
+}
+</style>
+<p>All three images should render the same</p>
+<img src="data:image/vnd.microsoft.icon;base64,AAABAAEAEBAAAAEAIABoBAAAFgAAACgAAAAQAAAAIAAAAAEAIAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA+QAkAP4A3gD+AN4A+QAkAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACAAAEA/QBcAP0AXACAAAEAAAAAAP0AegD/AP8A/wD/AP0AegAAAAAAgAABAP0AXAD9AFwAgAABAAAAAAAAAAAA/QBdAP8A/wD/AP8A/gCdAKoAAgD+AIAA/wD/AP8A/wD+AIAAqgACAP4AnQD/AP8A/wD/AP0AXQAAAAAAAAAAAP0AXQD/AP8A/wD/AP8A/wD+AJ0A/gCBAP8A/wD/AP8A/gCBAP4AnQD/AP8A/wD/AP8A/wD9AF0AAAAAAAAAAACqAAIA/gCdAP8A/wD/AP8A/wD/AP4AzgD/AP8A/wD/AP4AzgD/AP8A/wD/AP8A/wD+AJ0AqgACAAAAAAAAAAAAAAAAAIAAAQD+AJcA/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD+AJ4AqgACAAAAAAAAAAAA+QAkAP0AfgD+AIgA/gCIAP4AzgD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD+ANIA/gCJAP4AiAD9AH4A+QAkAP4A3QD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP4A3QD+AN0A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD+AN0A+QAkAP0AdQD9AHcA/QB4AP4AyAD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD+AMsA/QB4AP0AdwD9AHUA+QAkAAAAAAAAAAAAgAABAP4AlwD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP8A/wD/AP4AngCqAAIAAAAAAAAAAAAAAAAAqgACAP4AnQD/AP8A/wD/AP8A/wD+AM4A/wD/AP8A/wD+AM4A/wD/AP8A/wD/AP8A/gCdAKoAAgAAAAAAAAAAAP0AXQD/AP8A/wD/AP8A/wD+AJ0A/gCBAP8A/wD/AP8A/gCBAP4AnQD/AP8A/wD/AP8A/wD9AF0AAAAAAAAAAAD9AF0A/wD/AP8A/wD+AJ0AqgACAP4AgAD/AP8A/wD/AP4AgACqAAIA/gCdAP8A/wD/AP8A/QBdAAAAAAAAAAAAgAABAP0AXAD9AFwAgAABAAAAAAD9AHoA/wD/AP8A/wD9AHoAAAAAAIAAAQD9AFwA/QBcAIAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA+QAkAP4A3gD+AN4A+QAkAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA/D8AAIQhAACAAQAAgAEAAIABAADAAwAAAAAAAAAAAAAAAAAAAAAAAMADAACAAQAAgAEAAIABAACEIQAA/D8AAA==">
+<img src="data:image/bmp;base64,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">
+<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAMAAAAoLQ9TAAAAS1BMVEUAAAAAgAAAqgAA+QAA/QAA/QAA/QAA/QAA/QAA/QAA/QAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/gAA/wDNmTtpAAAAGHRSTlMAAQIkXF11d3h6foCBiImXnZ7Iy87S3d5CsE4PAAAAZ0lEQVQY02VPwRaAIAijSMvSsrLa/39pil6KPQ5s8MYgEvDzMDV0w9CRA1ztiAwQ+wVY+giYKiB6wGcuQhkgSeVVMdnQsBfOdpyOys9ptEw3Pri1wHMIqZJrDTNrU31WBVPR1XO/918rbA2ED1iblwAAAABJRU5ErkJggg==">
