Update pixels::unmultiply_inplace to support RB swap and use it in canvas_state (#35313)

* update unmultiply_inplace to handle reversed RGB

Signed-off-by: Euclid Ye <yezhizhenjiakang@gmail.com>

* Reuse unmultiply_inplace instead of manual compute; remove unused

Signed-off-by: Euclid Ye <yezhizhenjiakang@gmail.com>

---------

Signed-off-by: Euclid Ye <yezhizhenjiakang@gmail.com>

components/canvas/webgl_thread.rs

@@ -2785,7 +2785,7 @@ fn prepare_pixels(
         },
         Some(AlphaTreatment::Unmultiply) => {
             assert!(pixel_format.is_some());
-            unmultiply_inplace(pixels.to_mut());
+            unmultiply_inplace::<false>(pixels.to_mut());
         },
         None => {},
     }

components/pixels/benches.rs

@@ -20,7 +20,7 @@ fn bench(c: &mut Criterion) {
     c.bench_function("unmultiply_inplace", move |b| {
         b.iter_batched(
             || data.clone(),
-            |mut data| pixels::unmultiply_inplace(&mut data),
+            |mut data| pixels::unmultiply_inplace::<false>(&mut data),
             BatchSize::SmallInput,
         )
     });

components/pixels/lib.rs

@@ -197,7 +197,7 @@ pub fn detect_image_format(buffer: &[u8]) -> Result<ImageFormat, &str> {
     }
 }
 
-pub fn unmultiply_inplace(pixels: &mut [u8]) {
+pub fn unmultiply_inplace<const SWAP_RB: bool>(pixels: &mut [u8]) {
     for rgba in pixels.chunks_mut(4) {
         let a = rgba[3] as u32;
         let mut b = rgba[2] as u32;
@@ -209,9 +209,15 @@ pub fn unmultiply_inplace(pixels: &mut [u8]) {
             g = g * 255 / a;
             b = b * 255 / a;
 
-            rgba[2] = b as u8;
-            rgba[1] = g as u8;
-            rgba[0] = r as u8;
+            if SWAP_RB {
+                rgba[2] = r as u8;
+                rgba[1] = g as u8;
+                rgba[0] = b as u8;
+            } else {
+                rgba[2] = b as u8;
+                rgba[1] = g as u8;
+                rgba[0] = r as u8;
+            }
         }
     }
 }

components/script/canvas_state.rs

@@ -58,7 +58,6 @@ use crate::dom::offscreencanvas::{OffscreenCanvas, OffscreenCanvasContext};
 use crate::dom::paintworkletglobalscope::PaintWorkletGlobalScope;
 use crate::dom::textmetrics::TextMetrics;
 use crate::script_runtime::CanGc;
-use crate::unpremultiplytable::UNPREMULTIPLY_TABLE;
 
 #[cfg_attr(crown, crown::unrooted_must_root_lint::must_root)]
 #[derive(Clone, JSTraceable, MallocSizeOf)]
@@ -320,12 +319,7 @@ impl CanvasState {
         self.send_canvas_2d_msg(Canvas2dMsg::GetImageData(rect, canvas_size, sender));
         let mut pixels = receiver.recv().unwrap().to_vec();
 
-        for chunk in pixels.chunks_mut(4) {
-            let b = chunk[0];
-            chunk[0] = UNPREMULTIPLY_TABLE[256 * (chunk[3] as usize) + chunk[2] as usize];
-            chunk[1] = UNPREMULTIPLY_TABLE[256 * (chunk[3] as usize) + chunk[1] as usize];
-            chunk[2] = UNPREMULTIPLY_TABLE[256 * (chunk[3] as usize) + b as usize];
-        }
+        pixels::unmultiply_inplace::<true>(&mut pixels);
 
         pixels
     }

components/script/lib.rs

@@ -67,7 +67,6 @@ pub mod test;
 #[allow(dead_code)]
 pub mod textinput;
 mod timers;
-mod unpremultiplytable;
 mod webdriver_handlers;
 mod window_named_properties;