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Accumulate subpixels through stacking contexts

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Instead of simply rounding layer origins and discarding subpixel
offsets, accumulate them by transforming them into the space of the
next child stacking context. This is an attempt to eliminate subpixel
differences that are caused by different stacking context boundaries in
reference tests.

Currently these accumulated subpixels are only used for text
positioning, but the plan is that they can be used for all drawing in
the future.
This commit is contained in:
Martin Robinson 2016-07-21 15:26:35 +02:00
parent c1409af543
commit 47e69d1a95
5 changed files with 58 additions and 25 deletions
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56
components/gfx/display_list/mod.rs
View file

@ -382,7 +382,11 @@ impl DisplayList {
current_item_index: start,
last_item_index: end,
};
self.draw_stacking_context(stacking_context, &mut traversal, paint_context, transform);
self.draw_stacking_context(stacking_context,
&mut traversal,
paint_context,
transform,
&Point2D::zero());
}
fn draw_stacking_context_contents<'a>(&'a self,
@ -390,6 +394,7 @@ impl DisplayList {
traversal: &mut DisplayListTraversal<'a>,
paint_context: &mut PaintContext,
transform: &Matrix4D<f32>,
subpixel_offset: &Point2D<Au>,
tile_rect: Option<Rect<Au>>) {
for child in stacking_context.children.iter() {
while let Some(item) = traversal.advance(stacking_context) {
@ -399,7 +404,11 @@ impl DisplayList {
}
if child.intersects_rect_in_parent_context(tile_rect) {
self.draw_stacking_context(child, traversal, paint_context, &transform);
self.draw_stacking_context(child,
traversal,
paint_context,
&transform,
subpixel_offset);
} else {
traversal.skip_past_stacking_context(child);
}
@ -417,12 +426,14 @@ impl DisplayList {
stacking_context: &StackingContext,
traversal: &mut DisplayListTraversal<'a>,
paint_context: &mut PaintContext,
transform: &Matrix4D<f32>) {
transform: &Matrix4D<f32>,
subpixel_offset: &Point2D<Au>) {
if stacking_context.context_type != StackingContextType::Real {
self.draw_stacking_context_contents(stacking_context,
traversal,
paint_context,
transform,
subpixel_offset,
None);
return;
}
@ -431,18 +442,35 @@ impl DisplayList {
&stacking_context.filters,
stacking_context.blend_mode);
// If a layer is being used, the transform for this layer
// will be handled by the compositor.
let old_transform = paint_context.draw_target.get_transform();
let transform = match stacking_context.layer_info {
Some(..) => *transform,
let pixels_per_px = paint_context.screen_pixels_per_px();
let (transform, subpixel_offset) = match stacking_context.layer_info {
// If this stacking context starts a layer, the offset and transformation are handled
// by layer position within the compositor.
Some(..) => (*transform, *subpixel_offset),
None => {
let pixels_per_px = paint_context.screen_pixels_per_px();
let origin = &stacking_context.bounds.origin;
transform.translate(
origin.x.to_nearest_pixel(pixels_per_px.get()) as AzFloat,
origin.y.to_nearest_pixel(pixels_per_px.get()) as AzFloat,
0.0).mul(&stacking_context.transform)
let origin = stacking_context.bounds.origin + *subpixel_offset;
let pixel_snapped_origin =
Point2D::new(origin.x.to_nearest_pixel(pixels_per_px.get()),
origin.y.to_nearest_pixel(pixels_per_px.get()));
let transform = transform.translate(pixel_snapped_origin.x as AzFloat,
pixel_snapped_origin.y as AzFloat,
0.0).mul(&stacking_context.transform);
let inverse_transform = transform.invert();
// Here we are trying to accumulate any subpixel distances across transformed
// stacking contexts. This allows us transform stacking context with a
// pixel-snapped transform, but continue to propagate any subpixels from stacking
// context origins to children.
let subpixel_offset = Point2D::new(origin.x.to_f32_px() - pixel_snapped_origin.x,
origin.y.to_f32_px() - pixel_snapped_origin.y);
let subpixel_offset = inverse_transform.transform_point(&subpixel_offset) -
inverse_transform.transform_point(&Point2D::zero());;
let subpixel_offset = Point2D::new(Au::from_f32_px(subpixel_offset.x),
Au::from_f32_px(subpixel_offset.y));
(transform, subpixel_offset)
}
};
@ -455,6 +483,7 @@ impl DisplayList {
clip_rect: Some(stacking_context.overflow),
transient_clip: None,
layer_kind: paint_context.layer_kind,
subpixel_offset: subpixel_offset,
};
// Set up our clip rect and transform.
@ -469,6 +498,7 @@ impl DisplayList {
traversal,
&mut paint_subcontext,
&transform,
&subpixel_offset,
Some(transformed_tile_rect(paint_context.screen_rect, &transform)));
paint_subcontext.remove_transient_clip_if_applicable();