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st: Generate shadows from the silhouette of the source texture

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As first mentioned in commit 672171093, the CSS spec defines shadow
colors independently from the colors casting the shadow. It's not
a physical light simulation so a shadow is allowed to be a different
color from texture casting it.

This means we only care about the shape of the source where alpha
values of zero are adjacent to alpha values of non-zero. And all such
non-zero pixels should be treated as fully opaque for the purpose of
shadow generation. While this would be wrong for a physical light
simulation it does allow us to cast shadows around semi-translucent
shapes and better support CSS.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4477
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1918>
This commit is contained in:
Daniel van Vugt 2021-07-16 18:34:11 +08:00 committed by Marge Bot
parent b3c7a855c6
commit 918c063693
1 changed files with 31 additions and 4 deletions
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35
src/st/st-private.c
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@ -394,7 +394,10 @@ _st_create_shadow_pipeline (StShadow *shadow_spec,
float sigma;
int src_height, dst_height;
int src_width, dst_width;
CoglPipeline *texture_pipeline;
static CoglPipelineKey texture_pipeline_key =
"st-create-shadow-pipeline-saturate-alpha";
static CoglPipeline *shadow_pipeline_template = NULL;
g_return_val_if_fail (shadow_spec != NULL, NULL);
@ -433,10 +436,34 @@ _st_create_shadow_pipeline (StShadow *shadow_spec,
});
/* Texture */
texture_node = clutter_texture_node_new (src_texture,
0,
CLUTTER_SCALING_FILTER_NEAREST,
CLUTTER_SCALING_FILTER_NEAREST);
texture_pipeline = cogl_context_get_named_pipeline (ctx,
&texture_pipeline_key);
if (G_UNLIKELY (texture_pipeline == NULL))
{
CoglSnippet *snippet;
snippet = cogl_snippet_new (COGL_SNIPPET_HOOK_FRAGMENT,
"",
"if (cogl_color_out.a > 0.0)\n"
" cogl_color_out.a = 1.0;");
texture_pipeline = cogl_pipeline_new (ctx);
cogl_pipeline_add_snippet (texture_pipeline, snippet);
cogl_object_unref (snippet);
cogl_context_set_named_pipeline (ctx,
&texture_pipeline_key,
texture_pipeline);
}
/* No need to unref texture_pipeline since the named pipeline hash
* doesn't change its ref count from 1. Also no need to copy texture_pipeline
* since we'll be completely finished with it after clutter_paint_node_paint.
*/
cogl_pipeline_set_layer_texture (texture_pipeline, 0, src_texture);
texture_node = clutter_pipeline_node_new (texture_pipeline);
clutter_paint_node_add_child (blur_node, texture_node);
clutter_paint_node_add_rectangle (texture_node,
&(ClutterActorBox) {