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lightbox: Use variable magnitude for dithering noise

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The magnitude of the noise in the final output scales with:

 1.0 - cogl_color_out.a

A fixed value for the magnitude relative to cogl_color_out.a might be
too small for large cogl_color_out.a or too large for small
cogl_color_out.a (or possibly even both).

Compensate for this to achieve constant magnitude in the final output.
The coefficients were determined experimentally as a compromise between
preventing colour banding with dark background content and tolerable
noise with bright background content.

This helps for the issue described in
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7795 .

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3572>
This commit is contained in:
Michel Dänzer 2024-12-12 17:55:08 +01:00 committed by Marge Bot
parent cbfcf17002
commit 60dd2cd7a8
1 changed files with 2 additions and 1 deletions
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3
js/ui/lightbox.js
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@ -24,7 +24,8 @@ vec2 position = cogl_tex_coord_in[0].xy - 0.5; \n\
float t = clamp(length(1.41421 * position), 0.0, 1.0); \n\
float pixel_brightness = mix(1.0, 1.0 - vignette_sharpness, t); \n\
cogl_color_out.a *= 1.0 - pixel_brightness * brightness; \n\
cogl_color_out.a += (rand(position) - 0.5) / 100.0; \n';
float noise_magnitude = (2.0 / ((1.0 - cogl_color_out.a) * 255.0)); \n\
cogl_color_out.a += (rand(position) - 0.5) * noise_magnitude; \n';
const RadialShaderEffect = GObject.registerClass({