Bug 205444 – [GTK][WPE] Wrong visualization of Conic gradients in high resolution displays

• Component: CSS
• Status: RESOLVED FIXED

[Diego Pino]

Created attachment 386091 [details]
Conic gradient test 400x400 (taken from https://leaverou.github.io/conic-gradient/). Contains html code and visualizations in hi-res and lo-res displays.

There's a bug in https://trac.webkit.org/changeset/253685 when a Conic gradient is displayed in a high resolution display (i.e 3840x2160). In such displays, a conic-gradient like in the example attached shows a clear separation between sections. However, in a low resolution display (i.e 1360x768) the conic-gradient shows just fine.

[Diego Pino]

Created attachment 386092 [details]
Patch

[Carlos Alberto Lopez Perez]

Comment on attachment 386092 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=386092&action=review

> Source/WebCore/platform/graphics/cairo/GradientCairo.cpp:86
> +    const double pixelWidth = 0.1;

I find confusing this variable name. Its the width of a pixel 0.1??
maybe pixelSeparationWidth is a better name?
Also I think its worth adding a comment about why this is needed (because at high resolution a white line appears)

> Source/WebCore/platform/graphics/cairo/GradientCairo.cpp:99
> +        cxOffset = 0 * pixelWidth;
> +        cyOffset = -1 * pixelWidth;
>      } else if (from.offset >= 0.25 && from.offset < 0.50) {
> -        cxOffset = 0;
> -        cyOffset = 0;
> +        cxOffset = 0 * pixelWidth;
> +        cyOffset = 0 * pixelWidth;
>      } else if (from.offset >= 0.50 && from.offset < 0.75) {
> -        cxOffset = -1;
> -        cyOffset = 0;
> +        cxOffset = -1 * pixelWidth;
> +        cyOffset = 0 * pixelWidth;
>      } else if (from.offset >= 0.75 && from.offset < 1) {
> -        cxOffset = -1;
> -        cyOffset = -1;
> +        cxOffset = -1 * pixelWidth;
> +        cyOffset = -1 * pixelWidth;
>      } else {

Instead of doing this, perhaps it will be clear to do the multiplication below only once?

cx = cx + cxOffset * pixelWidth;
cy = cy + cyOffset * pixelWidth;;

[Carlos Alberto Lopez Perez]

I also wonder if its possible to write a regression test for this? can the current behaviour be triggered at low resolutions like the one used for layout tests? (i think its 800x600)

[Diego Pino]

Created attachment 386114 [details]
Patch

[Diego Pino]

I only managed to reproduce this issue when displaying the gradient in a high resolution monitor. I didn't manage to trigger this issue on a low resolution display, even trying different sizes for the gradient (3000x3000, 800x800). In conclusion, I think it's going to be hard to have a test for this issue considering LayoutTests run on a 800x600 window.

[Diego Pino]

Created attachment 386121 [details]
Patch

[Diego Pino]

Created attachment 386186 [details]
Patch

[Carlos Alberto Lopez Perez]

Comment on attachment 386186 [details]
Patch

r=me

[WebKit Commit Bot]

Comment on attachment 386186 [details]
Patch

Clearing flags on attachment: 386186

Committed r253817: <https://trac.webkit.org/changeset/253817>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.

[Radar WebKit Bug Importer]

<rdar://problem/58114649>