1617039 - Implement conic-gradient for Cairo graphics backend (printing support)

Status: NEW

(Core :: Graphics, enhancement, P5)

Description

[Tim Nguyen :ntim]

Cairo is used exclusively for printing.

According to jrmuizel, the easiest way to do this is to paint the gradient using a different backend then send the image over to Cairo for printing.

Comment 1

[Tim Nguyen :ntim]

Attachment: Bug 1617039 - Implement conic-gradient for Cairo graphics backend.

Comment 2

[Tim Nguyen :ntim]

Markus, do you think you could provide a bit of guidance ? I'm not sure this is on the right path. Thanks!

Comment 3

[Markus Stange [:mstange]]

The approach is looking pretty good to me! The hard part is going to be the computation of the surface bounds, I think. Unless there's some "get current clip extents" helper around.

Comment 4

[Markus Stange [:mstange]]

Oh, actually, I think it would be best to handle the transform in Skia. For example, let's say you have a 50x50 DrawTargetCairo, set a scale(5) transform on it, and then draw a 10x10 user space conic gradient that should fill the entire DrawTarget. In that case, you'll get blurry rendering if you let Skia render the conic gradient into a 10x10 surface and then scale that surface up to 50x50. It's better if you make a surface of the device space bounds (50x50), pass the current transform to Skia, let Skia fill the 50x50 surface with the correct transform, turn that into a SurfacePattern with the inverse transform set on it, and then draw that SurfacePattern into the DrawTargetCairo so that it's drawn without any transform.

Comment 5

[Mohamed H]

Attachment: Bug 1617039 - Implement conic-gradient for Cairo

Comment 6

[Mohamed H]

Attachment: Bug 1617039 - Implement conic-gradient for CairoDifferential Revision: https://phabricator.services.mozilla.com/D92949

Comment 7

[[e-mail]]

Cairo can draw conic gradients using the mesh gradient API. See this test case for an example.

https://gitlab.freedesktop.org/cairo/cairo/-/blob/master/test/mesh-pattern-conical.c

This is preferable to embedding an image as it uses vector output on the PS/PDF output.

You just need to ensure there is at least one patch every 90 degrees to maintain a good approximation of a circle as described here.

https://stackoverflow.com/questions/1734745/how-to-create-circle-with-b%C3%A9zier-curves

Comment 8

[Diego Pino]

FWIW, the WebKitGTK/WPE WebKit ports feature an implementation of conic gradients using Cairo as graphics backend. Perhaps that could be useful.

https://webkit-search.igalia.com/webkit/source/Source/WebCore/platform/graphics/cairo/GradientCairo.cpp