Drawing rectangles with interpolated colors (different color for each corner)

[Jean-Claude Monnin]

I try to draw rectangles filled with an interpolated color form the corners (a different color assigned at each corner). 

I have a grid of data points that I would like to visualise on a map. I would like to fill the area of this grid with interpolated colors in between the data points. In OpenGL it's straightforward to achieve that with a quad- or tri-strip and assigning a color to each vertex.

I didn't find an obvious way to do that in Skia. How can this be done in Skia?

Thanks for your help.

Jean-Claude

[Mike Reed]

canvas->drawVertices(...)

> --
> You received this message because you are subscribed to the Google Groups
> "skia-discuss" group.
> To view this discussion on the web visit
> https://groups.google.com/d/msg/skia-discuss/-/kny49fXx27YJ.
> To post to this group, send email to skia-d...@googlegroups.com.
> To unsubscribe from this group, send email to
> skia-discuss...@googlegroups.com.
> For more options, visit this group at
> http://groups.google.com/group/skia-discuss?hl=en.

[Jean-Claude Monnin]

Oops, I missed that while checking the docs. This is exactly what I need. Thanks.

On Oct 22, 2012, at 14:11 , Mike Reed <re...@google.com> wrote:

> canvas->drawVertices(...)

[Dave M]

hi, I have same trouble on it, but drawVertices can not make it, can  you   help me?

see https://groups.google.com/g/skia-discuss/c/eeBmFD7aKWs

[Mike Reed]

A 4-corner gradient (inside a rect) is not a linear problem -- so it can't be done with just a couple of triangles. It is sort of bi-quadratic, in the same way that bilerp (image filtering) is quadratic (involves the product of u and v).

SkCanvas::drawPatch() can do this. It is more general, in that it can handle a cubic curve on each side of the 'rectangle', but you can easily specify the cubics to be straight lines.

--
You received this message because you are subscribed to the Google Groups "skia-discuss" group.

To unsubscribe from this group and stop receiving emails from it, send an email to skia-discuss...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/skia-discuss/f6e55b64-b030-4186-84a4-98a8686447a4n%40googlegroups.com.

--

"I think I can safely say that nobody understands quantum mechanics." -- Richard Feynman

"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so." -- Mark Twain

"I am pleased to see that we have differences" -- Surak

[Dave M]

Dear mi...@reedtribe.org   ,Thank you for the suggestion on  drawPatch() ,i make 4 straight lines, while the result is not i want ,  I feel depressed 。

i wander if i shader can make it?

[Brian Salomon]

The spacing of the interior control points on the edges of the quads affects how the color interpolation works out:

https://fiddle.skia.org/c/053d04121f4870db1bcb5f638e661a3a

Maybe that's where the problem lies?

To view this discussion on the web visit https://groups.google.com/d/msgid/skia-discuss/963bdd93-362b-4e44-a08f-78fd0a57a72cn%40googlegroups.com.

--

Brian Salomon |

Office Hours: go/bsalomon-office |

bsal...@google.com

[Dave M]

Dear BS,thank you for your demo, i try it while produced little effect,  

maybe the problem is part of image data quality, can skia do image filter or smooth?