[Pixman] Is circle rendering possible?

Indi Kernick kerndog73 at gmail.com
Wed Feb 6 10:27:51 UTC 2019

I thought pixman allowed you to choose between premultiplied and not
premultiplied but I checked and apparently not. I think I'm getting my
libraries mixed up. The problem I have with premultiplied alpha is that you
lose precision when you have to un-premultiply. That's something I need in
my application. I want to get the color of a pixel without losing precision
with semi-transparent pixels.

So if I don't want to lose precision, I'll have to store images in regular
alpha and then multiply the alpha every time I want to composite. That's
kind of annoying.

To render a polygon, I could divide it into triangles and render those with

Thanks for your help. I think I'll explore other libraries.

On Wed, 6 Feb 2019 at 20:24 Pekka Paalanen <ppaalanen at gmail.com> wrote:

> On Wed, 6 Feb 2019 19:25:48 +1030
> Indi Kernick <kerndog73 at gmail.com> wrote:
> > The only problem I can find with Cairo is that it uses premultiplied
> alpha.
> Pixman uses premultiplied alpha. Also Wayland uses premultiplied alpha
> FWIW. I think you will find it quite common in general.
> > That's so annoying! Also, I'm not sure if the Cairo renderer is GPU or
> > based.
> You choose through the API which one you use. Use the image API, and
> you get CPU rendering.
> > I'm using Qt to render the images. Qt can render surfaces so I can use
> > pixman to manipulate a surface and then Qt to upload it to the GPU when
> > it's time to render. Cairo can do the same thing but then I have to worry
> > about premultiplied alpha. I could use pixman to do the compositing and
> > Cairo to do other stuff but that's just a mess.
> >
> > I'm guessing that using a GPU backend for Cairo and then making it play
> > nice with the Qt layout engine will be a nightmare. If I have to go back
> > and forth between CPU and GPU, it might actually be faster to use CPU
> > rendering. The only thing I'm worried about is compositing. Compositing
> is
> > much faster on the GPU (I presume) but if I use Cairo, I'll have to
> fiddle
> > around with the alpha channel. That means a round trip between CPU and
> > memory.
> You don't need to fiddle with alpha. Just write your fragment shaders
> and blending state to expect it.
> The point of using premultiplied alpha is to pre-compute values you
> would be computing in any case during blending.
> > I only really need non-antialiased circle rendering (antialiasing is a
> > nice-to-have). I can figure out to do that myself (Bresenham comes to
> > mind). I think I'll use pixman and do everything on the CPU.
> >
> > Another question: can pixman render a filled polygon without
> antialiasing?
> > If not, I might have to do that myself too. 😢
> That I don't know. I have never used Pixman beyond
> pixman_image_composite32(). I am guessing the trapezoids or triangles
> API could be used, but there does not seem to be anything about polygons
> per se. You can see the header yourself, that is all the documentation
> there is, which is practically none. Cairo at least has documentation.
> I believe there are also other CPU rendering libraries, but I wouldn't
> know to point to them.
> Thanks,
> pq
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