[Pixman] Getting started with Pixman

[Erico P]

Thanks for the information!

I wanted to read the pixel for my own save function that I am writing, so
even if it was slow I would not bother. I want to use pixman to replace
Allegro4 bitmaps.

>From your reply, I think that I need to convert the full bitmap to a
different format that a .bmp file accepts and then write these converted
bits to the stream all in one go. I still don't know how to do that. :/


I am sorry, I didn't mean a direct reply, I wanted to email the mailist
actually but I misclicked the reply to all.

On Mon, Jul 13, 2020, 04:29 Pekka Paalanen <ppaalanen at gmail.com> wrote:

> On Sun, 12 Jul 2020 23:24:51 -0300
> Erico P <eri0onpm at gmail.com> wrote:
>
> > Thanks!
> >
> > I am trying to get started but I need to read and write bitmaps to see
> what
> > I am doing. I made a function to write bitmaps from my own, and made up
> > some function to read. I can't seem to find anything readymade that
> > supports all formats available in pixman (r8g8b8a8, ...).
>
> Hi,
>
> Pixman's format support is so wide and wild (from 1-bit formats
> where endianness affects also bit order within a byte, to floating
> point formats nowdays IIRC), that I doubt you will find even a
> file format that supports everything as is - or if you do, it's
> really complicated.
>
> > I have a small problem that I can't seem to figure out, is there a
> function
> > to read or write a single pixel? Maybe it returns an int, or a color
> (for a
> > 32bit bmp), or the index of the palette in case it's an indexed bitmap?
> It
> > would be really useful for me to be able to read a single pixel, is
> there a
> > function for this I am overlooking?
>
> I don't think there is, maybe because using it would make things
> slow so no-one wanted it. Instead, if you need to, you get the
> pointer to the beginning of the data, compute the appropriate
> offset from stride, format and position, and access the bits
> yourself according to the format.
>
>
> https://ppaalanen.blogspot.com/2016/02/a-programmers-view-on-digital-images.html
> and
> https://afrantzis.com/pixel-format-guide/
> might help.
>
> You might also mimick that by doing a blit with
> pixman_image_composite32() from your source image into a 1x1 target
> image in your preferred inspection format.
>
> I'd recommend using just few carefully selected formats for your
> input/output and use Pixman itself to convert to/from those.
>
> If you want to continue the chat, I would like to keep it on the
> mailing list to benefit everyone and you might get more help, too.
> It would be nice to CC this reply to the mailing list.
>
>
> Thanks,
> pq
>
> >
> > Em qua., 8 de jul. de 2020 às 13:23, Pekka Paalanen <ppaalanen at gmail.com
> >
> > escreveu:
> >
> > > On Wed, 8 Jul 2020 11:43:59 -0300
> > > Erico P <eri0onpm at gmail.com> wrote:
> > >
> > > > Hello,
> > > >
> > > > I foundout about pixman recently as a recommended library for
> efficiently
> > > > drawing pixels, apparently used when writing a rasterizer and some
> other
> > > > use cases in GUIs too.
> > > >
> > > > I want to learn how to use it, but unfortunately the webpage states
> > > "There
> > > > is currently no documentation besides the source code itself."
> > > >
> > > > Is there any documentation available at all, maybe something
> someone
> > > wrote
> > > > some day for someone, anything other than just the source? Maybe a
> manual
> > > > from someone who ported it to other language - but I do intend to
> use it
> > > in
> > > > a mixed C/C++ environment.
> > > >
> > > > I appreciate any suggestions on getting started with it!
> > >
> > > Hi,
> > >
> > > I'm not aware of anything particularly about Pixman, unfortunately.
> > > You'll have to go through code that uses Pixman.
> > >
> > > If you want to do rectangular operations,
> > > pixman_image_composite32() is the main function for practically
> > > everything. I've used that one, but I never looked into the
> > > trapezoid or font compositing stuff.
> > >
> > >
> > >
> https://gitlab.freedesktop.org/wayland/weston/-/blob/master/libweston/pixman-renderer.c
> > > is one of Weston compositor's renderers that has a relatively
> > > simple task of just copying and blending window images. Weston also
> > > uses pixman_region32 for most region tracking and operations.
> > >
> > > http://ssp.impulsetrain.com/porterduff.html
> > > explains the Porter/Duff compositing operations. Some or all of
> > > those are found in pixman_image_composite32(). It doesn't mention
> > > the use of a mask though.
> > >
> > > Since Cairo can use Pixman underneath, I think some Cairo
> > > documentation about the image operations are applicable as well:
> > > https://www.cairographics.org/tutorial/
> > >
> > >
> > > Thanks,
> > > pq
> > >
>
>
> --
> Pekka Paalanen
> http://www.iki.fi/pq/
>
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