[cairo] CAIRO_FORMAT_A1 image source color

Uli Schlachter psychon at znc.in
Sat Mar 29 11:48:55 PDT 2014


On 28.03.2014 17:51, sthustfo wrote:
> Thanks for the clarification regarding the A1 and A8 formats. I did not
> notice this in the documentation. Further, when I run the sample program
> provided by you, the generated out.png is 1x2 image in complete black. I do
> not see the white pixel.

Oh, yay, bugs!

This was fixed in git half a year ago and will be gone with the next release:


> I would like to generate a white surface and then draw some text over it in
> black using pango layout api (pango_layout_set_text). And then make use the
> raw data of the surface, and not really interested in the png output. Is
> this possible using the cairo?

Sure, but why do you do this with A1 surfaces?

Anyway, attached is a quick example for this.


> On Fri, Mar 28, 2014 at 6:15 PM, Uli Schlachter <psychon at znc.in> wrote:
>> Hi,
>> On 28.03.2014 07:01, sthustfo wrote:
>>> I am using cairopango library to render text on monochrome image on
>> Ubuntu
>>> 13.10. Now before rendering text, I create a surface of type
>>> CAIRO_FORMAT_A1, create a cairo context and set the source to solid
>> white.
>>> However, the color of the image remains black even though I have set the
>>> color source to solid white. I want to render black color text on white
>>> background.
>>> surface = cairo_image_surface_create (CAIRO_FORMAT_A1, 320, 280);
>>> cr = cairo_create (surface);
>>> cairo_set_source_rgba (cr, 1.0, 1.0, 1.0, 1.0);
>>> cairo_paint (cr);
>>> cairo_surface_write_to_png (surface, filename);
>>> The saved PNG image always is black color. How do I achieve an image of
>>> solid white color for a monochrome image (CAIRO_FORMAT_A1)? If suppose,
>> the
>>> surface is created using CAIRO_FORMAT_ARGB32, then everyrhing is fine.
>> Please note that CAIRO_FORMAT_A1 and CAIRO_FORMAT_A8 are *not* monochrome
>> images. They describe images without any colors at all which only contain
>> an
>> alpha channel. Only when generating a PNG are these images interpreted as
>> monochrome. The contents of the color components in your call to
>> cairo_set_source_rgba() won't make any difference at all.
>> Also note that the default operator (CAIRO_OPERATOR_OVER) doesn't allow to
>> "remove transparency", it only ever is added.
>> Attached is a program which generates a PNG with a white and a black
>> pixel. It
>> uses the operator "SOURCE" to set the alpha-value of the target pixel. I
>> hope
>> this can help you.
>> Cheers,
>> Uli
>> --
>> No matter how much cats fight, there always seem to be plenty of kittens.

<alanc> I think someone had a Xprint version of glxgears at one point,
    but benchmarking how many GL pages you can print per second
    was deemed too silly to merge

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