[cairo] How to write to a CAIRO_FORMAT_A8 image surface?

[Arjen Nienhuis]

Maybe you can create a 1x1 A8 surface, write a byte to the data, and use it
as a source.
On Oct 2, 2012 11:33 AM, "Manfred Kogler" <manfred.kogler at gmail.com> wrote:

> Hi,
>
> Currently I am rendering postscript commands using Cairo to a
> CAIRO_FORMAT_RGB24 image surface. This image then I have to transfer/map to
> the framebuffer of a display device. One pixel in that display framebuffer
> is an 8-bit index into a predefined color table. Hence, when transferring
> the CAIRO_FORMAT_RGB24 image surface into the 8-bit display framebuffer
> each RGB-pixel is mapped to the respective 8-bit color index and this index
> value is written to the framebuffer at the respective position. This works
> fine, only it is a bit slow since rendering a frame always involves
> copying/transferring the whole image into that 8-bit framebuffer.
>
> To get rid of copying/transferring the image I thought of using
> cairo_image_surface_create_for_data(frame_buffer, CAIRO_FORMAT_A8, ...). My
> plan is to not write an intermediary RGB image anymore but to somehow
> directly write the RGB-mapped color-index-table-values to that
> A8-byte-buffer. I tried several approaches but could not figure out how to
> write one byte (color index) to that CAIRO_FORMAT_A8 image surface and am
> not sure whether it is possible at all.
>
> Effectively, my question reduces to the following point:
> Is there a way to write (stroke/fill/paint/...) some byte unchanged to a
> CAIRO_FORMAT_A8 image surface?
>
> Is there some way to do what I want?
>
> Many thanks and best regards,
> Fred
>
>
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>
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