[Pixman] pixman transform

[Thilo Cestonaro]

Hey pq,

thanks for your explanation!

Regards
Thilo

Am 16.03.2015 12:42, schrieb Pekka Paalanen:
> On Mon, 16 Mar 2015 12:11:28 +0100
> Thilo Cestonaro <thilo at cestona.ro> wrote:
> 
>> Hey!
>> 
>> I try to understand how weston's fbdev compositor trys to transform 
>> the
>> screen.
>> As it uses pixman to do this, I come here to hopefully get some 
>> answers
>> :).
>> 
>> Can someone briefly explain the following functions (what they tend to
>> do and what the parameters mean):
>> 
>> pixman_transform_init_identity
>> pixman_transform_rotate
>> pixman_transform_translate
>> pixman_image_set_transform
>> 
>> That would be awesome!
> 
> Hi,
> 
> a pixman_transform_t is a 3x3 matrix which represents a projective
> (much more often an affine) 2d transformation. You can see this in the
> Pixman header. Rotate and translate modify the transformation.
> 
> Pixman_image_set_transform sets the destination-to-source
> transformation in the image, to be used when you use that image as a
> source in a composite call.
> 
> The transformation is in the opposite direction than what one might
> expect, but the rendering code really iterates over the destination
> coordinates and transforms those into source coordinates to sample from
> the source image.
> 
> That's all just an impression I got when working with Weston's
> pixman-renderer.
> 
> If you look at weston master branch which contains
> http://cgit.freedesktop.org/wayland/weston/commit/?id=8870a23fbd6a54d2290d2f5d9c4696e24cec4770
> you might have easier time understanding it, I hope.
> 
> 
> Thanks,
> pq