On 04/24/2015 07:31 AM, Pekka Paalanen wrote:
Would it be good to set a rule of thumb, that when you are scaling
images, first compute the final size, and then compute the
transformation paramaters for Pixman, regardless from which one you
start with?
If I understand what you are asking for, the answer is no.
When scaling images do not be confused by the pixel size or pixel
centers. The correct transform is the one that places the outer corners
of the source image (ie integers 0,0 and w,h for the entire contents of
a w,h sized image) at the outer corners of the desired destination
quadrilateral.
This will produce the correct filtering and the correct position for the
image. If you gradually change the destination rectangle, the image will
scale smoothly with it. Another way to look at is that this rule is the
one so that if you transformed to an intermediate image and then did a
further transform to the final, the result will not be shifted or scaled
differently (it may be blurrier but that blur will be centered with no
visible shift).
The confusion is that filtering will sample outside the source
rectangle, and the filtering can produce non-zero outside the
destination rectangle. This effects the calculation of bounding boxes,
and yes it gets complicated if you want an accurate bounding box. But
you should *not* use this to determine the desired transformation.
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