Current algorithm used to calculate the bounds of the boxes in the source
image to which the pixels in the target (rescaled) image are mapped to, has
flaws which cause a non-uniform distribution of the boxes in the source
image. And these miscalculations may result in the visible distortions
of the colours in the rescaled image.
New algorithm assures uniform distribution of the boxes in the source image
and thanks to the integer arithmetic is faster than currently used
and not prone to the problems with floating point representation.
Closes#18012.
When wxImage is rescaled with wxIMAGE_QUALITY_BILINEAR, wxIMAGE_QUALITY_BICUBIC or wxIMAGE_QUALITY_BOX_AVERAGE algorithm then for proper interpolation there is necessary to uniformly (linearly) map entire pixel range in the new image to the entire pixel range in the current image, i.e. pixels positions (integer values) in range [0..newDim-1] has to be mapped to "virtual" pixel positions (not necessary integer values) exactly in the range [0..oldDim-1].
[0..oldDim-1] range of target mapping is required because for proper interpolation every "virtual" pixel has to be located between two physical pixels in the current image.
Thus scaling ratio used to find corresponding pixels in the current image should be (oldDim-1)/(newDim-1) and not oldDim/newDim (which is used now).
When image is e.g. upsampled oldDim/newDim ratio then some new rightmost/botommost pixels are mapped to old pixels > (oldDim-1) and their values are improperly interpolated (in the current implementation their positions are just truncated to (oldDim-1) to bypass the problem and upsampled image looks like it was shifted left/up). The larger scaling ratio the effect is more visible.
Note:
Because reference images used currently in the unit tests were created with improper scaling there is necessary to upload new reference images created with fixed scaling.
Closes#17594
Check that resizing the test horse image produces the same results in the
future as it does now, by saving the current results in files and verifying
that images resized directly and loaded from these files are the same.
See #15281.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74318 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
