improved high quality image downscaling

[Peer Sommerlund]

Hello.

The following patch to the image sample shows a different method for downscaling images, that provides a higher quality image than "high quality" scaling.

Regards,
Peer

[tierra]

What kind of algorithm does this use and how does it compare to wxIMAGE_QUALITY_HIGH in regards to speed when either upsampling or downsampling?

[Peer Sommerlund]

The algorithm only works for downscaling. It is slower than wxIMAGE_QUALITY_HIGH, since it uses float math. (wxIMAGE_QUALITY_HIGH is blur + sample which is integer math only)

The process is simple: For each target pixel, compute the average colour of the corresponding area on the source image. Note that this may include fractions of pixels.

Example:

Source image:

Code: Select all

+---+---+
|   |   |
+---+---+
|   |   |
+---+---+

With target pixel area overlayed:

Code: Select all

+---+---+
|  TTTT |
+--TTTT-+
|  TTTT |
+---+---+

thus the two rightmost pixels of the source image contribute more than the two leftmost pixels.

Please note that the code has not been optimized so it may be possible to reorganise to get more speed, but it will always be slower than wxIMAGE_QUALITY_HIGH.