PNG Optimizer

PNG Optimizer is a tool for automatic correction of PNG files. Usage example:

pngopt.exe Data\Graphics\*.png

The image data of all files found by the specified wildcard will be expanded one pixel beneath the alpha channel boundary. So images will render without artifacts when scaled, stretched, rotated or rendered into not integer coordinates.

Note that for sprites you plan to transform, you also should ensure that they have at least one free pixel around them on the texture.