When blending a partially transparent image on top of a background image you can get artifacts in areas which are not fully transparent and not fully opaque. The problem stems from the use of texture filtering, which will interpolate values from nearby pixels without taking transparency into account.
Use premultiplied alpha; usually this means:
- multiply RGB values by the alpha component when loading textures
This allows texture filtering to combine the RGB and alpha components separately and produce accurate blending results.
Note that if sRGB textures are used (and they probably shouldn't be) then the process is different; see the article on gamma correction for more information.