Disabling texture filtering is sometimes useful if you need very crisp look of textures or you have low-resolution textures that get blurry when seen at a grazing angle. Texture filtering is independent from Blur and Blur offset values in the bitmap's settings.
To globally disable or enable texture filtering, go to the "Development/Experimental Stuff" rollout and find "Texmap filtering". From the list you can pick All, None or All but opacity.
1. Bitmap texture - texture filtering on (All).
Note visible blurriness of the texture where it's stretched on the torus knot object.
2. Bitmap texture - texture filtering off (None).
Note that blurriness on the torus knot object disappears.
3. Bitmap as opacity map - texture filtering off (All but opacity - default setting).
Rendering time: 01:45
4. Bitmap as opacity map - texture filtering on (All).
Rendering time: 02:35
There is little visual difference, but rendering is significantly slower when texture filtering is enabled for opacity maps.