Unfortunately I have never used du/dv maps in my work, but if I understand how they are used by engine correctly, then this steps to achieve resulting texture seem way too strange.
I mean, isn't just removing info from the blue channel a better way to do this? GIMP will be just fine to set the channel to gray/to black. Though, the normalmap might become slightly - or more than slightly - unnormalized as a result (don't know if they should be normalized at all), you have still more control over the texture you get in the end, and may increase, or decrease contrast as you wish.
Err... And just to be more exact - up to this day I thought that du/dv maps are normal maps without "blue axis". You know, if the length of normal vector is equal to 1, then obviously values of two projections are enough to fully describe it (i.e. you can restore the blue channel from this data - with small errors because of the limited precision, of course). If I this is right, then removing blue channel is the natural way to get du/dv's from tangent-space normal maps. --Halfgild Wynac 03:28, 21 Sep 2008 (PDT)