OK, this has recently been posted elsewhere, but everyone here seems quite open to new ideas, so I thought this might be of some value here. While in the process of doing a repaint for my own aircraft design, creating panel lines and such, I came across a way to make highlighted reflective areas using Dxtbmp. This may well be known to the more seasoned on this forum, but having just found out how to do it myself, I thought this may come in handy for some of the newbies attempting their own repaints here.
1. Create/open your main image using your favourite paint editor. For the bare-metal parts, paint in the associated areas using a neutral grey (Pic-1).
2. Once done, copy the image, and re-paint in the same metallic areas using a lighter shade of grey. Erase the remaining areas of the image and paint-fill with either pure white or a very light shade of grey - pure white will show absolutely no reflection (e.g., completely matte finish), and very light grey will render a satin finish. To create realistic vent holes, use pure white, in order to have no reflection showing. Experiment with varying shades of grey to obtain your desired effect. What you end up with here will be your Alpha Channel (Pic-2).
3. Next, open up your main image (pic-1) using Dxtbmp. Click "Alpha" and then "Import Alpha Channel". You will see a message indicating that the currrent alpha channel will be replaced. Click OK and then navigate to where your Alpha Channel (pic-2) is. In the preview window at top-right of the screen, this image will be displayed. Click on "Alpha" then "Apply Alpha to Image". Save in DXT3 format. That's it - you now have reflective highlights on your aircraft (pic-3, 4).
SPAMMERS MUST BE KILLED