Question about VirtualDub (and AviSynth) Levels command

[mpwr5]

Could somebody explain in more detail how to use the Levels command in Vdub (or AviSynth). Specifically, how to the parameters relate to brightness and contrast.

For example, if I just wanted to raise the brightness, and not adjust contrast, do I just move up the black (left) point on the output and leave everything else alone?

When would you change gamma?

Finally, do these adjustments affect hue?

[EddyH]

Input levels: whatever value you choose for black and white here, anything below it on the source will end up as flat black, anything above will be flat white, and the remainder in between stretched to fit. If you have something that's particularly bad contrast but not "centred" (etc, etc) and can check out the min/max brightness with a histogram or paint program then it's far quicker and easier than playing around with normal brightness and contrast. Particularly seeing as VDubs "normal" bright/cont controls are so coarse and tricky.
(works very well for quickly colour correcting a batch of digi photos too, if you have RGB level control).

Output levels: Whatever in your source, or your input-altered source is flat white, will be adjusted down to where you put the white slider, and similar for the black one. Quite why you'd add in this artificial lack of contrast is beyond me.. suppose there's reasons like playback on crappy monitors that dont properly show anything below luma 16 or over 240, or for effects.

Gamma correction - can be very handy! This alters the "midpoint" of the lightness range (so whatever was 50% before shifts a bit), and all points inbetween to black and to white on some kind of mathematical curve thing. Basically its something to do with the relationship between how monitors percieve and display the signal they recieve, and ditto for your eyes. Anyway. Basically what it does is lighten or darken the image without throwing away any of the top or bottom ends. Very good for getting extra detail in dark areas if you dont like turning your screen brightness up, or (far less common) de-washouting a pic or retrieving detail from *bright* areas.
1.2 is a good one for brightening many movies for the PC screen, if they need it.