Good text foreground color for a given background color

I'm drawing a color selection button and I'm looking for a nice and simple formula to get a good text color (foreground) for a given background color in RGB.

A simple try would be to just take the complement color but this will produce an odd looking button for colors like pure blue or pure red.
Is there something well known that does this?

If it matters at all, I'm using QT.

For maximum legibility, you want maximum brightness contrast without getting into hues which don't work together. The most consistent way to do this is to stick with black or white for the text color. You might be able to come up with more aesthetically pleasing schemes, but none of them will be more legible.

To pick between black or white, you need to know the brightness of the background. This gets a little more complicated, due to two factors:

• The perceived brightness of the individual primaries red, green, and blue are not identical. The quickest advice I can give is to use the traditional formula to convert RGB to gray - R*0.299 + G*0.587 + B*0.114. There are lots of other formulas.

• The gamma curve applied to displays makes the middle gray value higher than you'd expect. This is easily solved by using 186 as the middle value rather than 128. Anything less than 186 should use white text, anything greater than 186 should use black text.

I'm no expert on programming things related to RGB, but from a designer's perspective, often the most readable color will be just a much lighter (if the background color is dark) or darker (if the background color is light) version of the same shade.

Basically you'd take your RGB values and if they're closer to 0 (dark) you'd push them each up by an equal amount for your foreground color, or vice versa if it's a light BG.

Complement colors can actually be really painful on the eyes for readability.

Leverage an outline for legibility

If by "good text color (foreground)" you intend it for legibility purposes when the user chooses any background colour, you can always produce white text having a black outline. It will be legible on any solid, patterned or gradient background, from black through white and anything in between.

Even if this doesn't hit the mark of your intention, I think it worthwhile posted here because I came looking for similar solutions.

Building on top of Mark's response, here's some Ruby code that'll do the work

rgbval = "8A23C0".hex
r = rgbval >> 16
g = (rgbval & 65280) >> 8
b = rgbval & 255
brightness = r*0.299 + g*0.587 + b*0.114
return (brightness > 160) ? "#000" : "#fff"