What shapes are you using? If your shapes are convex (like circles, squares, rounded rectangles), you can just draw versions of different size and clip them to a triangle whose diagonal goes from upper right to lower left.
I.e., zoomed in, light-to-dark:
Then clip that:
And then combine the two bezels:
and finally draw the original shape on top:
Now this example is rough and too coarse, but if each of the three rounded rectangles only is visible as a one pixel outline, this will look like a gradient from light to dark in the upper left, and in the opposite direction in the lower right.
Note that in this example the colors are too different, so you can see the various parts. Basically you should take the inner shape's color (in this case white) and lighten it 3 times for the transition to upper left, and darken it for the transition to lower right. The final effect you're going for (again, zoomed in) is something like:
In theory you could just use one larger shape, clip it, and fill it with a gradient, for each half, but it's quite difficult to get the gradients to line up properly for non-square shapes. Also, this doesn't work for concave shapes (like the letter "C"). The trick to a good bezel is a good algorithm for generating that larger version of your shape. But not scaled up. Ideally, you'd just "outset" the shape, which will change its proportions. If you're generating the shapes yourself, as pixels, just draw each pixel as 9 pixels and you have the perfect 1-pixel-outset for the next-larger shape in the algorithm in my answer.
Update: Another approach would be to first render your shape into a bitmap, and then loop over the pixels. If you find a pixel that is "on", check its surrounding pixels. If it has empty pixels below it or to its right, darken it. If it has empty pixels to its left or above it, lighten it. That would give a proper 1px bezel that lies inside the shape. You could then repeatedly apply this algorithm by mixing the light/dark colors with a percentage of your shape's color, and by looking for pixels you darkened/lightened in the previous iteration instead of transparent pixels. This is probably something that wouldn't be too fast until you manage to implement it e.g. as a shader on the graphics card.