Thank you for your answers.

"Of course, it all depends on your exact situation, but are you sure that it is worth precalculating and storing the values at all rather than just calculating them on the fly per fragment?"

My 'boss' asked the same question

Okay, this is what I'm doing: http://pastebin.com/YXP6j9vN

Short description: "grid" defines a distorted 2D grid of points like this one:

(-1.0 1.0) (0.0 1.0) (1.0 1.0)
(-1.0 0.0) (-0.2 0.0) (1.0 0.0)
(-1.0 -1.0) (0.0 -1.0) (1.0 -1.0)

Note: These four quads are not enough, I need at least 16x16, maybe even 24x24 or 32x32.

So, for every fragment, I need to find out in which one of the quads the current fragment lies (lines 88 - 105, pretty naive implementation with linear complexity) by dividing the current quad into two triangles and calculating the point's barycentric coordinates in this triangle (lines 19 - 30). If the correct quad is found, the point will be bilinearly interpolated and the value will be returned.

The resulting vec2 is what I want to store for every fragment.

I'm pretty sure a texture/SSBO lookup would be faster than 1-16 (assuming I come up with a faster way to find the correct quad) point-in-triangle checks and one bilinear interpolation, but I have no experience in that matter. What do you think?

@samoth: The second is the case. I chose the SSBO, because of its ability to store arrays of arbitrary length, which would be nice. My target platform (a virtual reality lab) fully supports OpenGL 4.3 But you are probably right: The drawbacks outweigh the benefits compared to a texture.

The lab's projectors have a resolution of 1400x1050 pixels, so the buffer would need 1400*1050*8 Bytes (two floats per pixel) = 11,760,000 Bytes = 11.21 MiB of space which is probably too much for the L1 cache of a GTX 460 :/

So, (assuming I really precalculate) you think I should go for a texture?