The specific context is here:
http://www.rorydriscoll.com/2012/01/11/derivative-maps/
It's an article about bump mapping. Specifically, the author mentions "texel-space height derivatives". I'm not sure what this means.
Texel-Space I believe refers to the width and height of the texture/image. The texture/image itself isn't colour values, it's height values. So, I imagine the texel-space height derivative would be the (x,y) slope on each pixel in relation to the neighbouring pixels in the x and y directions?
How would I even calculate this? I don't understand what the size of the texture has to do with the local slope, especially considering if the pixel is at the lowest of it's neighbours in the x and y directions, what would it's slope be?