In my 2D map application, I have 16-bit heightmap textures containing altitudes in meters associated to a point on the map.
When I draw these textures on the screen, I would like to display an analysis such that the pixel referring to the highest altitude on the screen is white, the pixel referring to the lowest altitude in the screen is black and the values in-between are interpolated between those two.
I'm using an older OpenGL version and thus do not have access to modern pipeline functionality like GLSL or PBO (Which somehow can make getting color buffer contents to CPU side much more efficient than glReadPixels, as I've heard).
I have access to ATI_fragment_shader extension which makes possible to use a basic fragment shader to merge R and G channels in these textures and get a single float grayscale luminance value.
Then I would've been able to re-color these pixels again inside shader (Map them to 0-1 range) based on maximum and minimum pixel luminance values but I don't know what they are.
My question is, between the pixels currently on the screen, how do I find the pixels with maximum and minimum luminance values? Or as an alternative, how do I find these values inside a texture? (Because I could make a glCopyTexImage2D call after drawing the texture with grayscale luminance values on the screen and retrieve the data as a texture).
Stuff I've tried or read about so far:
-If I could somehow get current pixel RGB values in the color buffer to CPU side, I could find what I need manually and then use them. However, reading color buffer contents with glReadPixels is unacceptably slow. It's no use even if I set it up so that it completes one read operation over multiple frames.
-Downsampling the texture to 1x1 size until the last standing pixel is either minimum or maximum value and then using this 1x1 texture inside shader. I have no idea how to achieve this without GLSL and texel fetching support since I would have to look up the pixel which is to the right, up and up-right of the current one and find a min/max value between them.
