Question

Situation: Generating N samples of a shape and corresponding edges (using Sobel filter or my own) with different transformations and rotations, while viewport (size=600*600) and camera remain constants. i.e. there will be N samples + N corresponding edges.

I am thinking to do like this,

Use One FBO with 2 renderbuffers [i.e. size of each buffer will be= (N *600) * 600]- 1st for N shapes and 2nd for edges of the corresponding shapes

Questions:

1. Which is the best way to achieve above things?
2. Though viewport size is 600*600pixels but shape will only occupy around 50*50pixels. So is there any efficient way to apply edge detection on bounding box/AABB region only on 2nd buffer? Also only reading 2N bounding box (N sample + N corresponding edges) in efficient way?

Answer 1

1 : I’m not sure what you call “best way”. Use Multiple Render Targets : you create two 600*N textures, bind them both to the FBO with glDrawArrays, and in your fragment shader, so something like that :

layout(location = 0) out vec3 color;
layout(location = 1) out vec3 edges;

When writing to “color” and “edges”, you’ll effectively write in your textures.

2 : You shouldn’t do this. Compute your bounding boxes on the CPU, and project them (i.e. multiply each corner by your ModelViewProjection matrix) to get the bounding boxes in 2D

By the way : Compute your bounding boxes first, so that you won’t need 600*600 textures but 50*50…

EDIT : You usually restrict the drawn zone with glViewPort. But ther is only one viewport, and you need several. You can try the Viewport array extension and live on the bleeding edge, or pass the AABB in a texture, or don’t worry about that until performance matters…

Oh, and you can’t use Sobel just like that… Sobel requires that you can read all texels around, which is not the case since you’re currently rendering said texels. Either make a two-pass algorithm without MRTs (first color, then edges) or don’t use Sobel and guess you edges in the shader ( I don’t really see how )