Subrect drawing to texture FBO within OpenGL ES Fragment Shader

Question

I'm trying to draw to a subrect of a texture based FBO, but am having difficulty. The FBO has dimensions of say 500x500 and I am trying to have the fragment shader only redraw say a 20x20 pixel subrect. Modiyfing the full texture works without difficulty.

At first I tried setting glViewport to the needed subrect, but it doesn't look to be that simple. I'm suspecting that the Vertex attributes affecting gl_Position and the varying texture coordinates are involved, but I can't figure out how.

Answer 1

原来，我正在尝试修改纹理坐标属性，但是使用着色器中的glViewport和gl_FlagCoord可以更轻松地修改视口。