问题
I have a Framebuffer with 2 textures attached is it possible to read from texture A and write to texture B, in the same fragment shader function ?
Thanks.
回答1:
This is trickier than I expected. I thought that as long as you don't sample from and render to the same texture you would be fine, no matter if the texture is attached to an FBO. But while trying to find some conclusive spec quotes to back this up, things became much less clear.
The 4.5 spec does contain a phrase that seems to confirm my initial instinct (emphasis added):
Specifically, the values of rendered fragments are undefined if any shader stage fetches texels and the same texels are written via fragment shader outputs, even if the reads and writes are not in the same draw call
The interesting aspect is that the "written via fragment shader outputs" does not appear in spec versions up to and including 4.4. I don't know if adding this in 4.5 was intended as just a clarification, or if the rules for feedback loops were relaxed in 4.5. I couldn't find anything in the change log that would provide more background on the change.
Up to 4.4, the spec says that if a texture is attached to the current draw framebuffer, and is sampled, you have a feedback loop. Since nothing says otherwise, this would include the situation where the texture is attached to the FBO, but not used as a draw buffer.
I wouldn't be surprised if things mostly work fine as long as you don't render to and sample from the same texture. But to be completely safe, particularly if you don't rely on having OpenGL 4.5, you should un-attach the sampled texture from the FBO. Not including it in the list of draw buffers would be insufficient.
回答2:
Extending Reto's answer.
You can read from texture A and write to texture B, BUT you cannot do this:
uniform sampler2D B; // Sampler for texture B.
layout (location = 0) out vec3 A; // Write to texture
A = texture(B,Texcoord);
This is invalid, and you must use a tertiary variable.
You cannot read and write to the texture on the same draw call, but you can read from one texture and write to another texture that both reside on the same framebuffer in the same draw call. You have to make sure when you bind the frame buffer you bind it like this.
glbindframebuffer(GL_FRAMEBUFFER, fboHandle)
This is because the framebuffer will be written to and read from and the `GL_FRAMEBUFFER, will allow you to do this.
来源：https://stackoverflow.com/questions/33025827/read-from-one-texture-and-write-to-a-separate-texture-with-in-the-same-frame-buf