Is it possible to copy data from one framebuffer to another in OpenGL?

Question

I guess it is somehow possible since this:

glBindFramebuffer(GL_READ_FRAMEBUFFER_APPLE, _multisampleFramebuffer);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER_APPLE, _framebuffer);
glResolveMultisampleFramebufferAPPLE();

does exactly that, and on top resolves the multisampling. However, it's an Apple extension and I was wondering if there is something similar that copies all the logical buffers from one framebuffer to another and doesn't do the multisampling part in the vanilla implementation. GL_READ_FRAMEBUFFER doesn't seem to be a valid target, so I'm guessing there is no direct way? How about workarounds?

EDIT: Seems it's possible to use glCopyImageSubData in OpenGL 4, unfortunately not in my case since I'm using OpenGL ES 2.0 on iPhone, which seems to be lacking that function. Any other way?

Answer 1

You can do it directly with OpenGL ES 2.0, and it seems that there is no extension neither.

I am not really sure of what your are trying to achieve but in a general way, simply remove attachements of the FBO in which you have accomplish your off-screen rendering. Then bind the default FBO to be able to draw on screen, here you can simply draw a quad with an orthographic camera that fill the screen and a shader that takes your off-screen generated textures as input. You will be able to do the resolve too if you are using multi-sampled textures.

glBindFramebuffer(GL_FRAMEBUFFER, off_screenFBO);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, 0, 0);

glBindFramebuffer(GL_FRAMEBUFFER, 0); // Default FBO, on iOS it is 1 if I am correct

// Set the viewport at the size of screen
// Use your compositing shader (it doesn't have to manage any transform)
// Active and bind your textures
// Sent textures uniforms
// Draw your quad

Here is an exemple of the shader:

// Vertex
attribute vec2 in_position2D;
attribute vec2 in_texCoord0;
varying lowp vec2 v_texCoord0;
void main()
{
   v_texCoord0 = in_texCoord0;
   gl_Position = vec4(in_position2D, 0.0, 1.0);
}

// Fragment
uniform sampler2D u_texture0;
varying lowp vec2 v_texCoord0;

void main()
{
    gl_FragColor =  texture2D(u_texture0, v_texCoord0);
}

Answer 2

Tested in OpenGL 4, glBlitFramebuffer not supported in OpenGL ES 2.0.

I've fixed errors in the previous answer and generalized into a function that can support two framebuffers:

// Assumes the two textures are the same dimensions
void copyFrameBufferTexture(int width, int height, int fboIn, int textureIn, int fboOut, int textureOut)
{
    // Bind input FBO + texture to a color attachment
    glBindFramebuffer(GL_READ_FRAMEBUFFER, fboIn);
    glFramebufferTexture2D(GL_READ_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, textureIn, 0);
    glReadBuffer(GL_COLOR_ATTACHMENT0);

    // Bind destination FBO + texture to another color attachment
    glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fboOut);
    glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_TEXTURE_2D, textureOut, 0);
    glDrawBuffer(GL_COLOR_ATTACHMENT1);

    // specify source, destination drawing (sub)rectangles.
    glBlitFramebuffer(0, 0, width, height,
                        0, 0, width, height,
                        GL_COLOR_BUFFER_BIT, GL_NEAREST);

    // unbind the color attachments
    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_TEXTURE_2D, 0, 0);
    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, 0, 0);
}

Answer 3

glBlitFramebuffer accomplishes what you are looking for. Additionally, you can blit one TEXTURE onto another without requiring two framebuffers. I'm not sure using one fbo is possible with OpenGL ES 2.0 but the following code could be easily modified to use two fbos. You just need to attach different textures to different framebuffer attachments. glBlitFramebuffer function will even manage downsampling / upsampling for anti-aliasing applications! Here is an example of it's usage:

// bind fbo as read / draw fbo
glBindFramebuffer(GL_DRAW_FRAMEBUFFER,m_fbo);
glBindFramebuffer(GL_READ_FRAMEBUFFER, m_fbo);

// bind source texture to color attachment
glBindTexture(GL_TEXTURE_2D,m_textureHandle0);
glFramebufferTexture2D(GL_TEXTURE_2D, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, m_textureHandle0, 0);
glDrawBuffer(GL_COLOR_ATTACHMENT0);

// bind destination texture to another color attachment
glBindTexture(GL_TEXTURE_2D,m_textureHandle1);
glFramebufferTexture2D(GL_TEXTURE_2D, GL_COLOR_ATTACHMENT1, GL_TEXTURE_2D, m_textureHandle1, 0);
glReadBuffer(GL_COLOR_ATTACHMENT1);


// specify source, destination drawing (sub)rectangles. 
glBlitFramebuffer(from.left(),from.top(), from.width(), from.height(),
                  to.left(),to.top(), to.width(), to.height(), GL_COLOR_BUFFER_BIT, GL_NEAREST);

// release state
glBindTexture(GL_TEXTURE_2D,0);
glBindFramebuffer(GL_READ_FRAMEBUFFER, 0);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER,0);