Qt5 OpenGL Multisampling using QOpenGLFrameBufferObject class

I need to perform multisampling in a Qt5 project but I am not sure how to use QOpenGLFrameBufferObject to perform FSAA. There is no example on how to do this as far as I searched and the documentation only mentions: “If you want to use a framebuffer object with multisampling enabled as a texture, you first need to copy from it to a regular framebuffer object using QOpenGLContext::blitFramebuffer().” My code currently looks like this:

//Enable FSAA for better output
int vp[4];
glGetIntegerv(GL_VIEWPORT, vp);
if(m_lpFBO == NULL)
{
    //MultiSampling set to 4 now
    QOpenGLFramebufferObjectFormat format;
    format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
    format.setMipmap(true);
    format.setSamples(4);
    format.setTextureTarget(GL_TEXTURE_2D);
    format.setInternalTextureFormat(GL_RGBA32F_ARB);

    //Create the FBO
    m_lpFBO = new QOpenGLFramebufferObject(vp[2], vp[3], format);
    m_lpFBOSurface = new QGLFramebufferObjectSurface(m_lpFBO);
}

QRect rc(0, 0, vp[2], vp[3]);
QGLSubsurface sub(lpPainter->currentSurface(), rc);


m_lpFBO->bind();
sub.setFramebufferObject(m_lpFBO);
lpPainter->pushSurface(&sub);

//Draw as usual
.
. 
.

lpPainter->popSurface();


//Now Copy
QOpenGLFramebufferObject::blitFramebuffer(lpPainter->currentSurface()->framebufferObject(), rc, m_lpFBO, rc);

Solution

You do not need to use a QGLFramebufferObjectSurface to perform the necessary downsampling, as you can just use two QGLFramebufferObjects. QOpenGLFramebufferObject::blitFramebuffer (which calls glBlitFramebuffer) will automagically manage downsampling (or upsampling) from a source framebuffer target to a destination frame buffer target. blitFramebuffer also lets you dictate how conversion is computed using (GL_NEAREST or GL_LINEAR) and what attachments are to be transferred (a bitwise combination of GL_COLOR_BUFFER_BIT, GL_DEPTH_BUFFER_BIT, GL_STENCIL_BUFFER_BIT).

So, for your needs you want to create 2 QOpenGLFramebufferObjects where one will contains the multisampled texture being rendered to, and one will contain the downsampled result texture. Then you will use QOpenGLFramebufferObject::blitFramebuffer to downsample the source texture into the result texture.

Here is a quick example:

// MultiSampling set to 4 now
QOpenGLFramebufferObjectFormat muliSampleFormat;
muliSampleFormat.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
muliSampleFormat.setMipmap(true);
muliSampleFormat.setSamples(4);
muliSampleFormat.setTextureTarget(GL_TEXTURE_2D);
muliSampleFormat.setInternalTextureFormat(GL_RGBA32F_ARB);
QOpenGLFramebufferObject multiSampledFBO(width, height, format);

QOpenGLFramebufferObjectFormat downSampledFormat;
downSampledFormat.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
downSampledFormat.setMipmap(true);
downSampledFormat.setTextureTarget(GL_TEXTURE_2D);
downSampledFormat.setInternalTextureFormat(GL_RGBA32F_ARB);
QOpenGLFramebufferObject downSampledFBO(width, height, downSampledFormat);

Then, every time you need to downsample (after rendering to your multisampled texture) simply do something like this. (using your preferred filtering)

QOpenGLFramebufferObject::blitFramebuffer(
    &downSampledFBO, &multiSampledFBO,
    GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT, GL_NEAREST);