I did in fact have to call glDrawBuffers myself. I assumed this was handled by the FBO binding, but apparently not. QOpenGLExtraFunctions* f = QOpenGLContext::currentContext()->extraFunctions(); GLenum bufs[2] = { GL_COLOR_ATTACHMENT0, GL_COLOR_ATTACHMENT1 }; f->glDrawBuffers(2, bufs); This seems strange to me that the FBO abstraction supports color attachments, but requires extra functions to use them.

Bump as I re-edited the topic with new info from "scratch"

Hi, I wouldn't say an oversight but likely you have a use case that has not happened before. Also QOpenGLFramebufferObject is a pretty new class compared to QGraphicsView. I'd recommend brining this to the interest mailing list. You'll find there Qt's developers/maintainers.

@SGaist Thank you for the reply, SGaist! I'll do it.

I solved this problem by creating a vertex- and fragment-shader because there was probably another shader active. Is there a way to mark this topic as solved?