I recently added framebuffer rendering to my game and rendering to the FBO works (verified with glGetTexImage), but when I go to render a quad to show the result nothing is drawn to the screen. I'm using OpenGL 3.3 Core, so I'm drawing using vertex buffers for the quad and I have two simple pass-through shaders to display the quad. I apologize for the amount of code that follows, but I felt it was all necessary for anyone who ends up looking at this.

Quad generation:

// create vertices
// Vertex(position, normal, tex coord)
quadVertices.Add( Vertex( Vector3( -1.0f,  1.0f, 0.0f ), Vector3(), Vector2( 0.0f, 1.0f ) ) );
quadVertices.Add( Vertex( Vector3( -1.0f, -1.0f, 0.0f ), Vector3(), Vector2( 0.0f, 0.0f ) ) );
quadVertices.Add( Vertex( Vector3(  1.0f, -1.0f, 0.0f ), Vector3(), Vector2( 1.0f, 0.0f ) ) );
quadVertices.Add( Vertex( Vector3(  1.0f,  1.0f, 0.0f ), Vector3(), Vector2( 1.0f, 1.0f ) ) );
quadVertices.SendData(); // gl*Buffer* methods wrapper

// create indices (quadIndices is a buffer of 16-bit integers)
quadIndices.Add( 0 );
quadIndices.Add( 1 );
quadIndices.Add( 2 );
quadIndices.Add( 0 );
quadIndices.Add( 2 );
quadIndices.Add( 3 );

Quad rendering:

glDisable( GL_DEPTH_TEST );
glDisable( GL_CULL_FACE );

// bind the texture
glActiveTexture( GL_TEXTURE0 );
glBindTexture( GL_TEXTURE_2D, framebuffer.GetColorHandle() );
_shader.Seti( "texture0", 0 );

 // projection matrix is glm::ortho( -1.0f, 1.0f, -1.0f, 1.0f, -1.0f, 1.0f );
_shader.Set( "proj", _projection );

// get attributes
int vertex   = _shader.GetAttribLoc( "vertex" );
int texCoord = _shader.GetAttribLoc( "texCoordV" );

// bind buffers

// enable arrays and point to data
glEnableVertexAttribArray( vertex );
glVertexAttribPointer    ( vertex,   3, GL_FLOAT, GL_FALSE, sizeof( Vertex ), (void*)( 0 ) );
glEnableVertexAttribArray( texCoord );
glVertexAttribPointer    ( texCoord, 2, GL_FLOAT, GL_FALSE, sizeof( Vertex ), (void*)( sizeof( Vector3 ) * 2 ) );

// draw!
glDrawElements( GL_TRIANGLES, quadIndices.Size(), GL_UNSIGNED_SHORT, 0 );

// disable arrays
glDisableVertexAttribArray( vertex   );
glDisableVertexAttribArray( texCoord );

// unbind everything
glBindTexture( GL_TEXTURE_2D, 0 );

glEnable( GL_CULL_FACE );
glEnable( GL_DEPTH_TEST );

Vertex shader:

#version 330 core

in  vec3 vertex;
in  vec2 texCoordV;
out vec2 texCoordF;

uniform mat4 proj;

void main()
    texCoordF = texCoordV;
    gl_Position = proj * vec4( vertex, 1.0 );

Fragment shader:

#version 330 core

layout(location=0) out vec4 fragColor;

in vec2 texCoordF;

uniform sampler2D texture0;

void main()
    fragColor = texture( texture0, texCoordF );

Maybe the projection matrix is wrong, but that should not matter. When rendering a fullscreen quad, you can discard the projection entirely.

Basically your vertex shader should be:

#version 330 core

in  vec3 vertex;
in  vec2 texCoordV;
out vec2 texCoordF;

void main()
    texCoordF   = texCoordV;
    gl_Position = vec4(vertex, 1.0);

The important bit is that you submit the vertexes in device coordinates (from [-1,-1] to [1,1]) but you are doing that already.

| improve this answer | |
  • \$\begingroup\$ Okay, this draws the quad, but somehow the quad is being affected by the matrices in the scene being rendered to it? As in I can move around my actual scene, and I also move around the rendered quad at the same time. \$\endgroup\$ – fastinvsqrt Jul 30 '14 at 16:05
  • \$\begingroup\$ I forgot to call glUseProgram for the pass-through shader... \$\endgroup\$ – fastinvsqrt Jul 30 '14 at 16:33

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