Rendering a second pass yields a different result - c++

Question

Currently I'm trying to render multiple passes with different shaders in a simple OpenGL application. Here's my (simplified) code:
void InitScene()
{
glViewport(0, 0, mWindowWidth, mWindowHeight);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, mWindowWidth, mWindowHeight, 0, -1, 1);
mFramebufferName = CreateFrameBuffer(mWindowWidth, mWindowHeight);
}
void DrawScene()
{
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
if(drawDirectlyToScreen)
{
// This works fine, image will fill the whole screen
// Directly draw to the screen
DrawFullScreenQuad();
}
else
{
// This does not work. The image from the first pass will only be a small quadrat
// Draw to frame buffer instead of screen
glBindFramebuffer(GL_FRAMEBUFFER, mFramebufferName);
DrawFullScreenQuad();
glBindFramebuffer(GL_FRAMEBUFFER, 0);
// Get ready for second pass
BindFrameBufferTextureAndActivateAnotherShader();
// Now draw to the screen
DrawFullScreenQuad();
}
glPopMatrix();
}
void DrawFullScreenQuad()
{
glBegin(GL_QUADS);
glTexCoord2f(0.0f, 1.0f); glVertex3f(0.0f, mWindowHeight, 0.0f);
glTexCoord2f(1.0f, 1.0f); glVertex3f(mWindowWidth, mWindowHeight, 0.0f);
glTexCoord2f(1.0f, 0.0f); glVertex3f(mWindowWidth, 0.0f, 0.0f);
glTexCoord2f(0.0f, 0.0f); glVertex3f(0.0f, 0.0f, 0.0f);
glEnd();
}
void CreateFrameBuffer(int width, int height)
{
// Generate and bind the frame buffer
mFramebufferName = 0;
glGenFramebuffers(1, &mFramebufferName);
glBindFramebuffer(GL_FRAMEBUFFER, mFramebufferName);
// Create and bind the render texture
glGenTextures(1, &mSecondPassRenderTexture);
glBindTexture(GL_TEXTURE_2D, mSecondPassRenderTexture);
// Give an empty image to OpenGL ( the last "0" )
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, NULL);
// Set "mSecondPassRenderTexture" as colour attachement #0
glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, mSecondPassRenderTexture, 0);
// Set the list of draw buffers.
GLenum DrawBuffers[1] = { GL_COLOR_ATTACHMENT0 };
glDrawBuffers(1, DrawBuffers); // "1" is the size of DrawBuffers
}
When rendering only one pass everything is fine. The image covers the whole screen. When rendering with two passes, the resulting image will only cover a small square area in the top left corner of the screen (see the attached images).
The problem seems to come from the first pass. The texture created in that pass is already wrong (i.e. the image is only in the corner, the rest of the texture is black). The second pass then works correctly (i.e. the broken texture is drawn correctly to the whole screen).
So my question is: why does my call to DrawFullScreenQuad() yield different results when
Rendering to the screen directly
Rendering to a frame buffer (which has the same size as the window)