OpenGL ES render to texture, then draw texture

Question

I'm trying to render to a texture, then draw that texture to the screen using OpenGL ES on the iPhone. I'm using this question as a starting point, and doing the drawing in a subclass of Apple's demo EAGLView.

Instance variables:

GLuint textureFrameBuffer;
Texture2D * texture;

To initialize the frame buffer and texture, I'm doing this:

GLint oldFBO;
glGetIntegerv(GL_FRAMEBUFFER_BINDING_OES, &oldFBO);

// initWithData results in a white image on the device (works fine in the simulator)
texture = [[Texture2D alloc] initWithImage:[UIImage imageNamed:@"blank320.png"]];

// create framebuffer
glGenFramebuffersOES(1, &textureFrameBuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, textureFrameBuffer);

// attach renderbuffer
glFramebufferTexture2DOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_TEXTURE_2D, texture.name, 0);

if(glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES) != GL_FRAMEBUFFER_COMPLETE_OES)
    NSLog(@"incomplete");

glBindFramebufferOES(GL_FRAMEBUFFER_OES, oldFBO);

Now, if I simply draw my scene to the screen as usual, it works fine:

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// draw some triangles, complete with vertex normals
[contentDelegate draw];
[self swapBuffers];

But, if I render to 'textureFrameBuffer', then draw 'texture' to the screen, the resulting image is upside down and "inside out". That is, it looks as though the normals of the 3d objects are pointing inward rather than outward -- the frontmost face of each object is transparent, and I can see the inside of the back face. Here's the code:

GLint oldFBO;
glGetIntegerv(GL_FRAMEBUFFER_BINDING_OES, &oldFBO);

glBindFramebufferOES(GL_FRAMEBUFFER_OES, textureFrameBuffer);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

// draw some polygons
[contentDelegate draw];

glBindFramebufferOES(GL_FRAMEBUFFER_OES, oldFBO);

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glEnableClientState(GL_TEXTURE_COORD_ARRAY);
glEnable(GL_TEXTURE_2D);
glColor4f(1, 1, 1, 1);
[texture drawInRect:CGRectMake(0, 0, 320, 480)];    
glDisableClientState(GL_TEXTURE_COORD_ARRAY);
glDisable(GL_TEXTURE_2D);

[self swapBuffers];

I can flip the image rightside-up easily enough by reordering the (glTexCoordPointer) coordinates accordingly (in Texture2D's drawInRect method), but that doesn't solve the "inside-out" issue.

I tried replacing the Texture2D texture with a manually created OpenGL texture, and the result was the same. Drawing a Texture2D loaded from a PNG image works fine.

As for drawing the objects, each vertex has a unit normal specified, and GL_NORMALIZE is enabled.

glVertexPointer(3, GL_FLOAT, 0, myVerts);
glNormalPointer(GL_FLOAT, 0, myNormals);
glDrawArrays(GL_TRIANGLES, 0, numVerts);

Everything draws fine when it's rendered to the screen; GL_DEPTH_TEST is enabled and is working great.

Any suggestions as to how to fix this? Thanks!

For the "inside-out" issue: if your front-facing primitives are culled, then you have an issue with vertex ordering, which is used to determine the facet normal. It's not an issue with texturing, unless I'm misunderstanding something. Do you have GL_CULL_FACE enabled? — prideout
GL_CULL_FACE was not enabled. Enabling it definitely improves the overall appearance. The object normals seem to be the right way out now, but a lot of front-facing faces that should be hidden are now visible, regardless of whether they are (or rather, should be) obscured by another face. Does that indicate an issue with the winding of my triangles? I'm using a script to export vertices from Blender; I'll make sure the winding is uniform when it's exported. Note that everything looks perfect when I draw directly to the screen (and GL_CULL_FACE being on or off makes no difference). — sb.

prideout prideout · Accepted Answer · 2009-10-30T21:21:40

The interesting part of this is that you're seeing a different result when drawing directly to the backbuffer. Since you're on the iPhone platform, you're always drawing to an FBO, even when you're drawing to the backbuffer.

Make sure that you have a depth buffer attached to your offscreen FBO. In your initialization code, you might want to add the following snippet right after the glBindFramebufferOES(...).

// attach depth buffer
GLuint depthRenderbuffer;
glGenRenderbuffersOES(1, &depthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, depthRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_DEPTH_COMPONENT16_OES, width, height);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, depthRenderbuffer);

OpenGL ES render to texture, then draw texture

1 Answers