'Render to Texture' and multipass rendering

Question

I'm implementing an algorithm about pencil rendering. First, I should render the model using Phong shading to determine the intensity. Then I should map the texture to the rendered result.

I'm going to do a multipass rendering with opengl and cg shaders. Someone told me that I should try 'render to texture'. But I don't know how to use this method to get the effects that I want. In my opinion, we should first use this method to render the mesh, then we can get a 2D texture about the whole scene. Now that we have draw content to the framebuffer, next we should render to the screen, right? But how to use the rendered texture and do some post-processing on it? Can anybody show me some code or links about it?

Answer 1

• render to a texture with Phong shading
• Draw that texture to the screen again in a full screen textured quad, applying a shader that does your desired operation.

I'll assume you need clarification on RTT and using it.

Essentially, your screen is a framebuffer (very similar to a texture); it's a 2D image at the end of the day. The idea of RTT is to capture that 2D image. To do this, the best way is to use a framebuffer object (FBO) (Google "framebuffer object", and click on the first link). From here, you have a 2D picture of your scene (you should check it by saving to an image file that it actually is what you want).

Once you have the image, you'll set up a 2D view and draw that image back onto the screen with an 800x600 quadrilateral or what-have-you. When drawing, you use a fragment program (shader), which transforms the brightness of the image into a greyscale value. You can output this, or you can use it as an offset to another, "pencil" texture.

Answer 2

I made this tutorial, it might help you : http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-14-render-to-texture/

However, using RTT is overkill for what you're trying to do, I think. If you need the fragment's intensity in the texture, well, you already have it in your shader, so there is no need to render it twice...

Maybe this could be useful ? http://www.ozone3d.net/demos_projects/toon-snow.php