I’m trying to implement a multi-pass rendering method using OpenSceneGraph. However, I’m not entirely certain my problem is theoretical or due to a lack of applied knowledge of OSG. Thus far, I’ve successfully implemented multi-pass shading by rendering to a texture using an orthogonal projection, but I cannot seem to make a perspective projection work.
It may be that I don’t quite understand how to implement multi-pass shading. Of course, I have to pre-render the entire scene with the multi-pass shaders to a texture, then use the texture in the final render. However, I’m not talking about creating a separate texture for each object in the scene, but effectively capturing a screenshot of the entire prerendered scene. Then, from that texture alone, applying the rendered effects to the individual geometries.
I assume this means I would have to do an extra conversion of the vertex coordinates for each geometry in the vertex shader. That is, after computing:
gl_Position = ModelViewProjectionMatrix * Vertex;
I would need to go a step further and calculate the vertex’s screen coordinates in order to map the vertices correctly (again, given that the texture consists of an entire screen shot of the scene).
If I am correct, then I must be able to pre-render the scene in a perspective view identical to the view used in the final render, rather than an orthogonal view. This is where I have troubles. I can make an orthogonal view do what I want, but not the perspective view.
Am I correct in my approach? The only other approach I can imagine is to render everything to a screen-filling quad (in effect, the same thing as converting to screen coordinates), but that doesn’t alleviate the need to use a perspective projection in the pre-render stage.
Thoughts? Links??
edit: I should also point out that in my successful attempts, I used a fragment shader only. The perspective projection worked, but, of course, the screen aligned quad I was using was offset rather than centered. I added a pass-through vertex shader and everything went blank.
As it turns out, my approach was correct. It’s especially nice as it avoids having to add another camera to my scene graph to render the final output – I can simply use the main camera. Unfortunately, it means that all of my output textures are rendered at the screen resolution, rather than a resolution appropriate to the size of the object. That is, if my screen is 1024 x 1024, then so is the output texture, one for each pre-render camera in the graph. Not exactly efficient, but it’ll do for now.