How to implement independent rendering layers in Direct3D9?

Question

I'm working on a windowed Direct3D data plotting application that needs to display multiple overlays on top of the data (similar to HUDs in games). Since there could be a large amount of data that needs plotting, and not all overlays will be changed every time, I figured it wouldn't be a good idea to replot verticies when only one overlay in the display changes.

This led me to the idea of rendering the textures and verticies of the overlays to multiple textures with transparent backgrounds that could be overlaid in the render loop and updated independently (similar to layers in Photoshop).

Before I embark on changing a large portion of this program to render to textures as opposed to surfaces, I was just wondering if using textures is the best approach.

Answer 1

RTT works well, I used it in a game I did recently. Each scene (scene refers to layer, "HUD" was a scene, "Main" was the main scene etc...) was rendered onto a texture, then each texture was rendering onto a quad, sorted back to front (for alpha blending). I chose this over just rendering the scenes directly onto the back buffer because it allowed me to do post-processing.

For your caching purposes this seems to be the best way to go, but just be aware that the textures can eat memory quickly, and sometimes its just better to render everything again, making sure you sort back to front.

Answer 2

Render to texture will certainly work and could be a good route but it is probably overkill. Modern 3D hardware is very fast and I'd suggest you verify whether performance is really an issue re-rendering when you need an update before investing significant time making major changes to your program.

If performance is an issue your time might be better spent optimizing the code that renders your plot since that will benefit updates that involve changes to the data as well as those that just change an overlay. I'm a graphics programmer for games and generally with realtime 3D you want to focus your optimization efforts on your worst case (you have to redraw everything) rather than your best (only one overlay needs an update).

Answer 3

Rendering to texture render target surfaces is a very good idea, and can be used for a lot of things e.g. optimization/caching, but beware of the blend operation with regular alpha (a*c1 + (1-a)*c2); if @ is ARGB blend, then l1@l2@l3 != l3@l1@l2; i.e. it's not commutative, but by using pre-multiplied alpha in all textures/layers the blend operation can be made commutative.

The ultimate reference is the Porter/Duff article "Compositing Digital Images" from 1984.