Hi all…


As some might have noticed in previous posts, I’ve been working on a deferred rendering pipeline for my engine…

I’m up to the phase where I’d like to have decals on the system.


The way I see it, there’s two different ways to implement decals on deferred rendering:


1)      The decals operates on the G-Buffer

2)      The decals operates on the frame buffer


The pros/cons I see in operating on the G-Buffer are:


-          Can get correctly lighted normal/emissive/specular mapped decals

-          Can’t have blend operations on the decals (writing on the G-Buffer has lots of data of different types, not all affected in the same way, so alphablending or any other blend operation could spoil my data). So all decals would have to “replace” what’s already on the G-Buffer.


The pros/cons I see in operating on the frame buffer (the already lighted scene, pre-glow pass):


-          Without checking the decal for every light, can’t get lighted decals

-          Can’t do decals that glow/remove glow from the scene (the glow intensity is on the G-Buffer, so that’s what’s used on the glow pass).

-          Blending operations are easier to define and control (and possible without hocus-pocus)


At first sight, I’d say that operating on the G-Buffer is much simpler and allows for more flexibility, but I still get the gnawing feeling that the lack of alpha blending operations will bite me in the ass…


Consider these three decal types: bullet holes, footprints and plasma residue.


On bullet holes, under the G-Buffer approach, I’d change the albedo of the G-Buffer to a dark gray/black, while changing the normals to distort the lighting on the bullet hole. On the frame buffer approach, I’d have to use a subtract blending operation to darken the “wall” in the shape of the bullet hole, without any distortion to the normals. The visual result might be good in both cases.

On the footprints, for the G-Buffer approach, I’d have to just change the normals to account for the footprint, without any change to the albedo, which would be a realistic footprint in “muddy” environments, but I think it would look silly in a dusty environment, which would be better done with an albedo change to dark-brown or something like that, which might look incorrect if the dust was on a metal floor. On the frame buffer approach, I’d just have to alphablend a mid-alpha black texture in the shape of a foot, using the alphablending hardware to get the correct footprint color on different floors. It wouldn’t look so good in muddy environments, but even so it would look great.

On the plasma residue, I’d change the albedo and emissive component on the G-Buffer to green and “high”, which would lead to a glowy green goo on where the plasma hit. On the frame buffer approach, I could only add a green color to the frame buffer in the shape of the plasma residue, which wouldn’t glow.


So, both approaches can be good or bad depending on what you’re trying to achieve… I even thought about having two kinds of decals (one that affects the G-Buffer and another that affects the frame buffer) for different usages, but to be honest I’m kind of at a loss on how to proceed…


The implementation seems pretty straightforward in both cases, it’s just a matter on where it goes in the pipeline, and all my searches on the Internet been fruitless, since apparently nobody discusses decals at this level (only building them and rendering them in the most primitive form).


So my question is: how do you guys handle decals in a deferred rendering context? Any ideas, suggestions, pointing-at-and-laughing appreciated!


Thanks in advance

Diogo de Andrade