It is an atlas texture and it looks wonderful when played – and in addition it’s loop-able!
But there’s no fire at all! I thought something must be missing until I remembered the concept of gradient mapping. More details can be read in this great article and watched in this small example:
So you can colorize the grey smoke texture into something with color by re-mapping colors. Here I defined the black values as orange and what was white before is now black:
Looks great but there are two problems left:
- The fire/smoke proportion stay the same but we want to have a lot fire at the beginning and only black smoke at the end
- We don’t see any cloud silhouette and if we apply the transparency information of the atlas texture (alpha channel) it looks like this:
I was confused because the alpha channel makes the whole texture mostly 50% transparent but in the game the clouds are almost opaque.
Again the solution seams to be gradient mapping: For cutting out the cloud shape, they do use the alpha channel of atlas texture but re-define its values via gradient mapping and achieve an opaque look:
Fallout stores its gradients in *tadaa* textures. What a surprise. Let’s have a look on it:
What’s that? For a standard gradient mapping you would only need a texture of 1 pixel height so why do they use a quadratic texture?? The reason is, that they sample different pixel-“lines” (from bottom to top) depending on the particles lifetime!
This is genius! It makes it possible to colorize the smoke with a lot fire at the beginning and the longer the particle raises into the air it gets more and more black.
They even changed the cloud shape a bit by modifying the alpha channel over time.
This is so awesome! I love that they have a loop-able animation so that can set the particle lifetime really high without having to bake more pictures into the atlas since the color variation is handled separately.
I think it’s fair to say:
Case closed!
For those who are interested, I’ve prepared some more experiments for a better understanding and to note some observations.
Here I colored the lower and the upper part of the gradient texture in different colors and like expected at some point of the particles lifetime the color jumps:
Here I did the same but with different colors right and left which means that one color overwrites the grey-scale values from 0 to 127 and the other the values from 128 to 255:
This one is a bit weird. I changed the alpha channel of the gradient texture (everything is black except one pixel line at the bottom). I would have expected that the particles disappear very early. But see the result with your own eyes:
The color changes like before (which means the shader reads from the center-position of the diffuse channel where the color change happens) but the particle is still visible! I guess the shader handles the alpha channel in a different and non-linear way and sticks a longer time to the first pixel-line.
In my last example I made the alpha channel hide more and more of the visible texture the longer the cloud “lives”:
Thank you for following me through all these clues and reading this dossier. If you have any feedback feel free to contact me!