Simple color-grading for games

Color grading is an easily overlooked, but extremely powerful way to add character to a game. Subtle color changes make day-night cycling much more atmospheric. Different areas can have their own signature “feel” based on how saturated the colors are. Dark games can shift the unlit areas of an environment to cool bluish tint that can remain visible but still feel like darkness. The possibilities are endless.

I haven’t given much thought to color grading before, until a friend (Samurai), told me of an extremely simple and powerful way to add color grading to a game. So simple in fact, that I had to try it as soon as possible!

The idea has two parts. First the obvious bit: Use a 3D texture as a look-up table, to map the RGB colors produced by the renderer to a different set of RGB colors which is the color-graded output. That translates to pretty much the following GLSL post-processing fragment shader:

uniform sampler2D framebuf;
uniform sampler3D gradelut;

void main()
{
    vec3 col = texture2D(framebuf, gl_TexCoord[0].st).xyz;
    gl_FragColor = vec4(texture3D(lut, col).xyz, 1.0);
}

And now the brilliant bit: write a bit of code to save a screenshot of the game with the “identity” 3D lookup-table serialized in the last few scanlines. Give that screenshot to an artist, and let him work his magic, color-grading it in photoshop or whatever… Did you get that? In the process of color-grading that screenshot, the artist automatically produces the look-up table which can be used to reproduce the same color-grading in-game, as part of the last few scanlines of the image! Feed that palette back into the game and it’s automatically color-graded!

So I used the dungeon crawler I’ve been writing recently to try out this algorithm. The output of my dungeon crawler as it stands, is not the best material to try color-grading on as it’s already very dark and highly tinted, leaving too little space for tweaking without banding everything to oblivion, but nevertheless I wrote the code, gave the screenshot to my friend Rawnoise to play with it in photoshop for a couple of minutes, fed it back into the game, and the result can be seen below.

Lower-left part of the screenshot produced by the game, with the identity palette attached:

Screenshot of the game before and after color grading:

Of course you can always opt for a completely bizarre effect just as easily. This is the result of me moving color curves in gimp randomly, and then feeding the resulting palette into the game:

Obviously this algorithm opens up all sorts of interesting possibilities, such as having two palettes and interpolating between them during sunset, or when a player crosses the boundary between two areas, etc. Simple, yet effective.

Dungeon crawler game prototype

I started writting a first person dungeon crawler game recently. Nothing ground-breaking, but I intend to fill up the void of the simplistic gameplay with over the top eye-candy. My main inspiration comes from eye-of-the-beholder-esque dungeon crawlers with awesome graphics (for their respective times) such as stonekeep and the legend of grimrock, without necessarily intending to stay true to the retro 90 degree grid-based movement.

Before starting, I wanted to try out a couple of things to see how they feel in practice, so I decided to make a prototype. The main thing I wanted to try out was a suggestion of a friend of mine, for keeping the level creation as simple as possible, to use a regular grid tile-based system with a couple of enhancments. Namely:

• Allowing multiple detail tiles on a single grid cell. Which makes it easy to lay down the whole level’s corridors and then add details such as torches on the walls, furnitures or whatever here and there.
• Allowing arbitrary geometry for each tile, not necessarily contained in the volume of the grid cell it occupies. This would allow, for instance, elaborate prefab rooms to be attached at various places of the dungeon.

It turns out I don’t like the extended grid-based idea that much, and for the actual game I will revert to a more powerful level organization, I came up with some time ago. More on that when I actually implement it.

The rendering is done with “deferred shading“, a neat technique I implemented once before in the Theseis engine, which makes it possible to have hundreds of actual dynamic light sources active at the same time. This is the cornerstone of my “lots of eye-candy” idea, because it enables each and every torch, spell effect, flame, or magical glow to illuminate the dungeons and its denizens dynamically.

Finally I implemented a nice positional audio and music playback system, on top of OpenAL. It keeps static audio sources in a kd-tree for efficient selection of the nearest ones within a certain radius around the player and enables/disables the appropriate ones automatically.

In case you are curious to see it, I just uploaded a video on youtube. The actual tileset is obviously placeholder, since I made it myself in blender, to be replaced by proper artwork later on. The music and sound effects are made by George Savinidis, who will be in change of all the audio production for this game.

S-ray goes public

S-ray is a photorealistic off-line renderer I’ve been writting on and off for the past few months. It’s basically a monte carlo ray tracer using the photon mapping algorithm to simulate global illumination effects, such as indirect diffuse lighting and caustics.

So, it’s time to open up the code and continue development in the open. The code is available only through subversion at the moment, no official “0.1″ release shall be made until at least the most important outstanding bugs are fixed. However, feel free to checkout the code and play around all you want.

The project is hosted at googlecode, and you can find it at: http://code.google.com/p/sray
As always, ideas, comments, or bug fixes are more than welcome.

s-ray: caustics through photon mapping

The main feature set that I had planned for my new s-ray renderer is almost complete. The renderer is now capable of producing caustics, that is light being focused on surfaces by curved mirrors or lenses (or otherwise transmission through refractive objects).

Standard recursive ray tracing is incapable of producing caustics; instead a two-pass algorithm called photon mapping is used. Photon mapping works by tracing random photons as they leave the light sources and bounce around the scene, finally ending up on various surfaces. The second pass works just like regular ray tracing, with the difference that on every ray hit, the density of stored photons around that point is used to estimate the amount of light that has arrived there after being reflected or refracted.

Here are a couple of images produced by s-ray, showing both reflective and refractive caustics:

reflective caustics

refractive caustics