Color space linearity and gamma correction

Ok so it’s a well known fact among graphics practitioners, that pretty much every game does rendering incorrectly. Since performance, and not correctness is always the prime consideration in game graphics, usually we tend to turn a blind eye towards such considerations. However with todays ultra-high performance programmable shading processors, and hardware LUT support for gamma correction, excuses for why we continue doing it the wrong way, become progressively more and more lame. :)

The gist of the problem with traditional real-time rendering, is that we’re trying to do linear operations, in non-linear color spaces.

Let’s take lighting calculations for example, when light hits a plane with 60 degrees incidence angle from the normal vector of the plane, Lambert’s cosine law states that the intensity of the diffusely reflected light off the plane (radiant exitance), is exactly half of the intensity of the incident light (irradiance) from that light source. However the monitor, responsible for taking all those pixel values and sending them rushing into our retinas, does not play along with our assumptions. That half intensity grey light we expect from that surface, becomes much darker due to the exponential response curve of the electron gun.

Simply put, when half the voltage of the full input range is applied to the electron gun, much less than half the possible electrons hit the phosphor in the glass, making it emmit lower than half-intensity light to the user. That’s not a defect of the CRT monitors; all kinds of monitors, tv screens, projectors, or other display devices work the same way.

So how do we correct that? We need to use the inverse of the monitor response curve, to correct our output colors, before they are fed to the monitor, so that we can be sure that our linear color space where we do our calculations, does not get bent out of shape before it reaches our eyes. Since the monitor response curve is approximately a function of the form: x^\gamma where \gamma = 2.2 usually, it mostly suffices to do the following calculation before we write the color value to the framebuffer: x^\frac{1}{\gamma}. Or in a pixel shader:

gl_FragColor.rgb = pow(color.rgb, vec3(1.0 / 2.2));

That’s not entirely correct, because if we are doing any blending, it happens after the pixel shader writes the color value, which means it would operate after this gamma correction, in a non-linear color space. It would be fine if this shader is a final post-processing shader which writes the whole framebuffer without any blending operations, but there is a better and more efficient way. If we just tell OpenGL that we want to output a gamma-corrected framebuffer, or more precisely a framebuffer in the sRGB color space, it can do this calculation using hardware lookup tables, after any blending takes place, which is efficient and correct. This fucntionality is exposed by the ARB_framebuffer_sRGB extension, and should be available on all modern graphics cards. To use it we need to request an sRGB-capable framebuffer during context creation (GLX_FRAMEBUFFER_SRGB_CAPABLE_ARB / WGL_FRAMEBUFFER_SRGB_CAPABLE_ARB), and enable it with glEnable(GL_FRAMEBUFFER_SRGB).

Now if we do just that, we’re probably going to see the following ghastly result:

The problem is that our textures are already gamma-corrected with a similar process, which makes them now completely washed out when we apply gamma correction in the end a second time. The solution is to make color values looked up from textures linear before using them, by raising them to the power of 2.2. This can either be done in the shader simply by: pow(texture2D(tex, tcoord).rgb, vec3(2.2)), or by using the GL_SRGB_EXT internal texture format instead of GL_RGB (EXT_texture_sRGB extension), to let OpenGL know that our textures aren’t linear and need conversion on lookups.

The result is correct rendering output, with all operations in a linear color space:
gamma-corrected rendering output

A final pitfall we may encounter is if we use intermediate render targets during rendering, with 8 bit per color channel resolution, we will observe noticable banding in the darker areas. That is because our 8bit/channel textures are now raised to a power and the result is again placed in an 8bit/channel render target, which obviously wastes color resolution and loses details, which cannot be replaced later on when we gamma correct the values again. Bottom-line is that we need higher precision intermedate render targets if we are going to work in a trully linear color space. The following screenshots show a dark area of the game when using a regular GL_RGBA intermediate render target (top), and when using a half-float GL_RGBA16F render target (bottom):

linear textures when using an 8bit/channel intermediate render target

linear textures when using an half-float intermediate render target

Color artifacts are clearly visible in the first image, around the dark unlit area.

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:

Original dungeon crawler output Dungeon crawler output after color grading test

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

dungeon crawler prototype videoI 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.