What They Are and How They Differ — KinematicSoup Technologies Inc.

Most images are stored with a gamma of 0.45 applied to them, which will have the effect demonstrated in the above left image. The darker regions of the image are now recorded using a greater range of values while bright ranges are compressed. Images stored like this are in “gamma space”. For example, a neutral grey in an image doesn’t have a numerical intensity of 0.5, rather it is around 0.73, while pure whites and blacks remain the same. This is the default behavior of nearly every digital camera, as well as image editing applications and so on. In fact, nearly every image you see on your computer has that gamma applied to it.

You may be wondering why images display correctly, and don’t look too bright. This is where the non-linear response of displays comes in. CRT screens, simply by how they work, apply a gamma of around 2.2, and modern LCD screens are designed to mimic that behavior. A gamma of 2.2, the reciprocal of 0.45, when applied to the brightened images will darken them, leaving the original image.

For example, the above left image represents what is stored on your computer, and when that is displayed, the result looks like the center image. The left image has been “gamma corrected”, which means it has a gamma value such that it will be displayed correctly after the screen’s gamma is applied to the image. If instead the computer is storing the center image, which has not been gamma corrected, the displayed result looks like the right image, clearly unwanted behavior.

Color Spaces and the Rendering Pipeline

When using the gamma pipeline in rendering, textures are passed into shaders, gamma corrected. Next the lighting is calculated. After, the final image is output to the display and adjusted by the display’s gamma value. This behavior, while simple, is not physically correct. In real life, light behaves linearly, which means that each contribution from multiple light sources added together give the correct intensity. Because of this, shading is done in linear space, but in the gamma pipeline the input colors and textures remain in gamma space. This means the result of the shading is not truly accurate, but after the display’s correction it is often good enough. However, with increasing demand for immersive, photo-real rendering, this is no longer a suitable method.

Therefore, typical practice in PBR is to use a linear pipeline. Here, the input colors and textures have their gamma correction removed before shading, putting them into linear space. When shaded, the result is physically correct because the shading process and inputs are all in the same space. After, any post effects should be computed while the frame is still in linear space, as post effects are typically linear, much like shading. Finally the image is then gamma corrected so it will have the proper intensity after the display’s gamma adjustments.