Followup: Normal Mapping Without Precomputed Tangents

This post is a follow-up to my 2006 ShaderX5 article [4] about normal mapping without a pre-computed tangent basis. In the time since then I have refined this technique with lessons learned in real life. For those unfamiliar with the topic, the motivation was to construct the tangent frame on the fly in the pixel shader, which ironically is the exact opposite of the motivation from [2]:

Since it is not 1997 anymore, doing the tangent space on-the-fly has some potential benefits, such as reduced complexity of asset tools, per-vertex bandwidth and storage, attribute interpolators, transform work for skinned meshes and last but not least, the possibility to apply normal maps to any procedurally generated texture coordinates or non-linear deformations. Continue reading

Branchless Matrix to Quaternion Conversion

(EDIT: This article is a more in-depth writeup of an algorithm that I developed around 2005, and first posted to Martin Baker’s Euclidean Space website. That time was the height of the Intel NetBurst architecture, which was notorious for its deep pipeline and high branch misprediction penalty. Hence the motivation to develop a branch-free matrix to quaternion conversion routine. What follows is the complete derivation and analysis of this idea.)

The original routine to convert a matrix to a quaternion was given by Ken Shoemake [1] and is very branchy, as is tests for 4 different cases. There is a way to eliminate these branches and arrive at a completely branch-free code and highly parallelizable code. The trade off is the introduction of 3 additional square roots. Jump the analysis section and the end of this article, or continue fist with the math bits.

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Cloud rendering and the relativity of whiteness

Here are some philosophical and rendering-related questions that I took home from the last vacation. What’s the color of clouds? The standard answer would be, white.
What’s the color of snow? Again, white. Ok, then look at the following picture, where the snow seems considerably whiter. This is the case in almost all photos that I took.

There is an image on Wikipedia from the same general area on which the brightness difference between clouds vs snow is even more pronounced. If you look at the directly lit parts of the snow and consider it white (#ffffff), then the directly lit parts of the clouds are at most 50% grey (#bbbbbb). Is that an evidence of air pollution? Unlikely! (At least not in Tyrol).

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