Monday, July 30, 2007

Bram's comments on the last article got me thinking about halftones. This is probably obvious to most people but it was news to me. Without good color profiles and calibrated monitors, you of course can't get predictable appearance. Things like sRGB will help the first but 99% of monitors are not calibrated. But with halftoning you can (assuming LCDs are well behaved about half tone patterns-- CRTs sure are not) get predictable greyscales. For example here are two images with a 25% and a 50% greyscale. (and RE the last article, not which is more of a middle grey). It just doesn't matter what your gamma is (assuming your black point is dark!) to predict what these greys are.

This basic idea is often used for gamma estimation-- the best example is Greg Ward's great gamma image.

Sunday, July 29, 2007

Gamma and textures.

Gamma correction issues are always a pain. A review: given 256 grey levels, it makes sense to make them perceptually uniform. Thus a 20% grey intensity (one fifth the physical intensity of white) should be around 127 as it is perceptually half way between black and white. But what happens when you want to use it as a texture map in a physically based renderer? You need to "de-gamma" the image. If you are using somebody else's renderer, should you do that as a preprocess or might they do it? For my own purposes in that situation I have made three textures of the grey squares in the macbeth color checker chart. The grey square in position 3 from the left (i.e., the third darkest) should look mid-grey in your final render. The 1.0 image has 0.19*255 in that square so it is not gamma corrected (i.e. the byte values are linear in intensity). Of course to make this all more complicated, a renderer might or might not do something different if there is something in the texture's color profile or not.

Saturday, July 21, 2007

New poll online

Here is a poll to narrow down what the non-random sample of people who look at this blog think. I am most interested in the percentage that say never (I am guessing 40%) but also interested in the distribution of the other answers.

Saturday, July 14, 2007

Plucker coords considered harmful

Christer Ericson has a terrific article on why scaler triple products are usually preferable to plucker coords here. I couldn't agree more. Christer has a book on collision detection I have been meaning to order for sometime and his blog content encourages me.

Sunday, July 1, 2007


I saw Ratatouille with the family this weekend. It is the first CG movie that we all really liked a lot. Brad Bird is really on fire-- I think I liked this movie better than the Iron Giant which was terrific. One thing I especially liked was the lack of big-name actors who are not voice actors. Watch the Toy Story movies and the watch some old Disney (e.g. Jungle Book) and you will see just how much using screen actors for voice talent hurts animated movies. Spongebob and the Simpsons would be awful if they used Tom Hanks and John Travolta. I hope Ratatouille makes a zillion dollars and will show that the director and studio is enough to bring in the crowds.

In addition to being a terrific movie, the rendering passed some threshold for me. Previous CG movies have always looked flat to me, with Shrek 2/3 being better due to their use of indirect bounces I assume. But Ratatouille had even more depth. It is the best looking CG movie I have seen by a long shot. As much depth as live action I think. I have no idea what is different, but I have to suspect global illumination given my prejudices. If it was done with hand-placed fill lights, then the artists must be on performance-enhancing drugs! If anybody knows please reply.