Given my track record of last year - a total of 4 blog posts - I do not have very high hopes that I'll do much better in 2011, but since I decided not to speak on any conferences this year I might have a little bit more time to share my experiments here instead of on stages only. I don't want to over-promise, but I hope that over the next weeks I'll find the time to finally explain and release the source codes for several projects that I have exclusively shown in my talks over the last years. Of course some of them have in the time since I've presented them been independently discovered by others, but I think I've still got a few aces in my pockets which are worth sharing.
I'll start with a simple visual effect: Radial Blur, which can give images the 200mph speed zoom look. In a naive approach you could achieve this effect by replacing every pixel with a weighted average of all neighboring pixels within the blur radius that lie on a line that goes through the center of zoom and the pixel itself. The problem with this method is that it is very slow since it would require many calculations to collect each neighbor and since the angle is different at every pixel there is not much room for other optimizations, like reusing already read pixels, which is key for every fast blur algorithm.
But using a neat trick that I remembered from my early Photoshop days allows to speed up this process a lot. Here is a demo:
The way this works is by converting the pixels from cartesian mapping (where the axes are x & y) to polar mapping (with axes angle & distance to the center) which arranges them in a way that all the neighbors that need to be averaged arrange themselves nicely in a straight line. This allows us to use a simple native Blur filter to quickly calculate the average. Depending if the blur is in x direction or y direction the final result will either be a Radial Blur (the well-known zoom effect) or a Circular Blur (which looks like a fast spinning wheel). The final step is to convert the pixels back from polar mapping to cartesian.
Open this link in a new window to see the process in a step by step demo.
By default the mapping uses the center of the image as the center of the polar coordinate system. This will result the center of the zoom to also be in the center of the image. So what if we want the zoom center to be at a different location? There are two ways of going about this - unfortunately I did not manage the faster way to work correctly yet which would be to simply use that center in the mapping algorithm. For some reason I have not found the right equations which work both ways. So for now I am using the slower method which does always work:
The trick here is to first make the image bigger by padding the borders so that the desired center of the zoom is temporarily the center of the image. So if you want the zoom center to be in the upper half of the image you add as many border pixels to the top that the chosen spot becomes the new vertical center. Then follows the cartesian-to-polar transform, the linar blur and the polar-to-cartesian mapping. The final step is to crop the image back to its original size by removing the previously added top pixels.
Another issue are artifacts that show due to the mapping. The problem is that since all the pixels that lie on a circle with a certain radius are mapped to a straight line, once the circumference of that circle gets bigger than the width of the mapped image pixel information gets lost. To fix this it is required to make the bitmap that holds the polar map as wide as the biggest possible circumference. Unfortunately that is not always possible since the temporary bitmaps could get pretty huge this way. The good news is that since we are blurring those artifacts will mostly not become too visible. Only for small blurs it improves the quality when using a bigger resolution for the cartesian image map.
You can find the source code for the RadialBlur class and the required pixel bender kernels as usual inside QuasimondoLibsPosted at January 31, 2011 07:45 PM | Further reading