Hi Jed, I believe the problem in your approach is in the assumption of how STMap works. STMap does a lookup for each pixel in the output image to find the coordinates in the input it needs to sample from to produce the final pixel value. In other words, it does not "push" pixels from the input into discrete output locations, but "pulls" pixels from the input for each pixel in the output. It's the difference between forward and backward warping.
To do what you're trying to do you would effectively need a UV map that's the inverse of that distortion. You can get an approximation of such an inverse UV map by displacing the vertices of a card, which would be a way of forward-warping. The only caveat is that you'll need a card that has as many subdivisions/vertices as possible, since the distortion values will be interpolated between vertices. That's why it's only an approximation at best. But given enough subdivisions, it should get you close enough. Once you have that inverse UV map, your distorted XY coordinate should just be the UV value at your undistorted coordinate, multiplied by width and height. (script attached as an example). P.S. The other "minor" thing you might want to look into is the way you're generating your UV map. The expression you're using "x/width" and "y/height" will result in a UV map that displaces the image by half a pixel from scratch when fed into an STMap. STMap samples pixels from the input at their centre (x+0.5, y+0.5), so for a more accurate UV map you should use U = (x+0.5)/width and V = (y+0.5)/height. Hope that helps. Cheers, Ivan On Sat, Aug 31, 2013 at 8:18 PM, Jed Smith <jedy...@gmail.com> wrote: > Greetings! > > *The Problem* > I am trying to write a tool to distort tracking data through a distortion > map output by a LensDistortion node. I have everything working, except > there seems to be inaccuracy in my method of calculating the distorted > pixel position from the sampled values of the uv distortion map, when > compared to a visual check. > > *My Method* > Say there is a pixel value at 1792,476 in a 1080p frame. I have a standard > UV Map, modified with a grade node through a mask, creating a localized > distortion when this map is plugged into an STMap node. The distorted pixel > value is 1821,484. > > The sampled uv map pixel values at the source pixel location is 0.916767, > 0.432918 (for width, and height offset, respectively). > > I am going on the assumption that the uvmap pixel values represent the > distorted location of that pixel, with the location being a floating point > percentage of the frame width and height. So a value of 0.916767, 0.432918 > would basically be telling the STMap node to set the output pixel location > for this pixel to a value of the difference between the 'unity' uvmap value > that would result in no transformation and the sampled uv value, multiplied > by the frame width. > > For horizontal distortion offset, this would be: > (pixel_coordinate_x / frame_width - uvmap_red) * frame_width, or (1792 / > 1920 - 0.916767) * 1920 = 31.807 > This would result in a distorted horizontal value of 1792+31.807 = > 1823.807. This value is close, but almost 3 pixels off. > > *Help!* > Can anyone here provide some insight into how exactly the math for the > STMap works to determine the output location of a pixel from the incoming > pixel values? I have attached a small nuke script demonstrating what I am > talking about. See the "Test_STMAP_Distortion_Calculations" node to see > the output results of the above algorithm. > > And if anyone is curious to check out the "DistortTracks" gizmo as it > exists so far, it lives here <https://gist.github.com/jedypod/6302723>. > > Thanks very much! > > > _______________________________________________ > Nuke-users mailing list > Nuke-users@support.thefoundry.co.uk, http://forums.thefoundry.co.uk/ > http://support.thefoundry.co.uk/cgi-bin/mailman/listinfo/nuke-users >
inverse_UV.nk
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