Luke Woodbury

Using chromakey with two movs of differing resolution.

One is a small sprite 300x400 on a keyed bground that I'm moving around on a much larger mov 2400x600.

Good frame rate (30FPS), but is very blocky and looks horrible.

If I convert the small one with [jit.matrix 4 char 2400 600] or [jit.qt.movie 2400 600] it looks great but my frame rate drops to about 13.

Any ideas on how to get around this would be much appreciated.

Using chromakey with two movs of differing resolution.
One is a small sprite 300x400 on a keyed bground that I'm moving around on a much larger mov 2400x600.
Good frame rate (30FPS), but is very blocky and looks horrible.
If I convert the small one with [jit.matrix 4 char 2400 600] or [jit.qt.movie 2400 600] it looks great but my frame rate drops to about 13.

Any ideas on how to get around this would be much appreciated.


mixed-resolutions-chromakey

Is the small sprite still meant to cover the entire 2400x600 space? Or do you use something like jit.rota for scaling it smaller? Seems like that might be an option..of course it's going to look blocky when it's that stretched out.

it depends on which inlet your movies are going in.

if your smaller resolution is going in the left inlet, the output will match the smaller resolution, which is probably what's happening.

in any case, 2400x600 is pretty big, and will probably only playback with good FPS if you are optimizing your patch. of course, CPU and movie codec are also considerations.

it depends on which inlet your movies are going in.
if your smaller resolution is going in the left inlet, the output will match the smaller resolution, which is probably what's happening.

in any case, 2400x600 is pretty big, and will probably only playback with good FPS if you are optimizing your patch. of course, CPU and movie codec are also considerations.


Should have said, yes I'm using jit.rota to scale it right down and move it around so it just covers a tiny bit of the 2400x600 space at a time, even so it looks crappy if a 300x400 matrix is used.

I think the small one has to go in the left inlet of chroma as its the one having its background keyed out? I have found using 'PhotoJPG' codec for the 2400x600 and 'Apple Intermediate Codec' for the small one yields the best frame rate

Sorry, yes just twigged, the chromakey object is indeed outputting at the smaller resolution. So it seems I have to change it to a 2400x600 before it goes in which will give me the lower frame rate.

If I play just the 2400x600 by itself I get a good frame rate of 30FPS. Hmm, any ideas?

Sorry, yes just twigged, the chromakey object is indeed outputting  at the smaller resolution. So it seems I have to change it to a 2400x600 before it goes in which will give me the lower frame rate. 

If I play just the 2400x600 by itself I get a good frame rate of 30FPS. Hmm, any ideas?


1. Use a videoplane for the sprite, simply calculating alpha channel with jit.chromakey, compositing on gpu.

2. Use jit.submatrix and the @out_matrix of jit.chromakey creatively.

Sorry I don't have time to make examples of this

That's OK, thanks very much for the tip, I'll see how I get on.

Ok, made a lot of mods to my patch, but importantly have keyed my foreground movie in After Effects and saved it with its alpha channel in the animation codec. jit.alphablend works nicely, but again we are mixing resolutions, so instead I am compositing with jit.gl.videoplane and have been trying to use 'jit.gl.slab @file co.alphablend.jxs'. I can't find any examples and it seems to be ignoring my alpha channel so I've still got a black background. I haven't got much experience with shaders, can anyone help out here? Is there an easier way to go about this?

OK, so if I plug the background mov into the right inlet of slab it works just like jit.alphablend, so still same problem.

Well pre keying the mov and using alphablend rather than chromakey has given me some frame rate back, replacing jit.scissors with jit.submatrix a bit more, and swapping some line objects for bline a bit more. Frame rate is acceptable now so I guess I'll live with it, but thanks for the clues.

OK, so if I plug the background mov into the right inlet of slab it works just like jit.alphablend, so still same problem. 

Well pre keying the mov and using alphablend rather than chromakey has given me some frame rate back, replacing jit.scissors with jit.submatrix a bit more, and swapping some line objects for bline a bit more. Frame rate is acceptable now so I guess I'll live with it, but thanks for the clues.


I would remove "jit.gl.slab display @file co.alphablend.jxs" since standard texture blending will use the alpha channel. You will need to disable depth buffering however, with @depth_enable 0 to both jit.gl.videoplane and jit.gl.mesh.

Hopefully this points you in the right direction. jit.gl.slab is best suited for compositing matrices of the same size, however with a similar GPU based strategy to what I suggested with submatrix, you could use slab for more interesting types of compositing than standard alphablending, but I don't think you need to take such a route for your application.

Thanks Joshua, been learning a lot about jit.gl lately! I just found this on one of Wes's posts which enables me to mix matrices of differing size and key out the alpha channel without detriment to my frame rate. I have implemented it in my patch and have gone from a pitiful 14FPS to around 30 in fullscreen. Seeing as I am rendering a 2400x600 movie with a keyed mov and a PNG on top on a mid range MBP I'm impressed! Thanks for all the help. 

mixed resolutions chromakey