Slit-scan Optimisation

I have spent some time experimenting with different parameters in the Quartz Composer slit-scan patch I have created. I’ve also tried alternative video compression codecs for the source video to see if any improvement to final performance can be made. The key factors appear to be

  • the number of image slices to be used to make the final image
  • the number of video frames held in memory from which the video slices are drawn, the level of video history available, if you like
  • the dimensions of the final image

I have made some small gains here and there, predominantly by turning off depth sorting in a couple of Quartz Composer modules, which saves some additional calculation required to arrange objects in 3D space (this project is currently 2D). A little optimisation of the maths functions seems to have helped too. The best source video codec I’ve found yet is the one I started off with – Apple ProRes 442 – which produces great quality image but is very high in datarate. One would think the high data rate would impact upon overall performance, but not so.

The base level I have come up with is 30fps output, using 36 slices (drawn vertically) which take image data from a ‘buffer’ of 1024 frames. The left most-slice takes its data from one of the oldest frames in the buffer, the right-most slice does so from one of the most recent. There is no ‘lattice’ effect with the realtime video being shown behind gaps between the slices, rather the slices are positioned with vertical edges meeting so there is a continuum from left to right.

The result is shown below.

Of course, all this optimisation is in an attempt to create realtime, responsive phenomena. Much of the work by other practitioners that I have been looking at collates and prints pixels to frames of a final video as an offline render process. I am keen to preserve the realtime nature of my slit-scan work, at least until the compositional relationship between audio and video has been formed. It may be that I add something extra in post production, this is certainly an opportunity to consider.

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