[PD] Gem Rutt-Etra / pix_data speed?

[Derek Holzer]

Dear PD folks,

I'm trying to cook up a Pd/GEM version of the Rutt-Etra scan processor, 
which sends audio signals to an analog CRT vector 
monitor/oscilloscope/etc to control the vertical and horizontal 
positions of its beam, plus its brightness.

The video posted here is webcam->MaxMSP->MOTU soundcard->Vectrex console 
display, using the [jit.matrix] and [jit.peek] objects:

https://www.instagram.com/p/BWk5imsAao1

To do this in GEM, I need to read the luma values of the pixels in a 
video stream according to their X and Y positions with the horizontal 
and vertical signal ramps which form the video raster on the CRT (using 
[phasor~] for the vertical scan and a multiplication plus a [wrap~] for 
the horizontal scan), and send that value out as a third audio signal to 
control the brightness of the scan line on the monitor.

Here is a block diagram of the original Rutt-Etra for the curious:

http://iasl.uni-muenchen.de/links/GCA_ill76.png

The issue that I have run into so far is that [pix_data] from GEM only 
responds to control rate messages.  PDP has the [pdp_scanxy~] object 
which does basically what I'm looking for: get luma values at given X Y 
coordinates of the image based on an audio signal input, and send those 
values out as an audio signal. I've gotten a rough raster with this 
already, but for various reasons I would prefer to stay clear of PDP.

Can anyone tell me how fast the [pix_data] object can respond? I have 
been using a [metro] operating at a rate of one bang per sample, and I 
have set a [block~ 1] in the subpatch doing this analysis.

[timer] reports an interval of 0.022 ms between bangs @44.1K, and 1.451 
ms between changed pixel values coming from [pix_data]. (Unchanged 
values are redundant.) In fact, the rate between the metro and the GEM 
object match until I go under 64 samples, then it becomes erratic. Is a 
64 sample block size hardcoded into [pix_data]?

Can anyone see how I could get audio rate data out of [pix_data], or if 
there might be another way to get luma values with an audio signal?

Thanks and best wishes!
Derek

-- 
derek holzer
noise.art.technology
http://macumbista.net