Difference between revisions of "PLR"

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Some attempts to reverse engineer the PLR high dynamic mode from CMV12000.
Some attempts to reverse engineer the PLR high dynamic range mode from CMV12000.


==Registers==
==Registers==

Revision as of 21:45, 30 January 2016

Some attempts to reverse engineer the PLR high dynamic range mode from CMV12000.

1 Registers

  • 79: Number_slopes: 1,2,3.
  • 75-78: Exp_kp1, Exp_kp2: exposure times for highlights (same formula as Exp_time)
  • 106: Vtfl2, Vtfl3: knee point locations (range: 0-63; units: unknown)

Let's check the effect.

1.1 Register effects

I'll use an IT8 chart, exposed at 30 ms (normal exposure) and 100 ms (overexposed). A little dark in the lab today, but shouldn't be a big problem.

To analyze the images, I'll use octave 4.0, compiled with 16-bit image support. The scripts should run in Matlab as well, with minimal changes.

1.1.1 Linear exposures

30ms-lin.jpg 100ms-lin.jpg

Let's check if the first image is really exposed to the right, in octave.

a = read_raw('30ms-lin.DNG');
prctile(a(:),99) - 128        % note: black level is forced to 128 in raw2dng
ans =  2269                   % clipping starts at about 2400-2500 above black

Let's check if the clipping point is, indeed, where I say:

figure, hold on
colors = 'rgcb';                                  % meaning: red, gren1, green2 (plotted as cyan), blue (raw data from Bayer channels)
[g30,c30]   = sample_it8('30ms-lin.DNG', 0);      % read median RGGB swatch values from IT8 chart (first arg is grayscale, second is full color)
[g100,c100] = sample_it8('100ms-lin.DNG', 0);
for i = 1:4
  plot(c30(:,i), c100(:,i), ['.' colors(i)]);     % plot the 100ms image vs the 30ms one (extremely rough guess for response curve)
end

100ms-vs-30ms-lin.png

  • this approximates (but it's not equal to!) the response curve without PLR
  • this plot assumes the sensor response in the 30ms image (which was not overexposed) is linear (but it's probably not)
  • the sensor doesn't clip very harshly to white (could be the PLR circuits kicking in with a very low exposure time? to be checked)
1.1.2 2-segment PLR exposures

Let's start with a 2-segment PLR exposure, 100ms/10ms, vtfl2=32. That means, Number_slopes = 2, Exp_time = 8072, Exp_kp1 = 805, Vtfl=96.

100ms-10ms-32.jpg

Although the image may look a little overexposed at first sight (because of the magenta cast), the raw levels seem to be alright:

c = read_raw('100ms-10ms-32.DNG');
prctile(c(:),99)
ans =  1957

Let's check our "response curve" approximation:

[g100p,c100p] = sample_it8('100ms-10ms-32.DNG', 0);
figure, hold on
for i = 1:4
  plot(c30(:,i), c100p(:,i), ['.' colors(i)]);
end

100-10-32-plr-vs-30ms-lin.png

  • the knee point isn't as sharp as in the datasheet
  • its location is around 400 (todo: find the relationship between this and vtfl)
  • the second exposure segment appears a little noisy

Let's leave Vtfl constant and change exposure time for the second segment (1,5,10,20,50 ms).

100ms-1ms-32-tiny.jpg 100ms-5ms-32-tiny.jpg 100ms-10ms-32-tiny.jpg 100ms-20ms-32-tiny.jpg 100ms-50ms-32-tiny.jpg

Commands for plotting the "response curve" approximations are left as an exercise to the reader. Result:

100-x-32-plr-vs-30ms-lin.png

1.1.2.1 First models of the response curve for 2-segment PLR exposures

Notice the knee point appears to move to the right with higher Exp_kp1 values. Why would this happen?

If we assume the knee point threshold (Vtfl2) is only monitored while we still have time to fully expose the second segment (Exp_kp1), we can imagine a model of the PLR exposure that looks like this:

function y = plr_model_0(x, exp_time, vtfl_thr, exp_kp1)
   % regular exposure time
   y = x .* exp_time;
   
   % time needed to reach the Vtfl threshold (lower input levels => longer times)
   t_reach_vtfl = vtfl_thr ./ x;
   
   % which pixels reach vtfl?
   % assume this threshold is only monitored while we still have time
   % to fully perform the exp_kp1 (that is, before exp_time - exp_kp1)
   reached_vtfl = t_reach_vtfl < exp_time - exp_kp1;
   
   % how the exposure behaves in the highlights
   y_highlight = vtfl_thr + x .* exp_kp1;
   
   % copy highlight values only for those pixels that reached Vtfl
   y(reached_vtfl) = y_highlight(reached_vtfl);
end

Let's see how it compares to our real data.

100-x-32-plr-vs-30ms-lin-model0.png

Getting closer.

Notice the slopes of the second segment seem to be higher in the real data, compared to our model, as if the real exposure time would be a little higher. Let's extend it like this: assume the circuits that keep the pixel in "reset" state (after reaching vtfl) are not perfect - there may be still some light accumulating in the sensels. Let's call this gain "reset_leak", and we expect it to be quite a bit lower than 1 (which is the gain used during actual exposures).

function y = plr_model_1(x, exp_time, vtfl_thr, exp_kp1, reset_leak)
  
  % [ ... ]  
  
  % time while the pixel is kept in "reset" state, after reaching vtfl
  reset_time = exp_time - t_reach_vtfl - exp_kp1;
  
  % how the exposure behaves in the highlights
  y_highlight = vtfl_thr + x .* exp_kp1 + x .* reset_time * reset_leak;
  
  % [ ... ]  
  
end

100-x-32-plr-vs-30ms-lin-model1.png

Tuning parameters (trial and error): vtfl_thr = 900, reset_leak = 1/40.

This model appears to explain the actual response curves pretty well, but there's still room for improvement.

At this point, I would try to find some real response curves and reduce the noise of the test images by averaging multiple frames, then repeat the experiment. This will take a while.

[to be continued]