dear mario
i load a special gamma table, with fine steps over a limited range, in order to get better than 8-bit resolution in my low-contrast image, but I'm still getting 8-bit performance. i think i'm taking all the advice you've given me on this topic over the past 2 years, including the best macOS computer to buy. i was confident that this would work, and I'm at a loss to guess what i might be doing wrong.
MacBook Pro (Retina, 15-inch, Mid 2015)
AMD Radeon R9 M370X 2048 MB
macOS 10.12.3
MATLAB 9.1.0.441655 (R2016b)
Psychtoolbox 3.0.14 - Flavor: beta - Corresponds to SVN Revision 5452
I used a Cambridge Research Systems colorimeter to measure the luminance produced by each gamma value that I loaded into the CLUT, using EnableCLUTMapping and
Screen('LoadNormalizedGammaTable',...).
Here are the graphs of the luminance vs. gamma.
With 8-bit performance one expects 255 luminance steps across the whole range, and 4 steps over 1/64 of the range, and 2 steps over 1/128 of the range. That is exactly what I get, though I anticipated getting much better performance. I'd be happy with just dither, simulating better-than-8-bit performance.
PTB-INFO: OpenGL-Renderer is ATI Technologies Inc. :: AMD Radeon R9 M370X OpenGL Engine :: 2.1 ATI-1.48.21
PTB-INFO: Renderer has 2048 MB of VRAM and a maximum 1949 MB of texture memory.
PTB-INFO: VBL startline = 1800 , VBL Endline = -1
PTB-INFO: Beamposition queries unsupported or defective on this system. Using basic timestamping as fallback: Timestamps returned by Screen('Flip') will be less robust and accurate.
PTB-INFO: Measured monitor refresh interval from VBLsync = 0.000000 ms [inf Hz]. (0 valid samples taken, stddev=0.000000 ms.)
PTB-INFO: Small deviations between reported values are normal and no reason to worry.
PTB-WARNING: ==================================================================================================================
PTB-WARNING: DESKTOP COMPOSITOR IS ACTIVE! ALL FLIP STIMULUS ONSET TIMESTAMPS WILL BE VERY LIKELY UNRELIABLE AND LESS ACCURATE!
PTB-WARNING: STIMULUS ONSET TIMING WILL BE UNRELIABLE AS WELL, AND GRAPHICS PERFORMANCE MAY BE REDUCED!
PTB-WARNING: DO NOT USE THIS MODE FOR RUNNING REAL EXPERIMENT SESSIONS WITH ANY REQUIREMENTS FOR ACCURATE TIMING!
PTB-WARNING: ==================================================================================================================
PTB-INFO: All display tests and calibrations disabled. Assuming a refresh interval of 60.000000 Hz. Timing will be inaccurate!
PTB-INFO: Psychtoolbox imaging pipeline starting up for window with requested imagingmode 1027 ...
PTB-INFO: Will use 8 bits per color component framebuffer for stimulus drawing.
PTB-INFO: Will use 8 bits per color component framebuffer for stimulus post-processing (if any).
Compiling all shaders matching RGBMultiLUTLookupCombine_FormattingShader * into a GLSL program.
Building a fragment shader:Reading shader from file /Applications/Psychtoolbox/PsychOpenGL/PsychGLSLShaders/RGBMultiLUTLookupCombine_FormattingShader.frag.txt ...
EnableCLUTMapping: Using a 32 bit float CLUT -> 23 bits effective linear mapping precision per color channel.
By the way, using EnableCLUTMapping, I notice that what is returned by ReadNormalizedCLUT is not affected by whatever I load with LoadNormalizedCLUT.
My complete test program, 123 lines, is below. It produced the Figure above. Can you give me advice on how to achieve 10-bit performance?
Best
denis
function TestLoadGamma
% TestLoadGamma
% Measure the precision of the OUTPUT of the gamma table. This will include
% precision achieved by dither, as implemented by the video driver.
% Denis Pelli March 28, 2017
crsColorimeter=1; % ColorCAL II XYZ
if crsColorimeter
%% OPEN PORT FOR COLORIMETER
% Cambridge Research Systems
%
ob=instrfind;
fclose(ob);
portName='/dev/cu.usbmodem0001';
if exist('s1','var')
fclose(s1);
delete(s1)
end
clear s1;
% Open ColorCAL II virtual serial port.
s1 = serial(portName);
pause(0.1);
fopen(s1);
addpath(fullfile(fileparts(mfilename('fullpath')),'../ColorCAL MKII')); % folder in same directory as this M file
else
s1=[];
end
logBase2OfFractions=[0 6 7];
useFractionOfScreen=0.2;
BackupCluts;
Screen('Preference','SkipSyncTests',2);
try
%% OPEN WINDOW
screen = 0;
screenBufferRect = Screen('Rect',screen);
PsychImaging('PrepareConfiguration');
PsychImaging('AddTask','General','UseRetinaResolution');
PsychImaging('AddTask','AllViews','EnableCLUTMapping',256,1); % clutSize,high res
if ~useFractionOfScreen
[window,screenRect] = PsychImaging('OpenWindow',screen,255);
else
[window,screenRect] = PsychImaging('OpenWindow',screen,255,round(useFractionOfScreen*screenBufferRect));
end
%% MEASURE LUMINANCE AT EACH GAMMA VALUE
steps=31;
for j=1:length(logBase2OfFractions)
logBase2OfFraction=logBase2OfFractions(j);
for i=0:steps
if logBase2OfFraction>0
g=0.5;
else
g=0;
end
g=g+2^-logBase2OfFraction*i/steps;
assert(g<=1)
bits(j).test(i+1).gamma=g;
bits(j).test(i+1).L=LuminanceOfGamma(s1,window,g); % Read photometer
end
end
catch
if ~isempty(s1)
% Close the serial port
fclose(s1);
delete(s1);
clear s1
end
sca
psychrethrow(psychlasterror);
end
if ~isempty(s1)
% Close the serial port
fclose(s1);
delete(s1);
clear s1
end
Screen('Close',window);
close all
RestoreCluts
sca
%% PLOT RESULTS
figure
for j=1:length(logBase2OfFractions)
logBase2OfFraction=logBase2OfFractions(j);
subplot(1,length(logBase2OfFractions),i)
plot([bits(j).test.gamma],[bits(j).test.L]);
title(sprintf('Range/%d',2^logBase2OfFraction));
xlabel('gamma');
ylabel('Luminance (cd/m^2)');
xlim([bits(j).test(1).gamma bits(j).test(end).gamma]);
end
end
function L=LuminanceOfGamma(s1,window,g)
% L=LuminanceOfGamma(s1,window,g)
% MEASURE LUMINANCE PRODUCED BY ONE GAMMA VALUE
persistent CORRMAT
if ~isempty(s1) && isempty(CORRMAT)
% Get ColorCAL II XYZ correction matrix (CRT=1 WLED LCD=2 OLED=3):
CORRMAT = crsGetColorCALCorrectionMatrix(s1,2);
pause(0.2);
end
loadOnNextFlip = 2; % REQUIRED for reliable LoadNormalizedGammaTable.
gamma=[0:255;0:255;0:255]'/255;
iPixel=128;
for j=-4:4
gamma(1+iPixel+j,1:3)=[g g g];
end
Screen('LoadNormalizedGammaTable',window,gamma,loadOnNextFlip);
Screen('FillRect',window,iPixel);
Screen('Flip',window);
if ~isempty(s1)
XYZ=crsGetColorCALMeasurement(s1,CORRMAT)'; % Read photometer
pause(0.1);
L=XYZ(2);
else
% No photometer. Simulate 8-bit performance.
L=200*round(g*256)/256;
end
fprintf('g% .4f, %.4f cd/m^2n',g,L);
end
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