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How do I eliminate (minimize) display quantization with imshow?

Asked by Bruce Pillman on 19 Aug 2013

I see extra quantization when displaying a grayscale uint8 image. For example:

inRamp = uint8 ( round ( repmat( linspace(0,60,1200), 600, 1 ) ) );
imshow ( inRamp)
imwrite ( inRamp, 'inputRamp.tif' )

If I display this tif image in another display application, I see a series of steps. The image displayed in MATLAB using imshow has a couple of steps that are double-wide and double-deep. I have verified this using Windows screen captures of the image as displayed by other software and as displayed in MATLAB. The plots of this data show a clear stairstep pattern, with the MATLAB showing a couple of steps (near pixel value 10 and 40) that are double width and double height. I can email plots to anyone interested in seeing them.

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Bruce Pillman

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2 Answers

Answer by Bruce Pillman on 23 Aug 2013
Accepted answer

After discussing this with Mathworks technical support, I tried changing the renderer used from "painters" to "opengl". This appears to solve the quantization problem.

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Bruce Pillman
Answer by Image Analyst on 19 Aug 2013

You may be observing aliasing, possibly compounded by the Mach effect. Post your plots and images to snag.gy.

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Image Analyst on 20 Aug 2013

I did alt-printscreen and pasted into Photoshop and then saved out as PNG. Then I read it into MATLAB and extracted a line and plotted it. The plotted line (staircase ramp) did show some random jitter of plus or minus one gray level. I don't know why.

Bruce Pillman on 21 Aug 2013

Thanks for doing that test. That is interesting, since my results don't seem random. I am running 64-bit Windows 7. What are you running?

Image Analyst on 21 Aug 2013

The same.

Image Analyst

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