Strange resultat using imagesc() on a three-value matrix

I want to have a three-color-only visual on a matrix that contains many values which are one of three values. Here is the simplest way I could repeat the issue:

r=rand(1,2000);
r(r>0.6)=70;r(r<=0.3)=0;r((r>0) & (r<70))=35;
figure;imagesc(r);

Zooming on some part of the image, I see more than three colors: uploaded_files/82867/sh_244.jpg

I would expect the visual to be in accordance with the values in the matrix. I mean, a visual with only 3 colors. Strangely, that does not happen if I replace the value 2000 by 1000 or by 2100. Is it my video card creating this bug? Or something else?

"Ver" returns:

• MATLAB Version: 8.5.0.197613 (R2015a)
• Operating System: Microsoft Windows 7 Édition Familiale Premium Version 6.1 (Build 7601: Service Pack 1)
• Java Version: Java 1.7.0_60-b19 with Oracle Corporation Java HotSpot™ Client VM mixed mode

[JFLabrie]

An even simpler code that leads to the same issue:

r=round(rand(1,2000));figure;imagesc(r);

Answer 1

[JFLabrie]

The following command solved the issue:

set(gcf,'Renderer', 'painters')

Answer 2

[Image Analyst]

I do not see that. I see only 3 colors in your first example, and two colors in your second example, as expected. My guess is it's your video adapter and how it's trying to zoom in, like by pixel replication (which would not introduce any new colors), or bicubic interpolation (which would introduce more colors).

[JFLabrie]

Thank, it confirms to me that it is most likely hardware related.

Answer 3

[Walter Roberson]

See https://www.mathworks.com/matlabcentral/answers/157758-how-do-i-turn-off-antialiasing-in-matlab-r2014b

[JFLabrie]

I tried the following command without success:

set(gcf,'GraphicsSmoothing','off')

But your suggestion lead me to have a look at the Figure Properties. Then I tried the following command that solved the issue immediately:

set(gcf,'Renderer', 'painters')