# Images don't show properly with imshow after certain calculation

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Walker on 10 Dec 2019
Commented: Walker on 16 Dec 2019
Hello all,
The question is I need to calculate the matrix of a uint8 image. After the calculation (one order difference of the field) and normalization, I tried to figure out the image with imshow, however, the reults is a white blank or no longer smooth. The figure1.jpg can be any picture as you can find.
I should clarify that the values are uint8 after imread and I used "im2double" to transfer the type to double. The final imshow image is black and the final image will looks like some discrete dots when the matrix times 255.
clear all
deltax = 1;
deltay = 1;
AmpDistributionDouble = im2double(AmpDistribution);
AmpDistributionDouble(isnan(AmpDistributionDouble))=0;
[MSize, NSize, VSize] = size(AmpDistributionDouble);
DiffoverA = zeros(MSize,NSize,VSize);
for i = 2: MSize-1
for j = 2:NSize-1
for k = 1:VSize
DiffoverA(i,j,k) = (( AmpDistributionDouble(i+1,j,k) -AmpDistributionDouble(i,j,k))./(deltax) + (AmpDistributionDouble(i,j+1,k) -AmpDistributionDouble(i,j,k))./(deltay^2 ));
end
end
end
DiffoverA(isnan(DiffoverA))=0;
DiffoverA(DiffoverA==inf)=0;
MinValue = min(DiffoverA,[],'all');
MaxValue = max(DiffoverA,[],'all');
NormDelta = 255*(DiffoverA -abs(MinValue))/(MaxValue + abs(MinValue));
figure(1)
image(AmpDistribution);
figure(2)
imshow(AmpDistributionDouble,[]);
figure(3)
imshow(NormDelta,[]);

Bjorn Gustavsson on 10 Dec 2019
When you do things like this the errors are always (in my experience) in different normalizations and type-castings. To work these out I always go to the stage of looking at the individual R/G/B-planes of my images and check where they lie in intensity. Typically something like this:
subplot(2,2,1)
imagesc(NormDelta(:,:,1)),colorbar
subplot(2,2,2)
imagesc(NormDelta(:,:,2)),colorbar
subplot(2,2,3)
imagesc(NormDelta(:,:,3)),colorbar
subplot(2,2,4)
manualScale = @(I) (I-min(I(:)))/(max(I(:))-min(I(:)));
imagesc(manualScale(NormDelta))
Then you'll see where your normalised NormDelta are in intensity.
Finally: Your normalization step looks "very peculiar" to me, but I cant say if that is what you want.
HTH

Walker on 13 Dec 2019
@Bjorn thank you very much for the reply.I used the min non -zero value and max finite value to replace the negative and the infinite values and have got some reasonable results. I'll try your method and compare the two methods. thank you so much. Have a nice weekend.
Image Analyst on 13 Dec 2019
NormDelta = 255*(DiffoverA -abs(MinValue))/(MaxValue + abs(MinValue));
try using mat2gray(), rescale(), or imadjust(). And I would use imshow() with [] rather than image() or imagesc(). If the image still looks uniform, I'd examine the image in the workspace variable editor or look at its histogram. Maybe it really is largely the same value except for a very few pixels that are ruining the normalization.
Walker on 16 Dec 2019
@Image Analyst Thank you so much for your reply. imshow works well as you said. Yes, I'll try with other command. Until now, the results is basically resonable.@Bjorn Gustavsson @Image Analyst Thank you both very much.