What indicate the values in colorbar?
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Hello,
I would be very thankful, if someone could answer these two questions:
The image "RB.jpg" is read and shown via the following code:
A = imread('RB.jpg');
figure;
imshow(A);
1) What are the values (between 0 and 1) indicating, if the colorbar is, as shown, activated?
2) What is changed in the image "RB.jpg" with the following code, and what are the meaning of the values in the colorbar now?
A = imread('RB.jpg');
figure;
imshow(A);
AA=mean(A,3);
figure;
imagesc(AA);
Thanks in advance :-)
Yama
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Accepted Answer
DGM
on 10 Oct 2021
Edited: DGM
on 10 Oct 2021
In the first example, it doesn't mean anything. It's not referring to the image as displayed by imshow(), since imshow() is not using the relevant color map to display the image.
A = imread('peppers.png');
imshow(A)
colorbar
However, imagesc() renders the image using the current colormap, and so the colorbar shows the mean pixel value. As the image is uint8 class, it's understandable that the mean values are somewhere in the midst of [0 255]
clf % just for web view
imagesc(mean(A,3))
colorbar
This might get a bit confusing as the behavior varies between RGB and single-channel images. For example, if you repeat the first example with a grayscale version of the image, then the colorbar does refer to the image pixel values.
clf % for web view
imshow(rgb2gray(A))
colorbar
In this case, it adopts a grayscale colormap, but there's nothing stopping you from using another colormap.
clf
imshow(rgb2gray(A))
colorbar
colormap(parula)
2 Comments
DGM
on 20 Oct 2021
"mean pixel value" was specifically what you were calculating and displaying.
A = uint8(permute([64 128 255],[1 3 2])) % a single color tuple
B = mean(A,3) % dim3 average
This takes the simple average of RGB components, converting the image to a monochrome image with only one channel. Similarly,
C = rgb2gray(A) % 0.299*R + 0.587*G + 0.114*B
converts the RGB image to Y (luma), which is just a weighted average. The result is again a single-channel monochrome image. The value of the monochrome image is what's represented in the plot, and the colorbar represents whatever the monochrome image represents (e.g. the mean or a weighted mean)
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