What indicate the values in colorbar?

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

Answer 1

[DGM]

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)

[Yama Abawi]

Thanks for your answer :-) But to be honest, I am still a little confused. What is meant by "mean pixel value"? I thought the pixel color code is a vector of three values. so what does (for example) the value 250 in the color bar mean?

Regards

[DGM]

"mean pixel value" was specifically what you were calculating and displaying.

A = uint8(permute([64 128 255],[1 3 2])) % a single color tuple 

A = 1×1×3 uint8 array
A(:,:,1) =

   64


A(:,:,2) =

   128


A(:,:,3) =

   255

B = mean(A,3) % dim3 average

B = 149

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

C = uint8 123

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)