how are images stored in defaultimagecdata

Hi @Kees,

It is great to see your enthusiasm for learning. Let’s break down your code and clarify how it works, especially focusing on the manipulation of image data.

Default Image Data Retrieval

defaultCData = get(0,'DefaultImageCData');

This line retrieves the default color data for images in MATLAB, which is typically a 64x64 matrix. Each element in this matrix represents a pixel's color value.

Raising to a Power

defImage = pow2(get(0,'DefaultImageCData'),47);

Here, you are raising each element of the defaultCData matrix to the power of 47. The pow2 function computes ( x = f \times 2^e ), where f is the input matrix and e is the exponent. This operation significantly increases the values in the matrix, which may affect how the image is displayed.

Bit Shifting

for shift = 10:63
  img = bitshift(defImage, shift);
  imshow(img,'DisplayRange',[min(img(:)) 
  max(img(:))],'InitialMagnification',400)
  title(num2str(shift));
  pause(1)
end

In this loop, you are shifting the bits of the defImage matrix to the left by a specified number of positions (from 10 to 63). The bitshift function effectively multiplies the matrix by ( 2^{text{shift}} ), which can lead to a dramatic change in the pixel values, resulting in different visual representations of the image.

The bitshift function does not load different images per se; rather, it modifies the pixel values of the existing image matrix. Each iteration of the loop applies a different shift, which alters the brightness and contrast of the image displayed. The imshow function then visualizes this modified image. If you want to access individual columns of the defaultCData matrix, you can do so by indexing into the matrix. For example, to access the first column, you would use:

firstColumn = defaultCData(:, 1);

To display this column as an image, you can reshape it back to a 64x64 format if necessary, or simply display it directly if it represents a valid image format. Here’s how you might do that:

imshow(reshape(firstColumn, [64, 64]), 'InitialMagnification', 400);
title('First Column Image');

In nutshell, your code is effectively manipulating a single image matrix by altering its pixel values through bit shifting. Each shift results in a different visual representation of the same image. To access and display individual columns, you can index into the defaultCData matrix directly. Feel free to experiment with different shifts and explore how they affect the image.

If you have further questions or need clarification on specific points, don’t hesitate to ask!