CODEWITHSUNDEEP
image-processingmatlab
krishnab
krishnab

Reputation: 10070

change number of gray levels in a grayscale image in matlab

I am rather new to matlab, but I was hoping someone could help with this question. So I have a color image that I want to convert to grayscale and then reduce the number of gray levels. So I read in the image and I used rgb2gray() to convert the image to grayscale. However, I am not sure how to convert the image to use only 32 gray levels instead of 255 gray levels.

I was trying to use colormap(gray(32)), but this seemed to have no effect on the plotted image itself or the colorbar under the image. So I was not sure where else to look. Any tips out there? Thanks.

Upvotes: 6

Views: 18870

Answers (3)

mmgp
mmgp

Reputation: 19241

While result = (img/8)*8 does convert a grayscale image in the range [0, 255] to a subset of that range but now using only 32 values, it might create undesirable artifacts. A method that possibly produces visually better images is called Improved Grayscale Quantization (abbreviated as IGS). The pseudo-code for performing it can be given as:

mult = 256 / (2^bits)
mask = 2^(8 - bits) - 1
prev_sum = 0
for x = 1 to width
    for y = 1 to height
        value = img[x, y]
        if value >> bits != mask:
            prev_sum = value + (prev_sum & mask)
        else:
            prev_sum = value
        res[x, y] = (prev_sum >> (8 - bits)) * mult

As an example, consider the following figure and the respective quantizations with bits = 5, bits = 4, and bits = 3 using the method above:

enter image description here enter image description here enter image description here enter image description here

Now the same images but quantized by doing (img/(256/(2^bits)))*(256/(2^bits)):

enter image description here enter image description here enter image description here enter image description here

This is not a pathological example.

Upvotes: 3

Junuxx
Junuxx

Reputation: 14271

You can reduce the number of different values in an image by simple rounding:

I = rgb2gray(imread('image.gif'));
J = 8*round(I/8)

See imhist(I) and imhist(J) for the effect.

However, if you want to reduce image size, you might be better off using an image processing program like Photoshop, Gimp or IrfanView and save as a 32 color gif. In that way you'll actually reduce the file's palette, and I think that's something Matlab can't do.

Upvotes: 2

s.bandara
s.bandara

Reputation: 5664

Check if the the type of your image data is uint8 which I would suspect. If that's the case divide the image by 8 to abuse the flooring effect of integer division, multiply with 8 again, and you're set: I2=(I/8)*8. I2 will have only 32 gray levels.

Upvotes: 0

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