# How to compress 32 bit bmp images to 8 bit bmp?

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Amanda F on 8 Feb 2023
Commented: Jonas on 9 Feb 2023
I have thousands of 32 bit grayscale bmp images that I want to compress to 8 bit bmp grayscale images using a script to reduce the file size and save space while maintaining the image quality. I have looked at using the "im2uint8()", "rgb2ind()", and "imwrite()" functions to try and achieve this, but I either get fully black images or, when it compresses images that maintain the image quality, they turn out to be 24 bit and not 8 bit. I have two scripts I've tried posted below this. Any ideas why it isn't compressing to 8 bit? Also, is there a reason why it only compresses to 24 bit? Thank you to any replys.
Here are a couple scripts I've tried:
map = gray(256);
imwrite(im,map,output_filename);
This results in a 24 bit image.
Next:
im8 = im2uint8(im);
imwrite(im8,output_filename);
This results in a completely black image.
Luca Ferro on 8 Feb 2023
Amanda F on 8 Feb 2023
It helps clarify a few things, but I still can't seem to save an 8 bit file.

Jonas on 8 Feb 2023
you can stretch and compress it on your own, just as you want
im=rgb2gray(im);
class(im)
ans = 'uint8'
imshow(im);
im16=uint16(im)*2^8; % 16-8=8
imshow(im16)
im32=uint32(im)*2^24; % 32-8=24
imshow(im32)
% and similarly downwards
clearvars -except im32;
im16=uint16(im32/2^16);
imshow(im16)
im8=uint8(im16/2^8);
imshow(im8)
Amanda F on 9 Feb 2023
Thank you very much for suggesting I check the image for what color type was. I checked the image info and found the image as "True color". All my images were captured with no color, but the images still rgb and not grayscale, so using the rgb2gray() was required to make it work.
Here is the functional script:
im8gray = rgb2gray(im);
imwrite(im8gray,output_filename);
Jonas on 9 Feb 2023
btw, your initial rgb image is not 32 bit, but already 24bit (3x8 bit). so it is already uint8, that's why you do not need those divisions described above
size(im,3)
ans = 3
class(im)
ans = 'uint8'
im=rgb2gray(im);
class(im)
ans = 'uint8'

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