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12bit to 8 bit and Windowing

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David Großmann
David Großmann on 19 Jan 2020
Commented: David Großmann on 21 Jan 2020 at 19:27
Hallo,
I try to dicomread a 12bit-pic, change it to 8bit and windows it.
With
im = dicomread("CTA_Head.dcm");
im8 = uint8(im-1);
imshow(im8, [LOW,HIGH]);
Everything works fine. Matlab cuts all the values that aren't relevant and turns them into white or black.
But is there a function that does the same without directly displaying the picture?
When I use the following pictures do not look the same(it's darker):
im = dicomread("CTA_Head.dcm");
[nrows ncols] = size(im);
for c = 1:ncols
for r = 1:nrows
if (im(r,c) < LOW)
im2(r,c) = 0;
elseif (im(r,c) > HIGH)
im2(r,c) = 255;
else
im2(r,c) = im(r,c);
end
end
end
im8er = uint8(im2);
imshow(im8er);
Shall I cast to uint8 before or after cutting the values?
Thank You!

  1 Comment

Guillaume
Guillaume on 20 Jan 2020
If im is 12-bit (integer I presume), this is a very odd thing to do:
im8 = uint8(im-1);
If the 12 bit is signed (range [-2048, 2047]), this rescale im to the range [-2049, 2046] (assuming the 12-bits are stored in a 16-bit integer since matlab doesn't have 12 bit type) which is then clipped to [0 255] by the conversion to uint8. So it maps all values between -2048 and 1 included to 0, maps 1-256 to 0-255 and maps 256+ to 255.
If the 12 bit is unsigned (range [0 4095]), this rescale im to [0 4094] which is then clipped to [0 255]. So it maps 0 and 1 to 0, 2-256 to 1-255 and 256+ to 255.

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Accepted Answer

Walter Roberson
Walter Roberson on 19 Jan 2020
In older MATLAB see mat2gray, which has a deceptive function name and is not restricted to grayscale like the name suggests.
In newer MATLAB rescale()

  3 Comments

David Großmann
David Großmann on 20 Jan 2020
Thank you, but sadly I'm not allowed to use these. So have you got a different idea?
Walter Roberson
Walter Roberson on 20 Jan 2020
scaled = (double(im) - LOW) ./ (HIGH - LOW);
im8er = im2uint8(scaled);
David Großmann
David Großmann on 21 Jan 2020 at 19:27
Wow thanks, this looks perfectly fine!

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