Imshow() is odd in that it does actually support int8 images. It seems most other IPT tools which are class-sensitive don't. As usual though, the image needs to be properly-scaled for its class in order to display as expected.
The data needs to be rescaled as appropriate when casting in order for the image to be treated as intended. As Walter explained, simply casting a uint8() array using int8() will simply result in truncation without otherwise scaling or shifting the data. Consider the case with uint16 -> int16 conversion:
inpict = imread('cameraman16.png');
imshow(A,'border','tight')
imshow(B,'border','tight')
Why did I do this example with uint16/int16 instead of uint8/int8? Well remember that I said int8 support is an oddity in IPT? Yeah. There is no such im2int8() tool like there is for 16 bit images. You would either have to do it yourself:
inpict = imread('cameraman16.png');
ir = getrangefromclass(inpict);
scalefactor = diff(or)/diff(ir);
C = int8(round((double(inpict) - ir(1))*scalefactor) + or(1));
imshow(C,'border','tight')
outpict = imcast(inpict,'int8');
Either way, a workflow centered around int8 working images is going to be a huge hassle with no obvious benefit.
