Have a look at the imwrite() documentation:
If the input array is of class double, and the image is a grayscale or RGB color image, imwrite assumes the dynamic range is [0,1] and automatically scales the data by 255 before writing it to the file as 8-bit values.
Any negative double values you try to write will thus end up clipped at 0.
If you are trying to write the equivalent of the imagesc() then you need to do the scaling yourself before you imwrite() the result.
