In at least one of those cases (I'll let you discover which, it's very important in this pset), the sign of the variable (neg or pos) has a direct bearing on how the image is displayed by the computer. The sign of a variable in a structure is often used as a control feature this way. Consult the msft spec to explain these parameters.
And yes, using a signed int instead of an unsigned cuts the positive range in half, using the difference for the negative. For details, google "twos complement" notation.
It's also often the case that when one var in a structure is used in this way, the others will be made to be the same, simply to be consistent.
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