TIFFFieldReadCount() returns the number of values available to be read from the specified TIFF field;
that is, the number of arguments that should be supplied to TIFFGetField(). For most field types this is
a small positive integer, typically 1 or 2, but there are some special values:
• TIFF_VARIABLE=-1 indicates that a variable number of values is possible; then, a uint16_tcount
argument and a pointer data argument must be supplied to TIFFGetField().
• TIFF_VARIABLE2=-3 is the same as TIFF_VARIABLE except that the count argument must have type
uint32_t.
• TIFF_SPP=-2 indicates that the number of arguments is equal to the image's number of samples per
pixel.
fip is a field information pointer previously returned by TIFFFindField(), TIFFFieldWithTag(), or
TIFFFieldWithName().