The PNG Guide is an eBook based on Greg Roelofs' book, originally published by O'Reilly. 
Home PNG Options and Extensions Physical Scale (sCAL)  
Physical Scale (sCAL)
Table 115 shows the format of sCAL; it is quite simple.
Two units are defined: meters (unit specifier = 1) and radians (unit specifier = 2). The size of a pixel in the given units, both horizontally and vertically, is given by a pair of positive floatingpoint numbers encoded as ASCII strings and separated by a null byte. The most general form of a floatingpoint string includes an optional leading sign (+ or ), zero or more decimal digits (09, the ``integer part''), an optional decimal point followed by zero or more decimal digits (the ``fractional part''), and an optional e or E followed by an optional sign and one or more digits (the ``exponent part''). Either the integer part or the fractional part must contain at least one digit, but everything else may be omitted. Thus, 1 and .1 are valid floatingpoint numbers, as is +123.4567e089. Note that the exponent is interpreted as a power of 10 (10^{89} in the third example) to be multiplied by the integer and fractional parts; this is the computer version of what is sometimes referred to as scientific notation.


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