The PNG Guide is an eBook based on Greg Roelofs' book, originally published by O'Reilly. 
Home PNG Options and Extensions Physical Pixel Dimensions (pHYs)  
Physical Pixel Dimensions (pHYs)
The layout of the chunk is shown in Table 114.
If the unit specifier byte is 1, the units are meters; if it is 0, the units are unspecified, and only the relative dimensions are known. Currently, no other values are valid. Note that the format of the chunk precludes pixel sizes greater than one meter, which should not be a significant hardship for most applications, but it allows pixels as small as 4.7 Ångstroms, which is roughly the size of a single atom. For the previous scanning example, 600 dpi is equal to 23,622.05 pixels per meter, so both the x and y values would be 23,622, and the unit specifier would be 1. The second example is slightly trickier. First, it is necessary to know that practically all current computer displays have a physical aspect ratio of 4:3,[89] which means the viewable portion of the display (the glass) is threequarters as high as it is wide. Thus, the horizontal pixelsperunit in the case of a 1280 × 1024 display is proportional to (1280/4) or 320, while the vertical pixelsperunit is proportional to (1024/3) or 341.333333. Because we don't have an absolute scale, we are free to multiply these values by a common factor; doing so will preserve some of the precision that would otherwise be lost due to truncation of the decimal part of the second value (the .3333 part). One choice would be a power of 10, such as 1,000; then the stored values would be 320,000 and 341,333, respectively. But in this case, we can do better: we know that the fractional part is simply onethird, so multiplying both values by 3 will preserve the aspect ratio exactly. Thus the chunk would contain the values (3 × 1280/4) or 960, (3 × 1024/3) or 1,024, and 0 for the unit specifier. Values of 15, 16, and 0 would work equally well.


Home PNG Options and Extensions Physical Pixel Dimensions (pHYs) 