GIMP Question about PNG Resolution (but CC3 related - sort of..)

[RobA]

You can not fix it, as it is not broken

The PNG spec embedds resolution as:

4.2.4.2. pHYs Physical pixel dimensions

The pHYs chunk specifies the intended pixel size or aspect ratio for display of the image. It contains:

Pixels per unit, X axis: 4 bytes (unsigned integer)
Pixels per unit, Y axis: 4 bytes (unsigned integer)
Unit specifier: 1 byte

The following values are defined for the unit specifier:

0: unit is unknown
1: unit is the meter

When the unit specifier is 0, the pHYs chunk defines pixel aspect ratio only; the actual size of the pixels remains unspecified.

Conversion note: one inch is equal to exactly 0.0254 meters.

So, 300PPI= 300 pixels per 0.0254m, or 11,811.0236 pixels per meter. Since it can't do fractional (unsigned int) it gets rounded:
11,811 pixels/m = 299.9994ppi

jpeg and gif files, on the other hand specify in inches, not meters so they do not have the same issue.

If you have software that claims to create a 300ppi png file it is lying or rounding!

-Rob A>

[jaerdaph]

I knew someone here would know the answer - thanks Rob! This is exactly why I love (and have missed) this place!