pfsabsolute applies all necessary operations to convert an image
from relative luminance (tag LUMINANCE is RELATIVE) or
display-dependent luma (tag LUMINANCE is DISPLAY) to absolute
luminance values. When the luminance in an image is absolute (tag
LUMINANCE set to ABSOLUTE), the Y channel represents physical
luminance in cd/m^2. Absolute luminance levels are useful for some
tone mapping algorithms and also for image or video compression.
The argument <dest Y> denotes the level of luminance that
relative luminance <src Y> should be rescaled to. The luminance
is in fact multiplied by the ratio <dst Y>/<src Y>. <src Y> is
normally relative luminance checked with pfsview in the spot where the
absolute luminance <dest Y> is measured, known or guessed. If
<src Y> is omitted, the value 1 is assumed, so <dest Y> is
just a scaling factor.
In case if display-depended luma (tag LUMINANCE is DISPLAY), the
inverse gamma correction is applied (assuming sRGB color space). In
such case <dest Y> is usually the maximum luminance of a display
(e.g. 80 cd/m^2).
This command always sets LUMINANCE tag to ABSOLUTE.