Image Processing Reference
In-Depth Information
A word of warning, though: Note that none of the currently available compression
tools will allow you to key-frame the settings. These values tend to be applied globally by
the compression tool. Use Final Cut or After Effects if you want to process the luma and
key the changes to different segments of your movie.
Here are some final rules of thumb that apply when planning your luma correction.
Bear in mind that increasing the brightness also increases the noise floor, so any noise
in the black area of your image will become visible as you raise the overall luma level.
Brightness adjustment should not be needed for a clean source. But lowering the
brightness to correct the black levels also has the benefit of improving the coding effi-
ciency because of the noise reduction.
It used to be commonly recommended that the contrast should be increased by
27 before compression. Modern codecs should not need this adjustment since they have
built-in compensation. Any automatic compensation will be taken care of and you need
not worry. If you and the codec both apply
27 contrast enhancement, that is a 2
adjustment, which will lead to a severe crushing of the white levels.
Correcting the Gamma
The gamma value is not the same as the luma, although they are related. The luma prop-
erty describes the overall range, while the gamma property affects the linearity of that
intensity gradient. Often, the gamma value is adjusted upward on material being
processed on the Macintosh to make the mid-tones show up better on Windows PC dis-
plays, which have a different gamma setting.
Gamma correction is used to change the shape of the graph that maps the output
intensity levels of the luma signal and how they correspond to the input levels. This is nec-
essary because the apparent brightness of a pixel is not necessarily proportional to its
numeric value. The emissivities of CRT, Plasma, LED, OLED, TFT, and LCD display
devices are different. This is because the nature of the chemical or electromagnetic effect
that generates the light emits different wavelengths and intensities. While your eye may
see what you think is white, the actual value may vary considerably from one kind of
device to another.
The sensitivity of the human eye is affected by the perceived brightness. The entire
chain of processing, from the initial photography to viewing, will influence the apparent
brightness of a scene.
The Effects of Changing the Gamma
The area that is most affected by gamma settings is the lower mid-tones down to the
extreme blacks. As the gamma value is increased, any detail that is hidden in the darker
Search WWH ::

Custom Search