Color Correction - Nuke 101: Professional Compositing and Visual Effects - page 149

gamma thatitusestodisplayimages.Differentmonitorscanhavedifferentcurves,
but most often they have a gamma curve called sRGB . Because the monitor is not
showing the image as it appears in reality, images need to be “corrected” for this.
This is usually done automatically because most image capture devices are apply-
ing an sRGB curve too, in the opposite direction. Displaying a middle gray pixel
on a monitor only shows you middle gray as it's being affected by the gamma
curve. Because your scanner, camera, and image processing applications all know
this, they color correct by applying the reverse gamma curve on this gray pixel
that negates themonitor'seffect. Thisprocessrepresents basic color management .
However, if your image's middle gray value isn't middle gray because a gamma
curve has been applied to it, it will react differently to color correction and might
produce odd results. Most applications work in this way, and most people dealing
with color have become accustomed to this. This is primarily because computer
graphics is a relatively new industry that relies on computers that, until recently,
wereveryslow.Thecorrectwaytomanipulateimagery—inwhateverway—is be-
fore the gamma curve has been applied to an image. The correct way is to take a
linear image, color correct it, composite it, transform it, and then apply a reverse
gamma curve to the image to view it correctly (as the monitor is applying gamma
correction as well and negating the correction you just applied). Luckily, this is
how Nuke works by default.
Still confused? Here's a recap: Nuke creates very accurate representations of color and
canstorecolorsthatarebrighterthanwhiteanddarkerthanblack.Italsocalculatesallthe
compositing operations in linear color space, resulting in more realistic and more math-
ematically correct results.
Note
Nuke color values are displayed and are calculated in what's called “normal-
ized values.” This means that instead of defining black at a value of 0 and
white at a value of 255, black is still 0, but white is 1. It's a very easy thing to
remember that makes understanding the math easier.
Nuke has many color correction nodes, but they are all built out of basic mathematical
building blocks, which are the same in every software application. The next section looks
at those building blocks.