Know-how colors (part 5): the flexible gray axis and the gamma trick

In a color depth of 8 bits per channel, exactly 256 gray areas are located on the gray axis, so it always comes with 255 steps from black to woman. In episode 3, they have seen that this limited rate brings the risk of skipping visible color nuances, which makes itself noticeable as a banding.

The banding danger is all the rough, the gross the color space is, so you should use prophotorgb and the still larger lab color space only together with 16 bits color depth. It is really dramatic but only on the gray axis. Since the black and femoral point of an image are set only when playing from the playback medium, no one before, how far away from each other these two points will be last end. Photo paper and good inkjet printing can reach a contrast of 100: 1, which is called: the deepest black reflects about 1/100 of the light, which is reflected by the unprinted paper. The contrasting circumference depends on the black pigments alone, because the paperwear is specified.

Record 12-13 aperture steps

Let’s take this black brightness as a pace, we come in 100 brightness steps from the ink black to the paperweib – no problem for our 8-bit color depth with 255 possible steps. But each forest and meadow monitor today offers a (static) contrast scope of 500: 1 and more, digital projectors even significantly over 1000: 1.

Photographers like to calculate in aperture steps: 100: 1 are just under 7 diaphragm levels, 1000: 1 are 10 diaphragm levels. But also the image of a digital camera wants to be stored with its full contrast scope as JPG in 8 bits color depth – so our claim to the detectionable contrast scope to 12 to 13 lead stages, because that is the state of modern digital cameras. The brightest motive detail may then be more than 8000 times as bright as the darkest, just not in noise’s detailed detail.

Much contrast with little steps

To mit such a high contrast with possible little brightness levels, stabs two problems:

  • With increasing contrast circumference, with each of the allowed 255 steps, a large brightness difference will be changed – the banding danger in brightness ramps is rising.
  • A contrast of more than 8 diaphragm levels (255: 1) can no longer be detected with 255 equal steps.

Especially problem number two you should leave: why can we use 8-bit JPG images, which allow only 255 steps between black and female point, still use for the vast among photos?

The answer is: here is a trick again, this time an ancient, which comes from the start time of the television. He is usually briefly with gamma designated. Virtually all 8-bit and most of 16-bit pictures are "gamma verrated", your RGB values do not follow the physical brightness linear. Our sightiness is also uninfluenced and is less sensitive to gross brightnesses than for small – this supports the gamma trick and ensures that its negative consequences remain invisible.

Know-how colors (Part 5): The flexible gray axis and the gamma trick

With growing contrast circumference, not only schwarz- und fibalpunkt are further away from each other, even the differences between adjacent grayscale will be gross.

Like this post? Please share to your friends:
Leave a Reply

;-) :| :x :twisted: :smile: :shock: :sad: :roll: :razz: :oops: :o :mrgreen: :lol: :idea: :grin: :evil: :cry: :cool: :arrow: :???: :?: :!: