Re: Gamma
Re: Gamma
- Subject: Re: Gamma
- From: Andrew Rodney <email@hidden>
- Date: Wed, 25 Jul 2007 11:54:03 -0600
- Thread-topic: Gamma
On 7/25/07 11:16 AM, "Marco Ugolini" wrote:
> gamma
>
> The way brightness is distributed across the intensity spectrum by a monitor,
> printer or scanner.
My copy and paste (from Color Management for Photographers"):
Tone Response Curves and Gamma
The term gamma is used throughout imaging and color management. The use of
the term often describes the relationship between input values and output
values, but it¹s a bit more complicated than that. The correct term to use
is Tone Response Curve. In a perfect world, all devices would be linear. If
you double the input value, the output value doubles. Most devices are not
linear. Instead we require a complex curve to describe the relationship
between input and output. We call this a Tone Response Curve (TRC). The TRC
describes the relationship of the input, such as a digital value, voltage,
or light energy, to output. An example of a TRC is the response of a CRT
display. This TRC describes input amplitude (voltage) and the corresponding
light output (brightness). For example, a scanner¹s TRC would be the curve
that describes the relationship between the light energy that strikes the
sensor and the resulting digital value. Any TRC can be plotted on a grid;
this curve describes the entire range of the device. Imagine a device where
the input value and output value are indeed linear. An input value of 1
produces an output value of 1, 2 produces 2, and so on. If you plotted this
on a graph, you would see a straight line, hence the term linear. Since most
devices are not linear, we instead get a curve on this grid. Curves should
be familiar to anyone who has used Photoshop; it¹s got a curve dialog. This
dialog operates on another type of input/output curve.
Scanners, cameras, printers, and even color spaces have TRC. Tone Response
is simply a group of numbers that describes the relationship between input
and output values over the entire range of the device. How these curves are
plotted is somewhat important. Many people use the term gamma to describe
these curves. Unfortunately the term gamma often is used incorrectly, and my
technical editor, Karl Lang, who happens to be a true color scientist,
insisted I clear this up. Gamma is a letter of the Greek alphabet, and in
color science, the letter or term gamma represents a specific mathematical
formula of which I promise not to even begin to try and comprehend (although
I¹m told it¹s quite simple). Karl insisted I provide the formula:
output = input gamma
Various values for gamma produce different curves. So we can use a single
number to describe this type of curve. However, many devices do not follow
this formula. So technically they have no gamma. They do have a TRC. If we
were to plot the tonal input value of a device in comparison to the tonal
output value produced, and it followed the specific gamma formula, we¹d have
a true gamma curve and that curve would have a specific value (the result of
the gamma calculation). It just so happens that most CRTs behave this way,
so a single number can describe the tonal response of a CRT. The native
gamma of most CRT display is in the neighborhood of 2.0 to 2.5 and thus once
again, not close to linear, which would be a value of 1.0.
The relationship between input and output of devices, such as printers,
scanners, and digital cameras, often does not follow a simple gamma curve.
The values are far more complex and can¹t be described using this simple
formula. What we are talking about here is a concept that should be
described as the device¹s TRC, not as the device¹s gamma. Therefore, all
devices have a TRC but few TRC curves are gamma curves! Note that many
devices (LCD displays, scanners, etc.) map their original TRC to a TRC that
follows the gamma formula. These devices appear to the color management
system as following the gamma formula. When describing the response of a
system, the correct statement would be, ³My scanner has a TRC of gamma 1.8,²
instead of, ³My scanner has a 1.8 gamma.² It seems like silly semantics but
it¹s useful to understand.
Andrew Rodney
http://www.digitaldog.net/
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Colorsync-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden
| References: | |
| | >Re: Gamma (From: Marco Ugolini <email@hidden>) |