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Explanation of some terms needed
- Thread starter Gogu
- Start date
j1clark@ucsd.edu
Veteran
Hello guys.
Can you please tell me what all of the following mean : knee, toe, shoulder.
I know they are used to describe the various portions of a gamma curve but just when I think i've got it I read another post and they are used diferrently.
Help!
Materials that transform light into 'images', do not have a 'linear' relationship for all values of intensity. In very dark situations, photographic materials or CCD sensors will not respond with 'double' for each doubling of the light intensity. At some point, the relationship will become 'linear' and so for a certain range of light intensity, double the intensity, and one will have double the 'response'. At some point too much light will again not result in a doubling response, until finally the amount of light is so great, the sensor will not respond to any further increase in intensity.
The way the light intensity and sensor response is show graphically is called The Curve, and where the 'curve' becomes linear or non-linear ar given 'names.
The Toe would be on the 'dark' end of The Curve, the knee would be where the Toe ends and the linear portion of the resonse curve begins. The Shoulder would be on the high intensity portion of The Curve, and would be where the sensor no longer responds in a linear way with more intensity.
These points are not 'angular' but are 'rounded'...
The traditional name is 'exposure / density' curves, which make sense for 'real'(tm) Film... but for CCD sensors the initial sensor produces an analog which is then digitized to a number between 0-255 (for 8-bit digitization) or 0-1024 (10-bit digitization), or similar. Those digital numbers are then compressed via DV, or other codecs... but the idea is that at somepoint one can reconstruct an approximate graph of the 'exposure' vs. 'density', or 'light in', vs. 'digital value out', to graph the response.
The gama is the slope of the 'linear' portion of the graph. Adjustments to the 'gama' will change the overall contrast of the image.
If someone is using these terms differently, then perhaps you should post specifically the confusing usage.