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What you thought was a perfect image, is less than perfect at second glance.
Most images need to be processed.
When processing an image we are changing the pixel values of the array.
You may move and combined things or adjust the colors. There are allot of things that you can do to an image or part of it.
There is one thing that you must remember.
If you do something to an image and save it.
This type of process maps the values of input pixels to new output values.
The input values are at the top.
The output values are at the right.
The values are transformed from input to output by the diagonal line.
Look at the black arrows and the black line.
An input value of "0" has an output value of "0" There is no change in the output.
The violet arrows on the input are lower or darker than the output.
The output is brighter. The blue arrows show a darker output.
Contrast is changed by tilting the transform line more or less.
Looking at the blue arrows the input covers the entire range.
But the output only covers a small range in the center.
The output is compressed.
This is increasing contrast.
The opposite is true for the arrows lines.
A small area in the center of the input is stretched to cover the entire output
This is decreasing contrast.
Gamma is a non-linear mapping of the input to the output.
The transform line is curved.
Look at the violet line.
As the input increases there is a very rapid increase in output at first.
Higher values of input can change allot with little change in output.
This is increasing the Gamma.
Look at the violet line.
It is exactly the opposite.
At first large changes in the input cause small changes in the output.
Then small changes in the input cause large changes in the output.
This is decreasing Gamma.
C G Y R M B C
The color chart is helpful when changing color.
Across the top are the primary colors of light (Red Green Blue) and paint (Cyan Yellow Magenta).
Down the side to the middle is the color saturation.
From the middle to the bottom black is mixed in.
In many programs there are controls just like Brightness, Contrast and Gamma for each of the
RGB colors.
You can basically make transformation lines for R, G or B separately.
Saturation changes the amount of color. Hue adds more Red or Blue while green is unchanged.
For more information please see Digital Images
Looking at the above graphs we can see that when we adjust them we are loosing data.
If you look at the violet line in Brightness,
notice that the input values near 255 are all forced to 255 on the output.
This is called saturation.
If you then darken the image all of those pixels will have the same value.
Contrast works the same way.
Increasing the contrast decreases the number of values.
It's similar to changing a 24bit image to an 8 bit image.
Continually changing the brightness, contrast or gamma of an image
can seriously degrade an image.
TIP -- Always process a 24 bit image,
unless there is a reason to use a lesser image.
TIP -- Always keep The original image.
Save partially processed images under different names. They are easily deleted later.
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