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Image Compression


How many colors can your monitor show, and how many colors and details can your eyes distinguish? If you remove the colors and reduce the resolution of details that you can't distinguish, there's no visual loss of image quality, and that will improve the efficiency of compressing the image.
So, what can we see well and what not so well? For that we need to study the eyes and brains.

Information about the eye:
Dynamic range (light to dark)
Normal contrast ratio is about 100:1 (about 6 1/2 stops)
Maximum contrast ratio is about 1,000,000:1 (about 20 stops)
Acuity (sharpness, resolution)
Maximum is about 0.73 millimeters at a distance of one meter.
Optimal sharpness is only in the center, where you're looking at.
Only one distance at a time is in optimal focus (sharp). Other distances are more or less blurred (unsharp). More light = smaller pupil => greater "depth of field"
Rods and Cones
Rods and cones are both photosensitive, but respond differently to different frequencies of light. Rod cells are highly sensitive to light. However, they do not distinguish between colours, and have low visual acuity (a measure of detail). Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different colours (wavelengths) of light.
The human eye is sensitive to red, green and blue light. If all three forms of cones are stimulated equally, then white is seen. If none are stimulated, black is seen. Most of the time however, the three forms are stimulated to different extents - resulting in different colours being seen.

Cone type Name Range Peak sensitivity
Large R (Red) 500 - 700 nm 580 nm
Medium G (Green) 450 - 630 nm 544 nm
Small B (Blue) 400 - 500 nm 440 nm

This RGB (24 bits) gray spectrum on the left has all possible 256 gray tones in it.

Next are three gray spectrums, one with 128 tones, one with 64 tones, and one with only 32 tones. The 128 shows little loss, the 64 shows some loss, and the 32 is real bad.
By dithering the image, the illusion of more tones, makes the use of less tones possible.

But not all collors show the same loss at only 32 tones. Take a look at the following color gradients. I personaly can not see any loss in the yellow-to-white gradient.

256 tones

128 tones

64 tones

32 tones

Giesbert Nijhuis

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