AIM: Image-Gamma Induced Errors: Processing Space Variation

Accurate Image Manipulation for Desktop Publishing

Gamma Errors

Image-Gamma Induced Errors

Gamma Errors as the Function of Processing Gamma Space

The demonstration below shows the gamma induced errors for UnSharpMask in various processing gamma spaces. The original in 16-bit mode is linear 12-bit acquire using Canon eos*dcs 3 digital camera into Photoshop 15-bit space and then cropped.

The filter that is used in this demonstration is UnSharpMask at Amount=400, Radius=0.5 and Threshold=2. Images were processed as follows:

Editing
space
Step 1: convert to editing by applying inverse gamma Step 2: convert to Step 3 Step 4: convert to viewing gamma space (2.2) by applying inverse gamma

1.0 none 8-bit/color Apply USM 2.2

1.2 1.2 8-bit/color Apply USM 1.83

1.4 1.4 8-bit/color Apply USM 1.57

1.6 1.6 8-bit/color Apply USM 1.37

1.8 1.8 8-bit/color Apply USM 1.22

2.0
2.0
8-bit/color Apply USM 1.1

2.2 2.2 8-bit/color Apply USM none

In step 5 the images were saved as Jpeg at quality factor 4.

The overall gamma is gamma in step 1 multiplied with gamma in step 4, that is inverse gamma 2.2 for all the images.

Notice how your vision (as the person who is editing the image) adapts to the quality when you go slowly from linear editing down to gamma space 2.2 editing.

Then toggle between linear editing and some of the gamma space processed images. This is the difference in the first impression, when the image (one of them) is presented to a viewer for the first time. Pay attention e.g. to the alien appearance of the eye area and to the white gauze around numbers in gamma space processed images.

Please wait until the page is fully downloaded, the options buttons do work in a snap, but only after the download is complete.

Jpeg copy of the original.

Linear editing.
Gamma-space 1.2 editing.
Gamma-space 1.4 editing.
Gamma-space 1.6 editing.
Gamma-space 1.8 editing.
Gamma-space 2.0 editing.
Gamma-space 2.2 editing.

This experiment seems to show that a moderate processing gamma space like 1.2 to 1.3 could be a rational choice but it is not so because the gamma errors do cumulate, operation by operation. After a couple of operations the gamma errors are rather large when processing in gamma 1.2 space.

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Accurate Image Manipulation for Desktop Publishing