Accurate Image Manipulation for Desktop Publishing  
Gamma Errors
 
 

Image-Gamma Induced Errors

UnSharpMask, example 5, demonstrating Bruce Fraser's "sensical edits" or less "violant moves" as he put it on his appraisal at AdobeForums, color-management forum.

Backgound information and Mr. Fraser's full appraisal here.
 

(1) Jpeg copy of the original.
(2) Linear editing, Amount= 375%.
(3) Gamma-space 2.2 editing, Amount= 375%.
 (4) Linear editing, Amount =500%.

The original in 16-bit mode is linear 14-bit acquire using Umax PowerLook III scanner into Photoshop 15-bit space and then cropped.

According to Mr. Fraser's vision the options (3) and (4) : "obtain the same degree of visual sharpening".

Slowly toggle between the options (3) and (4) to get a feeling about how Mr. Fraser's vision is operating.

Naturally the fair way to compare the sharpening effect is first to toggle between the options (1) and (2) then to toggle between the options (1) and (3).
21th Sep 2001. When evaluating using Photoshop 6.0.x as the first step remove (unassign, Image/Mode/Assign... None) the embedded profile from the original. Photoshop 6.0.x behaves way differently (correctly) than the 5.x that was used when the original image was saved.

Linear space processing

  1. Open the original linear higher bit-depth image.
  2. convert to 8-bit mode.
  3. apply USM at amount=375%, radius=0.5, thereshold=0.
  4. apply inverse gamma 2.2.
  5. save as jpeg using quality factor 4.

Gamma-space processing

  1. Open the original linear higher bit-depth image.
  2. apply inverse gamma 2.2.
  3. convert to 8-bit mode.
  4. apply USM at amount=375%, radius=0.5, thereshold=0.
  5. save as jpeg using quality factor 4.

Click here to see this page with the image zoomed by 200%. Images are already in the cache of your browers so this is fast.

New, fully ICC color-managed demonstration with larger image size and best JPEG quality here.

Accurate Image Manipulation for Desktop Publishing 
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