Adobe Photoshop v.6.0.1 Slope Limiting

A practical example showing one part/aspect of the slope-limiting in Adobe Photoshop v 6.0.1.

Three copies of the original image (in 16-bit/c) (open it by preserving the embedded ICC profile, do not allow to Photoshop to convert the the colors) were converted to nativePC publishing space using Photoshop 6.0.1 Convert To Profile command with relative colorimetry (the blackpoint compensation selection does not not affect to this conversion). The following three ICC conversion engines were used:

1. ColorSync ICC engine on Mac version of Photoshop 6.0.1 on the first copy.
2. Microsoft ICM engine on PC version of Photoshop 6.0.1 on the second copy.
3. AdobeACE engine on PC version of Photoshop 6.0.1 on the third copy.

Then each of the copies were:

• up-sampled to 400% using nearest neighbor (to show the damage) ,
• converted to 8-bit/c and
• finally saved as JPEG at quality 12.

As always, it is far better have first hand experience so please download the 16-bit/color original in a 82kB ZIP file and experiment by yourself.

Conclusions

The slope-limiting in Photoshop has major affect on the image quality. Note that in this example only the slope-limiting in the ICC profile conversion was demonstrated, there is also slope-limiting in the on-the fly monitor conversion (and heaven only knows where else).

For color-space conversions in the RGB mode the Microsoft ICM engine on PC gives similar than the ColorSync engine on Mac, far better result than the AdobeACE

(it is not completely free from slope limiting either)

Note that there is compatibility issue between the Microsoft ICM and Adobe Photoshop in conversion of grayscale images.

Why Adobe ? Why? What good comes out of these slope-limiting "features" here and there?

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