DCRAW, a comparison between the VNG and AHD color-mask interpolation

In 2005 the color-mask interpolation algorithm in DCRAW was changed from the VNG to the AHD, both of these algorithms provide very high quality interpolation, however they do have their own characteristics. I demonstrate the most critical issues below.

All images on this page are JPGs saved at Photoshop quality 12 (the best) in order to preserve most of the image detail.

This Canon D60 picture is used in the below comparison, please click it to see in the original 6 MP size. Also the RAW image is available for download.

About the workflow of the comparison image

VNG version was converted using DCRAW version 7.19,
AHD version was converted using DCRAW version 7.84.

I used my fast workflow, in short:

1. Assign the proper ICC profile (Sunlight in this case)
2. Convert to Working RGB space (CIE 1931, D65, Gamma 1.0)
3. Levels correction
4. Density masking
5. Healthy USM (a=500,r=0.5, t=0)

Both the VNG and AHD versions had the very same editing. No noise reduction was performed.

The Comparisons

Please note that the below comparisons are:

• shown at 400% zoom in order to show the differences more clearly.
• rollovers, move the mouse on top of the comparison image to see the AHD version (the version of the image is shown in the Titlebar of the comparisons)

Comparison 1 shows the "zipper effect" of the VNG algorithm, it does this at all hard luminance edges. The AHD conversion is free from the "zipper effect". In addition this comparison shows the difference in the noise characteristics in the dark end of the range, the noise in the VNG version is easier to to reduce than the noise in the AHD version that has some similarities to strong compression artifacts of the JPG.

Comparison 2 shows the "edge fringe" properties (this phenomena/property is very often mis-attributed as the chromatic aberration of the lens). The VNG algorithms is quite fee from "edge fringe". AHD does produce some "edge fringe" and increasingly so when one of the sides of a slanted edge is overexposed. Spatially the noise characteristics are similar as in the dark end of the range but not that strong in luminance difference.

Conclusions

Does these differences matter? At normal (100% zoom) on a typical display (that shows images at about 100dpi) these differences are minute, in some images the zipper effect of the VNG can be faintly seen. Also in some images the "edge fringe" of the AHD version can be faintly seen.

Things become more critical when the images need to be enlarged (either for the screen or for printing). In my experience the VNG zipper effect is the most restricting (and very difficult to reduce by editing), it becomes visible in quite moderate enlargements. AHD allows slightly more enlarging but then the AHD noise becomes disturbingly visible, it of course is much more easier to reduce by editing than the "zipper effect".

I would very much like the to have the VNG performance but totally without the zipper effect. Until then I have chosen to use the AHD, it gives more freedom for post-processing.

More about DCRAW:

DCRAW2PS launcher/linker tool for DCRAW, easly launch DCRAW conversion from within image viewer like the IrfanView and have the converted PSD to open in Photoshop automatically.

DCRAW, general overview.