Hi everyone, I am new here and was referred by Sandy McGuffog from the thread I posted over at the luminous-landscape forums. In that thread I describe in great detail my observations about how different demosaicing algorithms render detail and the issues I observe with each.
To summarise: Overall I find AMaZE to give me the best quality, but it is plagued with artifacts that appear like 45-degree diagonal 'ringing', and it was Sandy that turned me on to the term 'diagonal interpolation correction'. I wonder if this is what is causing the artifacts that I see along diagonal high contrast edges when using AMaZE demosaicing.
To copy the relevant bits of my post from there to here: I have since noticed some disturbing artifacts with AMaZE demosaicing too - while it produces the best overall rendition of fine detail that has minimal mazing and zipper artifacts, some weird patterns form along diagonal edges of relatively high contrast. Note that you will not see these artifacts in edges that are vertical or horizontal or almost that. They are most obvious in diagonal edges (see first attached example, red arrows point to where one might see these artifacts). It is a sort of stair-stepped ringing artifact, with lines of varying contrast at exactly 45 degrees. I do not mean the different kind of digital jaggies, which is what happens with DCB demosaicing for example that is more coarse (you get 2x2 pixel blocks of steps rather than 1 pixel steps with AMaZE, which is the finest possible step). It's a single pixel wide line of darker dots, always a perfect 45 degree slant, and these lines often appear next to each other and overlap, similar to the kind of ringing artifacts one might see with Lanczos 8 interpolation. I wonder if the anti-zipper smoothing algorithm of AMaZE is the cause of this kind of artifact, making zipper artifacts - alternating dark pixels at a 45 degree angle - smooth perhaps means joining them up. Another artifact that is observed is a dark single pixel tends to appear in the corners of edges, like on the tip of a leaf. They are peppered throughout the provided example.
When I capture sharpen with Focus Magic, it enhances the stair-stepping to a very high degree, making edges look to have strong ringing artifacts. The black-dots artifact is also enhanced to a large degree. Using the blend-if sliders to tame the dark halo reduces the effect, but does not eliminate it as it is present already. I've noticed that using Topaz Detail's Deblur function, the ringing artifacts are hidden to a very large degree, except on some really problematic edges. I wonder if Focus Magic is creating these ringing artifacts - but a sanity check on a DCB demosaic version shows no such 45 degree diagonal lines. I also noticed that the thin-ness of edges, if there is indeed such a way of describing how crisp an edge is, is much finer than what I can achieve with even radius 1 in Focus Magic. The result is that edges appear to be more crisp, though at lower percentage viewing on monitors, the fatter halos with Focus Magic gives the sensation of a 'sharper' image, but when upsampled ~400% - 800% for printing larger and to serve 600ppi data to my Canon printer, Topaz Detail's Deblur (or Infocus) produces a visible cleaner edge which lends itself to more (better) output sharpening.
I am providing here a few full resolution examples to show what I am referring to, including links to the original raw test image. These are layered tiffs (warning: huge) containing the full resolution renditions, converted with different demosaicing from ACR 9.1.1, Raw Therapee and Capture One V10, and then a copy of each with Focus Magic sharpening applied. The layer names should be self-explanatory. The bottom set of layers, without the 'FM 1 300%' suffix in the layer name, is just converted from raw, with the white balance and exposures matched as best as I can, with zero curves, sharpening, noise reduction and all other adjustments zeroed out. You can try your own editing on the sample raw file if you wish, if you don't trust my layered tiff and wish to come to your own conclusions.
Raw file: https://drive.google.com/open?id=0B5AXKSbQEPuFZHBldXVSSi14eG8
Layered Tiff (1.3GB) This is the above raw image converted. I included the AMaZE rendition sharpened with Topaz Detail's Deblur as a comparison to the Focus Magic Sharpened layer. Toggle the layer visibility for best visual comparison (easier to see than the side-by-side example crops below) Note that I used Focus Magic at 300% amount to enhance the artifacts of the sharpening process, and I NEVER use such a high amount ever for capture sharpening, but the artifacts are still there, just less visible/amplified. That is, the width of the halos, and the amount of ringing, is the same but with less contrast only at lower amount percentages. Pan about at 300% view or even more - there is a fascinating minefield of artifacts to look at. Flip between the unsharpened and no noise reduction originals (bottom layers) and the sharpened layers. It's interesting to see how each demosaicing algorithm renders detail on a real world image, rather than on some synthetic target. Some leaves show significant mazing around them in the blue sky region, some leaves show significant zipper artifacts. Some don't show any. Complex results.
Layered Tiff (2GB) This is a different example, a frame from a Sony A7R II, underexposed and with extremely low contrast and shot at ISO 100 which makes it a good candidate to look at fine grained noise when the data is stretched quite a lot with a curves adjustment layer. AMaZE renders the grain of the image quite harshly compared to ACR, with random extra light and dark pixels peppered about. ACR has perhaps the best noise grain pattern, which is not just softer but the intensity of the salt and pepper noise is leveled out to around the average deviation. It looks like ACR has some noise filtering going on in the demosaicing, despite the noise reduction settings zeroed out. This example has large flat gray regions of almost no detail to allow a study of the rendition of grain patterns by the various demosaicing algorithms, but also some contrasty detail in the bottom corner to look at artifacts on high contrast edges too. Not as good as above example for looking at edge artifacts however.
Here is a crop comparison from the above TIFF, at 300% magnification of an area of interest. The red arrows in the AMaZE crop point to the diagonal artifacts:
For variety, here is another crop example, AMaZE vs DCB, sharpened with Focus Magic (Radius 1, Amount 175%), that show the diagonal line artifacts very clearly.
I would be most grateful if anyone has an explanation for the diagonal line artifacts with AMaZE, and if there is any possible resolution for this, like perhaps applying a less strong diagonal interpolation. Thank you!