Thursday, 15 March 2012

How dcraw demosaicing skews sharpness, part two

In the first post on this topic I provided some evidence of orientation-depended distortion of sharpness caused by the Bayer demosaicing algorithms found in dcraw. Now we will look at some samples from real photos.

I used MTF Mapper (http://sourceforge.net/projects/mtfmapper/) to produce maps of both Sagittal and Meridional MTF50 values of a Nikkor 35 mm f/1.8 AF-S G lens mounted on a Nikon D7000 body. The actual photo looks like this:

I captured this shot in both raw and JPEG. The .NEF was then processed with dcraw v9.10, with various demosaicing options.
First up, the results for Adaptive Homogeneity-Directed (AHD) interpolation (dcraw -q 3):

This is the image that started this whole thread. While I was developing the MTF surface map rendering option for MTF Mapper, I obviously used synthetic images to see whether MTF Mapper was producing the desired output. Once I was satisfied that the output look good, I tried running MTF Mapper on some .NEF shots developed with dcraw. The strange cross shapes that appeared in both the Meridional and Sagittal MTF maps did not really look like what I was expecting, which got me thinking about possible causes. 

I initially though that it was caused by something in the CMOS sensor pixel geometry. Maybe the pixels were not really square, and some edge angles thus produced biased MTF50 estimates. I have yet to test this, but it seems that the demosaicing algorithms are responsible for the bulk of the distortion.

Next, I modified the slanted edge MTF estimation algorithm used in MTF Mapper to use only pixels from a specific filter colour, e.g., only the red sites. Using the dcraw -D option to extract the raw Bayer image from the .NEF files, I could then directly measure only the red channel MTF without going through a demosaicing algorithm first.
The main disadvantage of this approach is that the red and blue filters cover only 25% of the pixels, meaning that effectively I have only 25% of the usual number of intensity observations around each edge. This has the effect of increasing the uncertainty in MTF50 estimates, and I still want to look at ways of improving performance --- maybe a test chart with larger squares will be the best solution. At any rate, here is the MTF surface of the raw red channel, without any demosaicing:

Notice how the meridional MTF map now appears much more symmetrical. The left side still looks a bit low, but then again, I did not take every possible precaution to ensure that the sensor is perfectly parallel to the test chart. Maybe the lens is even slightly decentered. Time will tell.

The sagittal MTF map also looks much more radially symmetrical. Although it is somewhat skew, it looks as if there is a donut-shaped region of maximum sharpness, something that is certainly possible if the field of this lens is not perfectly flat. The most important feature, though, is that the cross-shaped pattern seems to have disappeared, like we may expect based on the results derived from the synthetic images in the first post.

Next, I extracted the red channel from the in-camera JPEG. There are several important things to take into account with JPEGs, though. The chroma channels are downsampled by a factor two, meaning that MTF values should actually decrease, as this will involve a low-pass filter. Extracting only the red channel of a JPEG involves re-mixing the chroma and luminance channels, and then extracting the red channel from the resulting RGB image. This means that both the blue and green MTF values will be mixed in with the red channnel MTF, so if the red channel is actually softer than the other two, then the JPEG process will appear to increase the (relative) MTF50 values somewhat. Anyhow, here is the image

The most disturbing thing about these MTF maps is the grid of bumps that appear. I cannot explain their presence at the moment, but I will try to get some answers at some point. We can see that the centre of the meridional MTF map seems broader, and the sagittal map appears quite flat (other than the local bumps). There is only a very slight hint of a horizontal and vertical stripe through the centre --- more testing will be required to see if this is just random coincidence, or a real feature. Anyhow, the in-camera JPEG does not appear to suffer from the distortion we saw with the dcraw AHD demosaicing interpolation, so whatever the camera is using, it seems to be of better quality.

In future posts I will add some more plots, specifically for other popular raw convertors.

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