Apr

09

2019

Demosaic for Tintype or Blue-Spectra Photography

Code  Posted by / 3 comments

After reading my post on how to recreate the look of tintype and wet-plate collodion digitally, you may have been left wanting the code to appropriately demosaic/debayer.

This is a photography blog, not a coding blog, so please bear with me for my first go at making this accessible.

To recap from my earlier posts:

In looking at an image captured through a B25 glass, you will only capture 1 out of every 4 pixels correctly, thanks to the debayer color filter array (CFA) on top of your digital camera’s sensor.  That means 3 pixels are “dead” or, at least “incorrect”.  Here’s a quick script to throw them out.

As we can see in this close-up of the raw data, this image has a lot of dead pixels. Let’s get rid of ’em.

I know I will get some feedback that this is not a debayer, or demosaic, in any traditional sense.  You are technically right, haters. This is simply a downsample to strip out the three dead pixels.

You will quarter the resolution of your image with this process.  I’m sorry, but it has to happen.  The resulting clarity will play nicely with any upscaling processes you then wish to engage upon, however, and, especially in portraiture, artificial intelligence can work wonders.

I originally wrote this in MATLAB, but instead let me give it to you with freeware.  

Software Requirements

You will need the following command-line tools. These are free, but if you aren’t comfortable using a command line interface (CLI) then I have bad news for you- this is unfortunately how you’re going to have to do it.

ImageMagick – This is THE tool for altering an image from the command line. It exists for all platforms, so whatever computer you are running, you can use it.

DCRaw – This is THE tool for opening raw files from a digital camera. RawTherapee is a front-end for this tool. Unfortunately, you still will have to install it, even if you have RawTherapee, to use the command-line interface.

I recommend, if you are on a Mac/Linux, simply using homebrew to grab these-  open up a terminal window, and type 

homebrew install imagemagick

and then 

homebrew install dcraw

On PC, this might be a bit more complex, but these tools exist for all platforms, which is why I chose to code in this fashion.

Assuming these are installed correctly, you should be able to run this single line of code to demosaic down to the blue sensor pixel.

The Code

Open a terminal window, navigate to where your RAW file lives, and type:

dcraw -c -D -T -6 -g 2.4 12.92 -o 1 YOURINPUTFILE.(NEF.RAW) | convert - -roll +1+1 -sample 50% YOUROUTPUT.TIF

This tells DCRaw:

(-c) Output in a fashion that can be handed off to ImageMagick (STDOUT for piping)

(-D) Monochromatic, untouched output

(-T) Tiff

(-6) 16 bit

(-g 2.4 12.92) gamma 2.4 with a toe-slope of 12.92, which is, more simply put, the appropriate setting for sRGB.

(-o 1) Set the colorspace to sRGB with a d65 white point.  You can tinker with this, but ultimately it shouldn’t make too big of a difference, since we are compressing to a monochromatic pipeline here regardless.

Note that some tutorials will tell you to use a -4.  This will output a linear electro-optical transfer function, and unless you really know what you’re doing, you probably don’t want this.  I would argue you really don’t want this.

This then pipes ( | )the data to ImageMagick- or rather the utility it provides named “convert”:

(sample 50%) Downsample at 2:1, implied that you are dropping every other pixel in both X and Y directions

(-roll +1+1) Offset your image by the requisite number of pixels to place a blue pixel at the top left of the image.  Assuming RGGB, this is +1+1.  If you have BGGR, then this will be +0+0.  I recommend trying them both.  Heck, you can try +0+1 and +1+0 to see what would typically be the two green pixels.  The one that looks the brightest is your correct image.

To test these I ran:

dcraw -c -D -T -6 -g 2.4 12.92 -o 1 DigitalCollodion_D610-1297copy.nef | convert - -roll +0+0 -sample 50% OutputTest_A.tif
Close-up on the result: This looks dim and, dare I say, stupid. It looks like we’re peering at the data captured by the red pixel of the color filter array, which is the “leakiest” of the three color filters, accepting some blue light.
dcraw -c -D -T -6 -g 2.4 12.92 -o 1 DigitalCollodion_D610-1297copy.nef | convert - -roll +0+1 -sample 50% OutputTest_B.tif
Close-up on the result: Looking better, but still far dimmer than I’d expect. 
dcraw -c -D -T -6 -g 2.4 12.92 -o 1 DigitalCollodion_D610-1297copy.nef | convert - -roll +1+0 -sample 50% OutputTest_C.tif
Close-up on the result: Looks identical to the last one- this means that, as anticipated, we are looking at one of the two G pixels on the color filter array.
dcraw -c -D -T -6 -g 2.4 12.92 -o 1 DigitalCollodion_D610-1297copy.nef | convert - -roll +1+1 -sample 50% OutputTest_D.tif
Closeup of the result: Bingo! This is the blue pixel!

Not surprisingly, OutputTest_D matches the blue debayer pixel for my Nikon.

Note this is for a traditional bayer filter.  You might wish to review the description of color filter arrays, and see if there is a better fit for you and your sensor.  I don’t have the foggiest of how this might be accomplished with an x-Trans color array filter with free command-line tools, unfortunately.