De-vignetting

Overview

This page describes my software (written in C, Perl and bash script) which allows the removal of the vignetting from raw photos made with wide angle lenses (such as Sigma 10-20mm lens), with an optional neutral density filter (like B+W ND110). In the latter case, the variable color tint is also removed. The software can process multiple images (with or without filter) into one corrected HDR image.

Highlights:

It works only under Linux or Cygwin (under Windows).
To install and use it, you need to have some knowledge of Linux.
The included calibration files are only for the Sigma 10-20mm lens with the optional B+W ND110 neutral filter.
ND110 calibration was done only for one zoom setting of the Sigma lens - f=10mm.
You can do your own calibration - for any lens + (optionally) any filter. I include the calibration software.
Global ND110 tint was calibrated with my camera Canon 1000D; other cameras will have slightly different settings which can be easily derived using my software.
The output file(s) are in 16 bit (per color; 48 bit in total) lossless TIFF format, with the proper ICC profile (linear sRGB) attached to the file, and all the Exif information copied from the original image(s). Modern image processing software (Gimp, Paintshop, Photoshop, ACDSee etc) properly handle such files.
It doesn't work with manual lenses (because it relies on the zoom and F-number values stored as Exif data in the image.)
Now it has a new feature - chromatic aberration correction for calibrated lenses, using software package Hugin. I also include a script for calibrating other lenses. Soon I will add calibration files for my other lenses (Canon EF 50mm F1.8, Canon EFS 18-55mm, Sigma 55-200mm).
The package also contains useful general purpose scripts, e.g. hot.sh (to find your camera's hot pixels from a few dark shots), raw_convert.sh (uses the hot pixels' list to do clean RAW->TIFF or JPG batch conversion), tint.sh (matches the tint of the second image with that of the first one - useful for simple filter tint correction when no calibration was done).
I also included my other, unrelated program - bokeh calculator. (On that page you can directly download the Windows/Linux binaries - you don't need to install the whole package fo that.)

The software is provided as is; under no circumstances the author shall be found responsible for any damage resulting from the use of this software.

Examples

If you are a photo-hobbyist, you probably love wide-angle long exposure pictures like these:

These photos were made with the neutral density filter ND110, usually with wide-angle lenses. Of course, you immediately see the problems with these photos:

Very strong vignetting (becomes darker near the edges) - even more so in the original (rectangular) photos, so people often have to make them square (reducing the wide-angle wow factor), and
Strong color tint (different at the centre from that at the edges) - so often photographers have no choice but turning them into black and white ones.

Fortunately, all these defects can be fixed via software. Let's consider one example. I took a few long-exposure daytime photos of Galipeault bridge in Montreal with the Sigma 10-20mm lens and ND110 filter. Here are the photos made without the filter (left) and with the filter (middle):

Even without the filter the Sigma's vignetting at f=10mm is rather bad; with the filter it's much worse, and you get this terrible purplish tint. It's even hard to tell this is a color photo! Applying an auto white balance (right photo above) does not do any good, as the color tint is unnatural and cannot be fixed by changing the color temperature.

I decided to write software which would allow me to calibrate and fix these problems. As the first step, the software removes the global color tint (left photo):

As you can see, the tint is gone, but the strong vignetting is still there. (Tech info: I remove the tint at the RAW->TIFF conversion stage, before demosaicing; this ensures minimum color artifacts.) Next, I apply my de-vignetting code which was calibrated with this lens and the filter, at the specific values of the zoom (10mm) and aperture (the middle image above; de-vignetting is done separately for red, green, and blue colors). Now the photo looks the way it should - no vignetting, no tint.

Unfortunately, this is not a good result yet. The photo I obtained is much noisier near the edges than at the center. (You can see this effect amplified in the right photo above, where I dramatically increased the color saturation of the middle photo.) It is easy to understand why: in the original photo, the edges were much (almost 8 times) darker than the center, so it is not possible to find an exposure which would work well for both center and the edges: either the center will be well exposed, but the edges will be noisy (like in my example), or the center will be over-exposed, with well exposed edges.

To address this issue, one has to resort to HDR (high dynamic range) photography. To do that, you have to make a few shots in a row (with the filter on) with exposures both lower and higher than the normal exposure. (Camera has to be rigidly fixed using a tripod.) My software can process such a sequence of photos into one tint and vignetting corrected photo. Here's the tonemapped result (with normal and increased saturation):

(Click on the left image to see the full resolution version of it.) As you can see, the noise at the edges is practically gone. (It would be even better if I used more exposures; in the above example I used 4 photos with 1, 6, 19, and 160 seconds exposures.) (Tech info: in my software, I first remove the global tint, then merge all the photos into one HDR photo, and only then apply de-vignetting separately for the three colors. This produces an HDR photo with the least amount of noise.)

Even if you don't use dense ND filters (like ND110), you may still find the amount of vignetting produced by wide-angle lenses (like Sigma 10-20mm) unacceptable. This is especially relevant for low contrast scenes (like snowy plane). Let's condider the second example:

On the left, you see the original image, made with the Sigma 10-20mm lens with f=10mm and the f/18 aperture. (For such small aperture, vignetting should be the least noticeable; for f/4 it would be even more pronounced.) When I used the "auto-contrast" feature of my image editor (the middle image), vignetting became very dramatic, resulting in a non-acceptable quality of the image.

On the right, I show the image processed with my software (the command was devign.sh 0 0 image.CR2), with the auto-contrast applied. You can see that vignetting is gone, resulting in a nice quality image.

Software

My software works only under Linux, or under Windows with Cygwin. If you don't know anything about Linux and/or Cygwin, you probably shouldn't try to use it. In either case, you will need the following packages before installing my software:

If you are using Windows, and don't have Cygwin yet, install it from www.cygwin.com. In addition to the default packages, make sure the following ones are installed as well:
- gcc4-core, gcc4-g++, gcc4-fortran;
- pkg-config;
- wget;
- automake;
- make;
- netpbm, libnetpbm, libnetpbm-devel;
- ImageMagick, libMagick, libMagick-devel;
- libtiff, libtiff-devel;
ExifTools. Available in Linux distros, but can be easily installed under Cygwin by downloading the "Image" version of ExifTools, and then executing
```
tar -xvzf Image-ExifTool-8.03.tar.gz
cd Image-ExifTool-8.03
perl Makefile.PL
make install
```
Qtpfsgui. Needed for the HDR feature. Binaries are available both for Linux and Windows. (Install the Windows binary version if using Cygwin under Windows.)
Custom version of Pfstools (see below).
Hugin - required for chromatic aberration (calibration and removal) scripts, ca_calibrate.sh and ca_remove.sh. Binaries are available for both Linux and WIndows. (Install the Windows binary version if using Cygwin under Windows.)

If you have all of the above, then you can download the archive with my software,

vignette.tgz

and unpack it:

tar xvzf vignette.tgz

You will also need to download the source files for Pfstools. After you untar it in your home directory, copy my modified version of the program pfsgamma to the pfstools source directory, and then compile the tools:

\cp ~/Vignette/pfsgamma.cpp ~/pfstools-1.8.1/src/filter/
./configure --disable-static --disable-imagemagick  --disable-openexr --disable-qt --disable-jpeghdr  --disable-octave  --disable-opengl --disable-matlab --disable-gda
make install

(Under Linux, if you cannot compile with the default, gcc4 compilers, install the older, gcc3/g++3/g77 compilers, and add the corresponding CC, CXX, F77 switches at the end of the configure command; e.g., on Fedora 11, install compat-gcc34\* packages, and add "CXX=g++34 CC=gcc34 F77=g77".)

Then you can compile and install my software:

cd ~/Vignette
./install.sh

It is very advisable to edit the initially empty file deadpixels.txt, by putting there a list of your camera's hot pixels. This step will make your photos much cleaner, and will be helpful for the photo processing. You should put one pixel per line, the format is "x y 0", where x and y are the pixel coordinates. You can use my script hot.sh (included) to automatically generate the deadpixels.txt file. Just take two-three long (1-2 minutes with ISO 100; no dark frame subtraction in the camera) dark shots, and then run the script as

hot.sh Nsigma raw_dark_image1 raw_dark_image2 [ raw_dark_image3 ... ]

where Nsigma is the number of standard deviations. (Should be larger than 6; I use 10.)

The main script is

devign.sh

- run it without arguments to see all the options. Here are the typical examples:

To do devignetting for three photos IMG_001.CR2, IMG_002.CR2 and IMG_003.CR2 made without ND110 filter, execute
```
devign.sh 0 0 IMG_001.CR2 IMG_002.CR2 IM_003.CR2
```
This will produce corrected images IMG_001c.tif, IMG_002c.tif and IMG_003c.tif (16bit color; linear sRGB).
To merge these three photos into one HDR photo (again, made without ND110), do
```
devign.sh 0 2 IMG_001.CR2 IMG_002.CR2 IM_003.CR2
```
The result will be in IMG_003c.tif.
To correct IMG_001.CR2 and IMG_002.CR2 made with the ND110 filter under daylight conditions, do
```
devign.sh 1 0 IMG_001.CR2 IMG_002.CR2
```
To merge three images made with ND110 (IMG_001.CR2, IMG_002.CR2, IM_003.CR2) into one HDR image, using an identical photo made without the filter (IMG_004.CR2) for global tint calibration, do
```
devign.sh IMG_004.CR2 2 IMG_001.CR2 IMG_002.CR2 IM_003.CR2
```
In the above example, make sure that both IMG_004.CR2 and IMG_001.CR2 are well exposed (not over or under-exposed). The script will print the tint correction coefficients (RG and RB), which can be inserted into the devign.sh script - either as a new value of the "Mode", or by replacing my "Daylight" etc. values. (This will probably be required if your camera is not Canon 1000D.)

Now I include the chromatic aberration removal script, ca_remove.sh, with the calibration files for Sigma 10-20mm f4.0 lens. (More lenses will be added later, or you can do your own calibration, using the ca_calibrate.sh script - see below.) It can be used either with raw images (directly from your camera), or with 16-bit TIFF files with Exif information (e.g., output of my other scripts - devign.sh, raw_convert.sh etc.) Examples:

ca_remove.sh *.CR2
ca_remove.sh image1.CR2 image2.tif image3.TIFF

The output images are linear sRGB 16-bit tiff files, with the letter "a" added at the end of the image name.

Calibration

Chances are that you will want to fix (tinted) vignetting produced by other lenses plus filters. You can use my software for that, but first you'll need to calibrate the lens (+ the optional filter). Here are the steps:

The most crucial step is to arrange for a large evenly illuminated surface (like wall). Get with your camera on a tripod to the surface as close as you can without casting a shadow within your frame. It doesn't have to be perfectly white/grey, but you should avoid strong colors.
Prepare your camera: choose the lowest ISO (usually 100); set focus to manual (to infinity); enable long exposure noise suppression (which means dark frame subtraction); set to manual aperture, exposure, and white balance (doesn't matter which WB setting); choose raw image files (not JPEG) as the output.
For every aperture, always set proper exposure: there should be no highly exposed areas (in the top 90% of the brightness scale) in any color, but it shouldn't be very dark either. You don't have to be precise with exposures.
Start making exposures - without and (optionally) with the filter - for each F number your camera+lens have (say, 4.0, 4.5, 5.0, ..., 22.0). When you do filter calibration, always do the matching "without the filter" / "with the filter" shots one after another, to minimize the chance that your illuminated surface color changes between the shots. (Yes, unfortunately it means that you'll be doing a lot of screwing / unscrewing of the filter :^( ).
If it's a zoom lens, you'll have to repeat that for at least a few different zooms, including the two extreme ones.
For shots with dense filters (like ND110), you'll need a remote shutter release, as exposures can become very long (many minutes).
For ND110, the exposures should be roughly 400 times longer with the filter. For other filters, you will have to determine this factor yourself.
Once the flat-field images are done, copy them to your Linux/Cygwin computer with my software installed, and run my script "calibrate.sh", with the full list of image files being the argument. E.g., do
```
calibrate.sh *.CR2
```
to process all the CR2 files in the given directory. The script is smart enough to sort out photos made with and the without filter (but only if you use only one filter - don't mix shots made with different filters - the software has no way of detecting that!), and with different lenses (the order of photos is not important). All the "no-filter / filter" pairs of photos will be processed together to make full calibration files; if there is no pair, it is assumed to be a "no-filter" image. At the end, the calibration files will be copied into the standard location, used by devign.sh. You can test everything by running
```
devign.sh 0 0 *.CR2
```
- it should generate no warning about mismatched F number or focal distance, and produce nice flat (de-vignetted) tif files.
If you calibrate a different (not Sigma 10-20mm) lens and/or different (not ND110) filter for all zoom/aperture values (or calibrate Sigma 10-20mm + ND110 for f>10mm), please share the calibration files with me, so I can include them into my software archive on this page. Just execute
```
cd ~/Vignette/DAT
tar cvzf archive.tgz Your_lens_model
```
(replace Your_lens_model with the actual name of the directory), and send the file archive.tgz to me (you can find my email at the bottom of this page). This will be really appreciated!

Now I also include the chromatic aberration calibration script, ca_calibrate.sh, which is using the program tca_correct from the Hugin package. You can use it to calibrate chromatic aberration corrections for other lenses. Here is the procedure:

Find a large flat surface consisting of very sharp black and white features. (I used a gift wrap paper from a dollar store, attached to a wall.) Put you camera as far as you can from the surface (but it has to fill the whole frame), on a tripod, and use manual focus to make it as sharp as possible, especially at the edges, where the chromatic aberration is strongest.
Use the lowest ISO setting, raw image format, and mirror lockup feature (if available) for the sharpest images possible.
Start making well exposed photos for every value of the aperture. If it is a zoom lens, you have to repeat it for a few different zoom values, spanning the whole range.
Copy the images into your Linux/Cygwin home directory, and run the calibration script, e.g. do ca_calibrate.sh *.CR2 to process all the CR2 images in the current directory.
The tca_correct settings I put in the script are for the best possible quality, but super-slow (30min per image - so it can take days to process all your images). If it is unacceptable, and you are okay with a lower quality, change the arguments to the -g and -n settings in ca_calibrate.sh to lower values (I use 50 for both).
The calibration files will be automatically placed in the standard location, ~/Vignette/DAT1, and are immediately available for use with ca_remove.sh.

Algorithm

Here is a short description of my algorithm.

At the calibration stage, flat-field image is analysed by my C program vignette. It averages the pixel brigntess in 250 concentric rings centered at the image center. I average the three colors (R, G, B) separately within each ring. This is done in the linear space (where pixel value is proportional to the object brightness). The program writes out a file, vignette.dat, which containts a table of the ring radii and the corresponding average R, G, B pixel values.

Next, my program vign_fit reads the file vignette.dat, converts all the pixel brightness values to natural logarithms, and then fits polynoms of the seventh power of ring radius separately to the logarithms of R, G, and B brightness (using the least squares approach), and also for the average of the three colors (this will be used for "grey" de-vignetting - for the shots made without the filter). The computed polynomial coefficients are written to the file vign_params.dat, which is then copied to the standard location for the calibration files (~/Vignette/DAT/lens_model/focal_distance/F_number).

When you run devign.sh, the script fixes the global color tint, then finds the appropriate calibration file, and my C code vign_fix uses the polynomial coefficients to fix the vignetting (by dividing each image pixel value by the polynom value for this paricular radius; it is done separately for R, G, B colors if the Filter value is larger than zero).

Last modified: Fri Feb 19 16:32:06 EST 2010