Back to Dave Coffin's Home Page

Raw Digital Photo Decoding in Linux

Not affiliated with Canon, Nikon, Kodak, or any other company named herein.

Welcome! If you are wondering how to connect your digital camera and download images to a Linux PC, go to the gPhoto homepage. My software is for processing those images after downloading them.

If you're downloading JPEG files, you don't need my software at all. The image has already been processed inside the camera. Almost all digital cameras made since 1997 produce JPEG images, so why would you want to do it any other way?

Well, despite the convenience and ubiquity of JPEG, there are some disadvantages. JPEG is a lossy format -- to fit a big image into a small file, a lot of information is thrown away. That's why midrange and high-end digital cameras offer an alternative: Raw, unprocessed CCD data files, for which the camera manufacturer provides special decoding software.

Of course this software is for Windows and Macintosh only, with no source code. So it's useless to users of other operating systems, programmers hoping to design a better interpolation algorithm, and historians not yet born in an era when the only Windows machines will be in museums.

So here is my mission: Write and maintain an ANSI C program that decodes any raw image from any digital camera on any computer running any operating system.

That program is called dcraw.c, and it's become a standard tool within and without the Open Source world. It's small (about 6000 lines), portable (standard C libraries only), free (both "gratis" and "libre"), and when used skillfully, produces better quality output than the tools provided by the camera vendor.

Here's my resume. I do freelance consulting related to dcraw, and I'm also available for full-time work in BIOS engineering or UNIX system administration.

I can be reached by sending e-mail to cybercom dot net with the username "dcoffin".

My Code

There are no restrictions on this code, although I would like to receive credit for it. Donations are welcome, especially if you're using my code commercially.

Note to Linux distributors: The only executable files that should be installed by a dcraw package are "dcraw", "clean_crw", and maybe "fuji_green", "fujiturn", and "fujiturn16". My shell scripts are dangerous and should only be installed in a "doc" directory without execute permission.

For hackers only:

Other Raw Photo Decoders

Dcraw has made it far easier for developers to support a wide range of digital cameras in their applications. They can call dcraw from a graphical interface, paste pieces of dcraw.c into their code, or just use dcraw.c as the documentation that camera makers refuse to provide:

Frequently Asked Questions

I don't have a C compiler. Could you send me an executable?
No, but Francisco Montilla provides Mac OS and Windows executables on his website. And Benjamin Lebsanft has volunteered to maintain Windows executables optimized for specific CPUs.

If you're familiar with the DOS command line but don't know C, you can install this free C compiler for Windows and compile dcraw.c quite easily.

How can I read the EXIF data (shutter speed, aperture, etc.)?
Phil Harvey's ExifTool provides a unified Perl-based EXIF reader (and editor!) for all cameras and file formats. "dcraw -i -v" is much faster, but provides less information.

Do you have any specifications describing raw photo formats?
Yes, but they tend to omit important details, like how to decompress the raw image or decrypt private metadata. See the TIFF spec, the TIFF/EP spec, the Adobe DNG spec, the CIFF (CRW) spec, and the X3F spec.

Why is 16-bit output dark / flattened / unreadable?
If you want pretty pictures straight out of dcraw, stay with 8-bit output. 16-bit linear output is the best raw material for professional image editors such as PhotoShop and CinePaint, but it's no good for most image viewers.

Could you please add an option for TIFF / FITS / PNG / BMP / JPEG output?
No. Dcraw writes PPM only. To convert to other formats: 
dcraw -c crw_0001.crw | pnmtotiff -truecolor > crw_0001.tiff
dcraw -c crw_0001.crw | pnmtofits > crw_0001.fits
dcraw -c crw_0001.crw | pnmtopng > crw_0001.png
dcraw -c crw_0001.crw | ppmtobmp > crw_0001.bmp
dcraw -c crw_0001.crw | cjpeg > crw_0001.jpeg
I used the Netpbm toolkit in these examples. ImageMagick also does command-line format conversions. Both are free.

Why don't you implement dcraw as a library?
I have decided that dcraw shall be a command-line program written in C, and that any further abstraction layers must be added around this core, not inside it.

Library code is ugly because it cannot use global variables. Libraries are more difficult to modify, build, install, and test than standalone programs, and so are inappropriate for file formats that change every day.

There's a simpler way to make dcraw modular and thread-safe: Run it as a separate process. Eric Raymond explains this technique here.

What does the "-f" option do? What's "four-color RGB"?
If you see patterns like this in your output images, use the "-f" option. Otherwise, don't.

Why are there false colors along edges within the image?
Because of interpolation. This is a hard problem, easily defined:
  1. Take a three-color RGB image. At each pixel, set two color values to zero.
  2. Reconstruct the original three-color image as best you can from the remaining one color per pixel.
Dcraw currently gives a choice of three methods: Bilinear, Variable Number of Gradients (VNG), and Adaptive Homogeneity-Directed (AHD).

The Foveon X3 Capture chip requires a different kind of interpolation. Unlike CCD arrays, it captures three colors at every pixel location. But the colors are not well separated, so the raw data looks very gray. Much processing is needed to enhance color while suppressing noise.

How do I get my camera to take raw photos?
For some Nikon Coolpix cameras, you need to enable a special "DIAG RAW" mode.
For Casio cameras, see Maurice Delaney's website or read this discussion on dpreview.
For the Minolta DiMAGE G400, G500, or G600, go here (in Russian) or here (in English).
For the Minolta DiMAGE Z2 and Nikon Coolpix 2100/3100/3700, go here.
For SMaL cameras, see the camerahacking Forum.

For other cameras, refer to the User's Manual.

Does dcraw work with my camera?
Most likely, yes. The cameras listed below are supported. If your camera is not on the list, try dcraw anyway. If it doesn't work, post a raw image to a website and e-mail me the URL. If you don't have a website, use Dropload or YouSendIt.

Ideally, your sample image should show a standard white card or color chart in direct sunlight, with other colors in the background.

Patches

These are patches for software that is either no longer maintained, or maintained by authors who don't like my patches.
Super-patch for John Bradley's XV image viewer
Provides a better Smooth24() function, prevents segfaults when saving a color-adjusted image in PostScript format, and adds PNG support, raw photo decoding, non-destructive deletion, and simulated color-blindness.
Byte order patch for the coreutils "od" command
"od" cannot display multi-byte integers or floats correctly if they are stored in the opposite byte order of your CPU. With this patch, "od -I" is little-endian on all CPUs and "od -M" is big-endian everywhere.
Off-by-one patch for the coreutils "tail" command
"tail -n +3" skips the first two lines of input, and "tail -c +2k" skips the first 2047 bytes. This patch adds a "-z" option telling "tail" to skip the exact number of lines or bytes specified.
Acceleration patch for the FlightGear flight simulator
Time acceleration makes long simulated flights less tedious. Without this patch, FlightGear doesn't speed up the clock to match the accelerated aircraft, causing some instruments to malfunction, and allowing small planes to outrace the sun.