View Full Version : Anyone using DCRAW?

05-03-2005, 14:13
I am playing with DCRAW. Anyone has a good set of command line options to use for the brightness level etc.? I am getting fairly decent result currently with brightness level of 1.6 and then slap a Photoshop curve and staturation adjustment, but the result is not quite as good as the Epson RAW converter yet.


05-03-2005, 15:27
I've been using it (although I just sprang for the 70 bucks for RAW Developer.) I included it in a thread I started a while back on raw-file conversions for Macintosh computer users (click here. (http://www.rangefinderforum.com/forums/showthread.php?t=5549) )

If I understand the docs on author Dave Coffin's website correctly, there's no way to get dcraw to directly output a monitor-friendly image. dcraw is set up to produce a linear output -- i.e, there's a straight-line relationship between input values and output values. This is considered the best kind of file for post-processing, because there's no tonal compression -- but it means the "as is" file looks much too dark when viewed on your monitor.

To get an image that looks good on a monitor, the data has to be gamma-corrected -- mapped onto a curve somewhat like the "characteristic curve" that film has, in order to compensate for the contrast characteristics of the monitor and of the human eye. The command-line options in dcraw can make the image brighter or darker overall, but they don't alter its gamma -- so, you need to use another application to apply the gamma correction.

Photoshop does a good job of this -- I recorded an Action that would take a whole folder of dcraw output and simply apply the Auto Levels setting to it, and this would get almost any image into the ballpark of good viewability. Then I'd go back to the most promising-looking images and either tweak the dcraw-produced files in Photoshop, or re-convert them with the Epson plug-in, so I could tailor each one to give the best results.

I admit this multi-step process is kind of a nuisance, though, when you've just dumped a whole folder of raw images and simply want a quick look at whether or not you got anything decent. That's what makes the paid software packages (many of which are based on the code in dcraw) worth the extra money -- they can do the conversion and gamma correction in one step, along with saving in a wider variety of file formats than dcraw supports. Some of them also preserve your raw file's EXIF data, which gets stripped off when using dcraw.

05-03-2005, 15:49
As far as I know DCRAW applies gamma correction when producing 24-bits per pixel output. For 48 bits per pixel output it is linear, though.

I use the RawPhoto Gimp plugin (http://ptj.rozeta.com.pl/Soft/RawPhoto) together with DCRAW to process my Canon D30 files. DCRAW and the plugin can do all the hard work with white balance, contrast, white point/black point etc and apply it during the conversion, so that I only need to do resizing and apply some unsharp mask in Gimp to get a result suitable for my web album.


05-03-2005, 19:09
As far as I know DCRAW applies gamma correction when producing 24-bits per pixel output. For 48 bits per pixel output it is linear, though.

You're right -- I forgot that, since I never use the 8-bit-per-pixel option. Here's a quote from Dave Coffin's FAQ:

Why is 16-bit output dark, flattened, or 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.

The default, as it says in the man pages, is to "write eight bits per color value with a 99th-percentile white point and the standard 0.45 gamma curve. Double the height if necessary to correct the aspect ratio." However (unless there's some flag combination I'm missing,) this option writes only in the PPM file format which is popular in Linux environments, but not so accessible on some others. If you're doing your editing using The GIMP, then PPM of course is no problem.

However, if you want to run dcraw using the -3 switch, which causes it to write a Photoshop file instead of a PPM file, then you're constrained to 16 bits per pixel and linear gamma.