16bit workflow with hugin

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Revision as of 17:16, 9 February 2007 by Bruno (talk | contribs) (typo and clarification)

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This tutorial doesn't cover reasons why you might want to shoot with RAW and stitch in 16bit format. It is a simple HOWTO listing the tools available and how to use them with hugin.

Preparing the 16bit images

Start by using dcraw to read the RAW files and batch convert them into portable 16bit per channel PPM files. RAW pictures from my Nikon 8700 have a .nef extension, so the command looks something like this:

 dcraw -v -w -4 dscn*.nef

Note that although hugin, nona and enblend support HDR floating-point data, there is no real loss converting RAW data to 16bit colour depth as RAW images are typically only 12bit in the first place. If you need a greater dynamic range than found in RAW images, it is possible to combine bracketed exposures in a HDR workflow with hugin.

These PPM files are not readable by hugin or nona, so the next step is to use ImageMagick to create 16bit per channel TIFF files:

 convert -rotate 270 -gamma 2.2 dscn3088.ppm dscn3088.tif
 convert -rotate 270 -gamma 2.2 dscn3089.ppm dscn3089.tif

Note that I rotated them at the same time since these are portrait shots. Gamma correction is also applied at this stage since RAW data is generally linear and difficult to view without colour profile management (not yet supported by hugin).

Though you may find that you get better results leaving the gamma correction until after the final blending with enblend.

You can now delete the intermediate PPM files.

Alternatively UFRaw can be used for the entire conversion from RAW.

If necessary, correct chromatic aberration and vignetting with fulla at this stage.

Stitching with hugin

The TIFF images can be loaded into hugin as per usual except:

EXIF information about the field of view was lost during the PPM stage, so the field of view will need to be re-entered manually, re-optimised with PTOptimizer or transfered from the RAW file using exiftool:

 exiftool -tagsfromfile DSCN3088.nef -overwrite_original_in_place -exif:all DSCN3088.tif

Stitch the images as per usual into a TIFF file, you can use enblend as the final step.

Post processing

This TIFF file is in 16bit per channel RGBA format, which is not viewable in most image viewers or web-browsers, so there is an extra step needed to create portable images:

Open the file in a 16bit aware image editor such as cinepaint or krita, tweak the colours, retouch or apply a contrast mask and finally save in an 8bit per channel format such as JPEG.

Alternatively you can get very good results by simply applying the ICC colour profile for your camera and converting to sRGB colourspace, lcms provides the tifficc tool for doing just this:

 tifficc -i mycamera.icm myproject.tif output.tif

If you are not working with gamma corrected data, then you can apply this now:

 mogrify -gamma 2.2 output.tif