MS RFC 21: MapServer Raster Color Correction

Date:2006/10/13
Author:Frank Warmerdam
Contact:warmerdam at pobox.com
Status:adopted
Version:MapServer 5.0

Overview

To add support to MapServer to apply color correction curves (aka lookup tables) to raster images on the fly.

Technical Details

MapServer’s raster rendering engine (mapgdaldraw.c) will be extended to support a PROCESSING option indicating a color correction file to be applied on the fly when raster data is being read from disk. The PROCESSING option would take the form:

PROCESSING "LUT=<lut_specification>"

for a single LUT applied to all bands equally or:

PROCESSING "LUT_<color#>=<lut_specification>"

for a LUT applied only to one band. The <lut_specification> can be the name of a file containing a LUT, or an inline lut definition. The inline lut definition looks like:

<lut_specification>=<in_value>:<out_value>,<in_value>:<out_value>[,<in_value>:<out_value>]*

The text file takes the same form, except that it may be multiline (lines will be implicitly joined by commas). So some common forms would be:

PROCESSING "LUT=0:0,128:150,255:255"

PROCESSING "LUT_2=green_lut.txt"

Note that the LUT specification files will be searched for relative to the map file if a relative path is provided.

The LUT will be applied in LoadGDALImage(), after any scaling to 8bit. So currently only 8bit (0-255) input and output values are supported. Optionally we could consider non-8bit inputs, and allow the LUT to apply scaling to 8bit but this would be somewhat complicated to do in a computationally efficient manner.

A 256 entry LUT will be created by linear interpolation between the LUT control points. Note this is different than a true “curve” based approach where some form of curve fitting is done to the control points. For a significant number of control points the difference will be very slight, but it could be noticable for LUTs with only a few control points (ie. 3). Optionally we could compute a proper curve fit, but this will require extra research and development.

If no control points exist for input values 0 and 255 a mapping of “0:0” and “255:255” will be assumed.

Other Curve Formats

The GIMP’s ascii curve format will also be supported directly. Exact details of this are to be determined, and since a GIMP curve file contains several curves (for different bands) it may be necessary to select which is to be used in the PROCESSING statement. Perhaps something like:

PROCESSING "LUT_2=GIMP_red:gimp.crv"

Mapfile Implications

All new options are selected via new PROCESSING options. There is no change in the mapfile syntax. There should be no compatibility problems with old mapfiles.

MapScript Implications

There are no additions or changes to the MapScript API. The new options are controlled via PROCESSING information on the layers which I believe is already manipulatable from MapScript.

Documentation Implications

The new processing options will need to be documented in the Raster Access HOWTO (and possibly the mapfile reference).

Test Plan

New test cases for each mode will be incorporated in msautotest/gdal.

Staffing / Timeline

The new feature will be implemented by Frank Warmerdam and completed by November 30th, 2006, for inclusion in the MapServer 5.0 release. Funding provided by the Information Center of the Ministry of Agriculture and Forestry (Finland).