[gdal-dev] Color Columns in Raster Attribute Tables

[Ivan Lucena]

Hi Sam,

In GDAL color table is supported by the GDALColorTable http://gdal.org/classGDALColorTable.html not by GDALRasterAttributeTable http://gdal.org/classGDALRasterAttributeTable.html. 

The GDALColorTable is a little bit limited but it is pretty much consistent across all the drivers that support it. For example, when you say that HFA reports RGBA with values from 0..1 as RAT it probably reports the correct 0..255 as GDALGetColorTable or it doesn't report it at all, because of the GDALColorTable limitation, like data type or number of colors.

But IMHO, there is no reason to expect consistency on the representation of color table from a driver RAT. An RAT could be user defined or generated by some driver or application. 

What you are seem is that some drivers are using, or maybe abusing, the freedom format of RAT to report information than can be understood by some particular application or software (mea culpa on one of those).

Best regards,

Ivan

Date: Mon, 13 Jan 2014 13:06:01 +1300
From: gillingham.sam at gmail.com
To: gdal-dev at lists.osgeo.org
Subject: [gdal-dev] Color Columns in Raster Attribute Tables

Hi List,

There appears to be an 
inconsistency in the way color columns are handled within Raster 
Attribute Tables (RATs) by different drivers. Color columns are currently identified by their 'usage' setting - GFU_Red, GFU_Green, GFU_Blue and GFU_Alpha. The HFA driver presents 
color columns as doubles between 0 and 1 as this is how they are stored 
in the file. The IDRISI driver presents them as integers between 0 and 
255 like the Color Table API. Code that uses color columns from the RAT
 API currently needs to know which driver it is using, defeating one of the aims of GDAL. 


I propose that the RAT API be defined so that color columns 
always appear as Integer 0-255 no matter what type and range they are stored as. 
This would make the RAT API consistent with the Color Table API and would mean a change to the HFA driver

Another
 option is that flags are added to the RAT SetValue, GetValueAs* and 
ValuesIO calls to specify the conversion to/from the native type. This 
seems a bigger change.


Any thoughts?

Sam.


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