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I have an RGB GeoTIFF file and I want to convert a specific RGB colour to transparent. I would like to use GDAL for doing this, but external Python scripts are also an option (though I haven't learned the art of Python scripting yet).

My thoughts are:

  1. Add a 4th band to my GeoTIFF file and fill with 255.
  2. Insert 0 into the 4th band where RGB colour == target colour

The first part I think I can solve using

gdalwarp -dstalpha source.tif target.tif

The second though I don't know how to solve. matt wilkie suggest using a Python script he wrote, but I think that is done separately for each band and not for a specific colour.

How can I assign a value to the 4th band of my TIFF when the RGB colour is equal to a specific RGB colour?

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Which colour are you wanting to make transparent? Also, colour can be pretty arbitary because it is based on a set of numbers between 0 - 255 on 3 scales. You'd have to specify the exact range of colour you wanted to make transparent. –  Loxodrome May 9 '12 at 9:26
    
I want to make a specific colour - not a range of colours - transparent. A range would also be nice to have, but at the moment a specific colour will do. I specified that I am working with an RGB image, and it might be me simplifying stuff, but I assume that a colour is an RGB colour, thus a combination of values in all 3 bands. I apologise for any unclear asumptions :) –  Chau May 9 '12 at 13:13

2 Answers 2

Herein lies a misunderstanding: "...that is done separately for each band and not for a specific colour."

"Each band" and "specific colour" are in fact the same thing. That is, it is the values in each band that, when combined together, make a specific colour. For example the RGB triplet 255,0,0 is the specific colour of pure red, comprised of band-red at the top of the range, 255, and band-green at the bottom, 0, ditto for band-blue.

Here's a simple gdal python script to change the nodata values for a specific image with 3 channels, a.k.a RGB:

from osgeo import gdal

input = 'd:/gisdata/test.tif'
dataset = gdal.Open(input, gdal.GA_Update)  # open the raster for writing

# 1 corresponds to Red channel, 2 for Green, 3 for Blue
R = dataset.GetRasterBand(1)
G = dataset.GetRasterBand(2)
B = dataset.GetRasterBand(3)

print 'Initial nodata values (RGB):\t', R.GetNoDataValue(), G.GetNoDataValue(), B.GetNoDataValue()

# set null value to the background page colour used on GIS.se, change as desired
R.SetNoDataValue(251)
G.SetNoDataValue(250)
B.SetNoDataValue(247)

print 'New nodata values (RGB):\t', R.GetNoDataValue(), G.GetNoDataValue(), B.GetNoDataValue()

Output:

D:\>python setnull.py
Initial nodata values (RGB):  None None None
New nodata values (RGB):      251.0 250.0 247.0

A more generalized script that can handle arbitrary numbers of bands and used as a commandline utility can be found my code swamp, as gdalsetnull.py

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This won't work for GeoTiffs, at least persistently. GeoTiffs can't have different Nodata values for different bands. If you close the dataset and re-open it, all the no data values will be 247. You may need to wrap it with a vrt to get it to work. –  kyle Jan 27 at 19:50
    
That's interesting @kyle. What program are you closing and re-opening with where persistence doesn't work? –  matt wilkie Jan 27 at 20:21
    
It's a shortcoming (or not) of the geotiff format/driver, it looks like you submitted the ticket: trac.osgeo.org/gdal/ticket/2083 –  kyle Jan 27 at 23:15
    
@kyle hah! that's funny, and embarrassing. I guess that means I'm earning my grey hairs, don't remember some of my own work. :) –  matt wilkie Jan 28 at 18:49

here's a better answer, use gdalbuildvrt with either srcnodata or vrtnodata flag:

gdalbuildvrt -srcnodata "123 231 67" outfile.vrt input.tif

If the next application in line doesn't understand .vrt, translate to a new tif:

gdal_translate outfile.vrt final.tif
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