[gdal-dev] Giventiff with colortable, how do i assign one of the colors to be an alpha channel

[Even Rouault]

Le mercredi 16 janvier 2013 20:58:51, Isaac Gerg a écrit :
> Even,
> Thanks for your quick response.
> 
> I believe I understand that three options you presented me.  I believe
> option 1 is viable but like you mentioned will only work with GDAL
> software.  Options 2 and 3 are good but, especially with option 3, the
> resulting tifs are too large.
> 
> I have 20 .tif  which are 3MB a piece.  Together, they form trackline of a
> sensor data.  I would like to load this information into a WMS server for
> display in Google Earth or Arc Explorer.  Currently I am using Geoserver.
> 
> Anyways, I must mosaic these 20 .tifs together.  These tifs are indexed
> images with a colormap.  Here is my current algorithm:
> 1. Expand images to RGBA using gdal_translate
> 2. Rotate images north up using gdal_warp
> 3. Mosiac images using gdal_merge with -co COMPRESS=LZW -co PREDICTOR=1 -ot
> Byte -co TILED=YES -n 0
> At this point, I now have an RGBA image that is huge (300MB).  With all the
> tile collections I must process, 300MB is too big.  I was thinking the
> total would be around (20 tiles)*(3MB per tile) = 60MB.  300 is simply
> unacceptable.
> 4. Run the gdal_retile to make image pyramid.  This will yeild a size that
> is roughly twice that of my mosiac.
> 
> Is there any way I can make the RGBA file in step 3 smaller in size
> specifically to match the theoretical size of 60MB?

You can try rgb2pct.py on the mosaic

Or you can also try to gdal_translate -co COMPRESS=LZW on the mosaic, since 
I'm not sure how efficient the compression done by gdal_merge will be.