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? Thanks again, Isaac On Wed, Jan 16, 2013 at 2:21 PM, Even Rouault <even.roua...@mines-paris.org>wrote: > Le mercredi 16 janvier 2013 20:03:14, Isaac Gerg a écrit : > > Hi All, > > > > I have a tiff in which each pixel is a single byte (0-255) which > references > > a colortable. Each element of the colortable has 4 values (RGBA). > > > > I would like to convert my "black" color (entry 1 in the color table > > (0,0,0,255)) to be transparent (new colortable entry is (0,0,0,0)) > > > > Another way to say it is, how do I create a tiff with an indexed colormap > > but also have it contain an alpha channel? > > TIFF color palette are only RGB, not RGBA. What you can do, however, is > assign > a nodata value (note this is a GDAL specific TIFF tag that will only be > understood by GDAL based software) > > gdal_translate src.tif src_with_nodata.tif -a_nodata 0 > > You can create a TIFF with an indexed colormap and an alpha channel with > the > following extra steps : > > gdal_translate src_with_nodata.tif mask.tif -b mask > gdalbuildvrt tmp.vrt src.tif mask.tif -separate > gdal_translate src.tif src.vrt -of VRT > > the with a text editor copy the lines that look line the following ones > from > src.vrt > > <ColorInterp>Palette</ColorInterp> > <ColorTable> > <Entry c1="112" c2="120" c3="56" c4="255" /> > [...] > <Entry c1="204" c2="208" c3="164" c4="255" /> > </ColorTable> > > and paste them just below <VRTRasterBand dataType="Byte" band="1"> in > tmp.vrt > > Finally : > > gdal_translate tmp.vrt pct_with_alpha.tif -co ALPHA=YES > > That's it. But I'm not sure that this will be properly displayed by > viewers. > > A safest alternative would be to build a RGBA TIFF : > > gdal_translate -expand rgb src.tif rgb.tif > gdal_translate -b 1 rgb.tif r.tif > gdal_translate -b 2 rgb.tif g.tif > gdal_translate -b 3 rgb.tif b.tif > gdalbuildvrt rgba.vrt r.tif g.tif b.tif mask.tif -separate > gdal_translate rgba.vrt rgba.tif -co ALPHA=YES > > > > > Thanks in advance, > > Isaac >
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