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I'm attempting to use gdal_merge to stitch together four JP2 aerial orthophotos. the merge is successful, but the resulting image has washed out color, as you can see here:

enter image description here

All color in the four original tiles looks fine before the merge.

The line I'm using to merge them is:

gdal_merge -o merged.tif -of GTiff 16SEJ790950_201103_0x1500m_4B_1.jp2 16SEJ790965_201103_0x1500m_4B_1.jp2 16SEJ805950_201103_0x1500m_4B_1.jp2 16SEJ805965_201103_0x1500m_4B_1.jp2

GDALINFO color information shows:

COLORSPACE = RGB

COMPRESSION_RATE_TARGET=2

Band 1 Block=256x256 Type=Byte, ColorInterp=Red Description = Red Overviews: 5000x5000, 2500x2500, 1250x1250, 625x625, 312x312, 156x156

Band 2 Block=256x256 Type=Byte, ColorInterp=Green Description = Green Overviews: 5000x5000, 2500x2500, 1250x1250, 625x625, 312x312, 156x156

Band 3 Block=256x256 Type=Byte, ColorInterp=Blue Description = Blue Overviews: 5000x5000, 2500x2500, 1250x1250, 625x625, 312x312, 156x156

Band 4 Block=256x256 Type=Byte, ColorInterp=Undefined Description = AllOpacityPremultiplied Overviews: 5000x5000, 2500x2500, 1250x1250, 625x625, 312x312, 156x156

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    What does gdalinfo report about your source images?
    – user30184
    Jan 4, 2017 at 21:54
  • Added GDALINFO color-related info to the question.
    – D. Waschow
    Jan 5, 2017 at 13:25
  • 3
    Band 4 is interpreted to be alpha. Do you know if it is alpha or something else? For testing if alpha makes trouble drop it out from some images with gdal_translate. Use -b 1 -b 2 -b 3for creating plain RGB image.
    – user30184
    Jan 5, 2017 at 13:39
  • Alpha sounds like a good possibility. I hadn't come across premultiplied alpha before, that doesn't sound like the alpha channel behaviour I'd expect, which is like the 'straight alpha' described on that page. This might be a jp2 specific thing.
    – Steven Kay
    Jan 5, 2017 at 21:00

2 Answers 2

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It could just be that your combined image has a greater range of values and the colour stretch makes the new image look washed out.

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I ran into this issue too. I had 4 geotiffs that were 4 band orthoimagery. You can check the bands using the Raster->Miscellaneous->Information... command. Here I can see that the images are 4 bands: 

Band 4 Block=512x512 Type=Byte, ColorInterp=Undefined
Band 1 Block=512x512 Type=Byte, ColorInterp=Red
Band 2 Block=512x512 Type=Byte, ColorInterp=Green
Band 3 Block=512x512 Type=Byte, ColorInterp=Blue

4 Individual Tiffs

After merging them together they were washed out:

Washed out Image

The issue was the resulting geotiff bands were identifying that there was an alpha band. I noticed this using the information tool on the result: 

Band 1 Block=6000x1 Type=Byte, ColorInterp=Red
  Mask Flags: PER_DATASET ALPHA 
Band 2 Block=6000x1 Type=Byte, ColorInterp=Green
  Mask Flags: PER_DATASET ALPHA 
Band 3 Block=6000x1 Type=Byte, ColorInterp=Blue
  Mask Flags: PER_DATASET ALPHA

I could turn this off by going to Properties->Transparency. There I saw that the Transparency band was identified as Band 4 (which was NIR for my imagery)

Transparency Settings for Raster Images

If I set that to "None" the image will render correctly. However, if I want to merge them without the Mask Flags being added to the result, during the merge operation, I need to add a creation option for ALPHA with the value NO. Alternatively, you can edit the gdal command to add -co "ALPHA=NO".

This will prevent the Mask Flags from being added to the output, and the image will render correctly.

enter image description here

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