[Qgis-user] Four band raster displaying funny

[Jonathan Moules]

Hi Andy,
I guess that makes sense.

Relating to gdalwarp:
- Output files by default are larger than gdal_merge.
- But they can be much smaller. You have to set *both*  -wm and --config
GDAL_CACHEMAX - if you only set -wm, then the file is actually larger!
- gdal_merge seems to do something that results in some heavy blurring when
using -co PHOTOMETRIC=YCBCR - this doesn't happen with gdalwarp.

So the optimal filesize for an aerial photograph is rendered with something
like:

gdalwarp -of GTiff -wm 9999 --config GDAL_CACHEMAX 9999 -co TILED=YES -co
> BIGTIFF=YES -co COMPRESS=JPEG -co JPEG_QUALITY=80 -co BLOCKXSIZE=512 -co
> BLOCKYSIZE=512 -co PHOTOMETRIC=YCBCR input1.tif input2.tif output.tif


I've not tried the four-band stuff again; just trying to optimise by 3-band.

Thanks,
Jonathan


On 2 December 2013 16:10, Andrew Harfoot <ajph at geodata.soton.ac.uk> wrote:

>  I think QGIS is innocent in this - if a band is set as an alpha channel
> then it should be handled as such by default in a viewer (so mark down Arc
> for not using the alpha information!).
>
> GDAL is the culprit as it is adding the alpha interpretation without being
> prompted. I have just replicated this with some RGBI imagery myself: prior
> to passing through GDAL's hands the IR band is present, but isn't
> interpreted as an alpha channel. I can't get the -setci switch to do
> anything though :(
>
> Cheers,
>
> Andy
>
>
> On 02/12/2013 16:00, Jonathan Moules wrote:
>
> Hi Andy,
> Yep, that was it. I didn't know QGIS could do that; another good example
> of software trying to be "smart" and confusing the poor user. :-)
>
>  ====
>
>  I didn't know gdalwarp could do mosaicing too. I'll have to test it.
> I'll ask on the gdal list if I want to try the -setci parameter.
>
>  Many thanks!
> Jonathan
>
>
> On 2 December 2013 15:48, Andrew Harfoot <ajph at geodata.soton.ac.uk> wrote:
>
>>  PS. gdalwarp offers more flexibility when mosaicing rasters, and is
>> better at memory management. I have just noticed that in GDAL 1.10 and
>> above there is an gdalwarp option -setci that 'Sets the color
>> interpretation of the bands of the target dataset from the source dataset'.
>> This could be used to remove the assignment of the alpha channel to the IR
>> band on merging. Sadly there isn't an example of its usage!
>>
>>
>> Cheers,
>>
>> Andy
>>
>> On 02/12/2013 11:53, Jonathan Moules wrote:
>>
>>  Hi List,
>> I've got a 4 band raster aerial photography (RGBI) that comprises lots of
>> tiles. I've merged some of the tiles together with:
>>
>>  gdal_merge -o 1.tif -of GTiff -co TILED=YES -co BIGTIFF=YES -co
>>> COMPRESS=JPEG -co JPEG_QUALITY=50 -co BLOCKXSIZE=512 -co BLOCKYSIZE=512
>>> --optfile tiff_list.txt
>>
>>
>>  But the resultant file looks funny in QGIS.
>> This is what the source file looks like (correct):
>> [image: Inline images 1]
>>
>>  This is what the merged file looks like (wrong):
>> [image: Inline images 2]
>>
>>  All the shadows are a whitey colour. This doesn't happen with 3-band
>> (RGB) images.
>> I've tried comparing individual bands; they all look different in the
>> 4-band.
>>
>>  However, if I open the four-band in ArcGIS, it looks fine (both source
>> and original).
>>
>>  Anyone know what's going on? Is it a QGIS bug or is it doing something
>> "smart"; I can't see anything odd going on with symbology.
>>
>>  Thanks,
>> Jonathan
>>
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>> --
>> Andy Harfoot
>>
>> GeoData Institute
>> University of Southampton
>> Southampton
>> SO17 1BJ
>>
>> Tel:  +44 (0)23 8059 2719
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> --
> Andy Harfoot
>
> GeoData Institute
> University of Southampton
> Southampton
> SO17 1BJ
>
> Tel:  +44 (0)23 8059 2719
> Fax:  +44 (0)23 8059 2849
> www.geodata.soton.ac.uk
>
>

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