Even Rouault a écrit :
Well, actually, I can propose a 3rd solution that should work with any
recent GDAL version to produce a white & small raster.
1) Copy your source.vrt as white.vrt
2) Add a <NoDataValue>255</NoDataValue> line after each <VrtRasterBand>
3) Remove all existing <SimpleSource> content
So basically you get something as simple as that :
<VRTDataset rasterXSize="20" rasterYSize="20">
<SRS>PROJCS["NAD27 / UTM zone
11N",GEOGCS["NAD27",DATUM[&q
uot;North_American_Datum_1927",SPHEROID["Clarke
1866",6378206.4,2
94.9786982138982,AUTHORITY["EPSG","7008"]],AUTHORITY["E
PSG","6267"]],PRIMEM["Greenwich",0],UNIT["degree&q
uot;,0.0174532925199433],AUTHORITY["EPSG","4267"]],PROJECTIO
N["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],P
ARAMETER["central_meridian",-117],PARAMETER["scale_factor",0
.9996],PARAMETER["false_easting",500000],PARAMETER["false_northin
g",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]
],AUTHORITY["EPSG","26711"]]</SRS>
<GeoTransform> 4.4072000000000000e+005, 6.0000000000000000e+001,
0.00000000000
00000e+000, 3.7513200000000000e+006,
0.0000000000000000e+000,-6.0000000000000000
e+001</GeoTransform>
<VRTRasterBand dataType="Byte" band="1">
<NoDataValue>255</NoDataValue>
</VRTRasterBand>
<VRTRasterBand dataType="Byte" band="2">
<NoDataValue>255</NoDataValue>
</VRTRasterBand>
<VRTRasterBand dataType="Byte" band="3">
<NoDataValue>255</NoDataValue>
</VRTRasterBand>
</VRTDataset>
Then recreate your main VRT by adding white.vrt as the first dataset
(with GDAL 1.7.0 gdalbuildvrt, you would need to specify -srcnodata None
so that the nodatavalue of white.vrt is just considered as white and not
processed as a nodata value by the rendering layer of the main VRT... of
course !)
Greg Coats a écrit :
I do not think I want the black, opaque areas that gdalbuildvrt is
making up for areas where there is no imagery to become transparent
and remain black, I want them to be white and opaque. Surely, there
are many places in the RGB images where one or more bands is already
set to the value 255, and I do not want to change any legit 255 R, G,
B values in the imagery. I do not want to convert the 1699 RGB images
to RGBA. Essentially, I want for gdalbuildvrt what -init 255 does for
gdal_merge.py. But, for now there does not seem to be a way to do
that? Greg
No, basically there's no support in the VRT driver for initializing
the buffer to a non-zero value except if you set a nodata value. I
don't see a theoretical difficulty in extenting but that woud require
an addition to the XML VRT syntax.
I was a bit confused by what your 2 screenshots showed. I know suppose
that they show 2 VRTs layers. So, if I were you, I'd begin by setting
the vrtnodata to 255 and see if the result match your expectations.
You can already do that by manually editing the VRT and adding a
<NoDataValue>255</NoDataValue> just after each <VrtRasterBand>
element. If you blend the result onto a white background, I think this
should not alter visually the result.
There's another workaround I'm just thinking about, but it is a bit
more involved and may lower performance when displaying the VRT. You
can create a totally white GeoTIFF that has the extent of the whole
VRT, and add it first in the VRT. Something like this python script :
import gdal
ds = gdal.Open('source.vrt')
wkt = ds.GetProjectionRef()
gt = ds.GetGeoTransform()
xsize = ds.RasterXSize
ysize = ds.RasterYSize
ds_white = gdal.GetDriverByName('GTiff').Create('white.tif', xsize,
ysize, 3, options = ['TILED=YES', 'COMPRESS=LZW'])
ds_white.SetGeoTransform(gt)
ds_white.SetProjection(wkt)
ds_white.GetRasterBand(1).Fill(255)
ds_white.GetRasterBand(2).Fill(255)
ds_white.GetRasterBand(3).Fill(255)
ds_white = None
If you use a QGIS version that relies on GDAL 1.7.0, you can replace
the creation of the white dataset by :
ds_white = gdal.GetDriverByName('GTiff').Create('white.tif', xsize,
ysize, 3, options = ['TILED=YES', 'SPARSE_OK=YES'])
ds_white.SetGeoTransform(gt)
ds_white.SetProjection(wkt)
ds_white.GetRasterBand(1).SetNoDataValue(255)
ds_white = None
This will create a sparse GeoTIFF that should be very small and when
reading it, GDAL 1.7.0 will initialize the empty blocks with the
nodata value (previous GDAL version would initialize with 0). The
rendering of a VRT referencing that file should be much faster than
with the white geotiff produced by the first solution.
On Jan 4, 2010, at 4:08 PM, Even Rouault wrote:
Greg,
I don't think the issue is really about black or white, but more
about setting nodata appropriately. Basically, if I've well
understood your needs, you want to make some black padding in images
transparent. Assuming that your imagery is RGB, you could try adding
-srcnodata 0 to the gdalbuildvrt command line, so that pixels with
value 0 in source imaginery are not drawn in the VRT buffer. You
will also need to specify a -vrtnodata XXX (which is about like
-init XXX of gdal_merge.py, but does more than -init as it sets that
value as nodata), where XXX can be 255 if you want it to be nodata
white, but 0 should also do, but the value should not matter much to
QGIS : it should just ignore the pixels matching the nodata value
whichever it is.
You must be aware of the limitation of the nodata concept when
working with RGB imagery however. Nodata is considered band per
band, and not as a triplet. I mean if you have a blue pixel
(0,0,255), and you set a nodata value of 255 for each of the 3
bands, the blue component of (0,0,255) being 255, it will be
considered as nodata for that component... The only proper way to
deal with that is to work with RGBA imagery.
Best regards,
Even
By default, gdal_merge.py starts by creating an output image with
all black pixels, and then replaces these black pixels with pixels
from the images being read in. Passing to gdal_merge.py -init 255
pre-initializes the output image to all white, instead of all
black, pixels.
Similar to gdal_merge.py, gdalbuildvrt fills with black pixels. I
am seeking for gdalbuildvrt, the -init 255 option that
gdal_merge.py supports, but do not see -init 255 as a supported
option on the gdalbuildvrt man page. In GDAL 1.7.0, will
gdalbuildvrt support -init 255? If not, then in GAL 1.7.0 can the
option -vrtnodata be used in a way similar to how gdal_merge.py
supports -init 255? I need areas where there is no image to be
white, rather than the black that gdalbuildvrt is by default
setting them to be.
Greg
gdal_merge.py man page
http://www.gdal.org/gdal_merge.html
gdalbuildvrt man page
http://www.gdal.org/gdalbuildvrt.html
gdalbuild_black image
http://homepage.mac.com/gregcoats/jp2/images/gdalbuildvrt_black.png
gdalbuildvrt_white image (simulated)
http://homepage.mac.com/gregcoats/jp2/images/gdalbuildvrt_white.png
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