[gdal-dev] Anti-Aliasing

Chris Emberson chrisemberson at hotmail.com
Mon Oct 5 12:11:39 EDT 2009 

Thanks for the reply. This does improve the output (cubicspline works best), but still not great. So I increased the raster size by 10x and end up with an output of 500mb in size, which is difficult to work with.

Any other suggestions gratefully received.


> Date: Mon, 5 Oct 2009 17:03:04 +0200
> From: nowak at xpam.de
> To: chrisemberson at hotmail.com
> CC: chaitanya.ch at gmail.com; gdal-dev at lists.osgeo.org
> Subject: Re: [gdal-dev] Anti-Aliasing
> 
> Chris Emberson wrote:
> > The rasters do have colour tables applied, that are expressed using RGB 
> > values, using gdaldem:
> > 75%    107    0    0
> > 50%    196    109    27
> > 25%    247    195    72
> > 0%    252    233    109
> > nv    0    0    0 0
> > 
> > They are not derived from vector data however. I am unsure of your 
> > requirement in terms of the source of the raster data. All of the pixel 
> > values have been created and either fall between the values of 1-4 or 
> > contain no data.
> > 
> > The commands I am running on the files are:
> > gdal_translate -of GTiff -co "TILED=YES" -b 1 -b 1 -b 1 fileA.asc fileB.tif
> 
> Upsample fileB.tif here:
> gdalwarp -r bilinear -ts newwidth newheight fileB.tif fileB2.tif
> Where newwidth and newheight are a few times larger than original.
> 
> > gdaldem color-relief fileB.tif rgb.txt fileC.tif -alpha
> > gdalwarp -s_srs EPSG:27700 -t_srs EPSG:900913 -r bilinear fileC.tif 
> > fileD.tif
> > 
> > an example of how the bilinear is working (fileD.tif) can be seen in the 
> > attached file. I have tried running running the gdalwarp command above 
> > prior to gdaldem but this makes little difference.
> > 
> > TIA,
> > Chris
> > 
> > 
> > 
> >  > Date: Mon, 5 Oct 2009 16:46:55 +0530
> >  > Subject: Re: [gdal-dev] Anti-Aliasing
> >  > From: chaitanya.ch at gmail.com
> >  > To: chrisemberson at hotmail.com
> >  > CC: seth at pricepages.org; gdal-dev at lists.osgeo.org
> >  >
> >  > Chris,
> >  >
> >  > The images look like they have a colour table instead of RGB bands.
> >  > Also, they seem to be converted from vector to raster. Correct me if I
> >  > am wrong.
> >  >
> >  > Anti aliasing images with colour tables is a little more involved and
> >  > depends on the type of result required. Can you provide the source for
> >  > these images?
> >  >
> >  > One method would be to convert the image into RGB, apply bilinear or
> >  > any other interpolation method, and convert it back to the colour
> >  > table from RGB. If the source is a vector, we can smooth the shapes by
> >  > using curves in place of straight lines.
> >  >
> >  > On Mon, Oct 5, 2009 at 3:59 PM, Chris Emberson
> >  > <chrisemberson at hotmail.com> wrote:
> >  > > Seth,
> >  > >
> >  > > Thanks for the reply. The attached image hopefully will clarify the 
> > problem
> >  > > I am having. On the left of the image is the output I would like (I 
> > have
> >  > > produced this using a GIS). On the right is the output using mapnik and
> >  > > gdal. The problem is the pixellated effect between the raster 
> > values on the
> >  > > RHS.
> >  > >
> >  > > The GIS isn't that great as far as batch processing is concerned, 
> > hence why
> >  > > I am trying to use the GDAL utilities that ship with mapnik. The 
> > option in
> >  > > the GIS that produces the desired effect is "anti-aliasing (bilinear
> >  > > interpolation)", but it appears as though there is a more sophisticated
> >  > > smoothing algorithm being employed? I have tried running the various
> >  > > interpolation techniques with gdalwarp but none of them produce a crisp
> >  > > output. I have tried also increasing the resolution of the raster by 5x
> >  > > (both width and height) using "gdal_translate" and  "-outsize 500% 
> > 500%"
> >  > > this doesn't help either.
> >  > >
> >  > > Any suggestions gratefully received,
> >  > >
> >  > > Chris
> >  > >
> >  > >
> >  > >
> >  > >
> >  > >> Date: Fri, 2 Oct 2009 12:36:24 -0500
> >  > >> Subject: Re: [gdal-dev] Anti-Aliasing
> >  > >> From: seth at pricepages.org
> >  > >> To: chrisemberson at hotmail.com
> >  > >> CC: gdal-dev at lists.osgeo.org
> >  > >>
> >  > >> Could you give an example of the problem? Anti-aliasing generally only
> >  > >> applies when you are going from vector line drawings to rasterized 
> > images.
> >  > >>
> >  > >> If I was trying to smooth things I would first look at cubic or 
> > bilinear.
> >  > >> Any other resampling may enhance any noise in your image.
> >  > >> ~Seth
> >  > >>
> >  > >> On Fri, October 2, 2009 11:16 am, Chris Emberson wrote:
> >  > >> >
> >  > >> > I am looking for an anti-aliasing function to smooth out the 
> > pixellated
> >  > >> > effect between raster values. I have tried the various interpolation
> >  > >> > methods with gdalwarp and have also increased the resolution of the
> >  > >> > raster
> >  > >> > to try and minimise the effect, to no avail.
> >  > >> > Is this a function to be added soon?
> >  > >> >
> >  > >> > Thanks,
> >  > >> > Chris
> >  > >> >
> >  > >> > _________________________________________________________________
> >  > >> > Share your photos with Windows Live Photos – Free.
> >  > >> >
> >  > >> > 
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> >  > >>
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> >  > --
> >  > Best regards,
> >  > Chaitanya kumar CH.
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