[Qgis-developer] horizontal lines pdf/ps/image output

[Andreas Neumann]

Hi Andreas,

Although I am not the developer of the print composer (Marco Hugentobler 
is) I can provide some background on the issue.

Indeed it is a result of image partitioning. I believe there are 
horizontal stripes. With higher resolution this issue gets worth. The 
striping was introduced because people want to output high resolution 
map layouts at large sizes (e.g. A0). Of course rendering everything in 
a big raster (A0, 600 or 1200dpi) you would easily run out of memory.

The other issue is that people want to print high resolution from WMS 
sources. WMS servers are often limited to e.g. 1000 to 2000 pixel for 
one side of an image request. Therefore QGIS has to do several requests 
for getting the image on high resolution.

I think that there is a very small overlap of the stripes, because some 
printers would render small white stripes for adjacent images. Now, when 
transparency gets involved you will unfortunately see thoses overlapping 
stripes.

Question: are you rendering WMS or file-based images (e.g. tiff)? Would 
it help if you could configure the size of a stripe or even render 
everything in one tile? Can you provide a sample output to see how bad 
the issue really is? Do you have any idea to better solve the problem of 
outputting high resolution rasters at large paper sizes? I would also be 
interested to see the bathymetric map, btw.

I am sure Marco can provide more background on the issue. He is on 
holidays this week. I am sure he'll chime in on the issue and possible 
workarounds.

Andreas

On 10/20/10 7:33 PM, Andreas Plesch wrote:
> Hello,
>
> using qgis (1.5, I believe) to generate a bathymetric map from data
> for the Bering Strait, I came across horizontal lines when trying to
> output the map in print composer in any way. The process was a good
> success in that everything which was needed could be done and the
> result was very useful for the biologist who wanted a good looking map
> of the "sunken land bridge". There was not anything online. It might
> have even been faster than using ArcMap, apart from the issue below.
>
> The map has raster bathymetry with a custom color map, a transparent
> grey scale shaded relief on top, and contours. The horizontal lines
> show up after I compose a map and then output the map in any way. They
> do not show up in the main qgis window or the print compose. They
> become more frequent when I increase the resolution to say 600 dpi. At
> 300 dpi there is just one. At 150 dpi there is none. I believe it
> might have to do how images are partitioned into small rectangles, and
> perhaps the transparent layer ?  Since the output might be managed by
> Qt, there may not be a lot of options but it was an important issue.
> After all, in the end you need to output.
>
> Another, unrelated issue, with output was that the colors were only
> correct in the postscript. In the pdf and image output they were too
> dark, perhaps a transparency issue.Also text seemed rasterized when on
> top of images.
>
> It was a joy to be able to import easily the ASCII grid, to crop it,
> to easily warp it to a suitable projection, to easily generate
> contours and shaded relief, and to add some annotation in the print
> composer.
>
> Andreas
>
>
>