[mapserver-users] Unexpected result from overlapping images in tile index

[Stephen Woodbridge]

Right, you can not rely on the tileindex from doing anything more than 
say these are the objects that intersect with your bbox.

There are no specific controls over how the objects interact other than 
items are draw in the order they are found in the data sources. This is 
why sorting efected your example.

Explicitly burning NODATA or mask bands is the right way to control the 
images if your need that.

-Steve W

On 1/19/2017 1:08 PM, Peter Schmitt wrote:
> Thanks for the sortshp tip!  You are correct -- sorting the tileindex
> yields the result I expected.
>
> If the black pixels cover the full extents of black.tif (ie. with zero
> NODATA values), then Mapserver will render either an all black or an all
> gray image, regardless of the sorting order in the non-overlapping
> geometries in the tileindex.  When I internally tile the source images
> and zoom in, Mapserver does something curious: If the intersection
> between the tileindex and the requested bbox contain a single record,
> Mapserver will render the correct image.  When I zoom in, Mapserver
> renders an aliased image on the 256x256 WMS requests sent by the
> client.  I would guess the requested BBOX is intersected with the
> tileindex.  When the intersection yields a single item in tileindex, it
> renders the correct image.  When there are two items in the intersection
> of the requested BBOX and tileindex, the images are painted over the
> full requested BBOX in the order provided by the tileindex.
>
> I also could not find any docs/requirements on a tileindex guaranteeing
> ordering of the stacking images returned by the tileindex... so I guess
> it's best to not rely on this and just burn in NODATA/an external mask
> to my source images if I need such behavior.
>
> Thanks again,
> Pete
>
>
> On Thu, Jan 19, 2017 at 4:31 AM, thomas bonfort
> <thomas.bonfort at gmail.com <mailto:thomas.bonfort at gmail.com>> wrote:
>
>     Peter, if I understand correctly....
>     I'm not aware that there is any documentation or requirement that
>     using a tileindex with mapserver garantees any ordering in the
>     stacking of the images returned by the tileindex. Namely, if your
>     tileindex returns your gray (opaque) image after (i.e. on top of)
>     your black/white image, then your black/white image will be
>     completely obscured by your gray image.
>     This is what I'd try:
>     - try using the sortshp utility on your tileindex (once in ascending
>     order, once in descending) and see if the resulting image is affected
>     - use a postgis based tileindex layer with explicit ORDER BY clauses
>     to enforce that transparent images get rendered after opaque ones
>
>     --
>     thomas
>
>     On Thu, Jan 19, 2017 at 12:31 AM Peter Schmitt <pschmitt at gmail.com
>     <mailto:pschmitt at gmail.com>> wrote:
>
>         Hi,
>
>         I have a tile index to render a layer from two images.  The
>         images are black.png
>          https://github.com/pedros007/mapserver-tindex-problem/blob/master/black.png?raw=true
>         <https://github.com/pedros007/mapserver-tindex-problem/blob/master/black.png?raw=true>
>         which has NODATA values over the white areas and grayscale
>         pixels with intensity 1 and gray.png with intensity 128 and zero
>         NODATA values in the image
>         https://github.com/pedros007/mapserver-tindex-problem/blob/master/gray.png?raw=true
>         <https://github.com/pedros007/mapserver-tindex-problem/blob/master/gray.png?raw=true>
>
>         I have a
>         tileindex http://bl.ocks.org/d/968e0f11f1fd9b02444c3765bbb285a1
>         <http://bl.ocks.org/d/968e0f11f1fd9b02444c3765bbb285a1> that
>         basically stacks/flattens the black image on top of the gray
>         one.  However, when I make a WMS request to the layer:
>
>         LAYER
>                 NAME my_layer
>                 TYPE RASTER
>                 TILEINDEX tindex.shp
>                 TILEITEM "location"
>                 OFFSITE 0 0 0
>                 STATUS OFF
>             END
>         END
>
>         I get only the gray image.  In other words, I expected to get this:
>         https://github.com/pedros007/mapserver-tindex-problem/blob/master/expected.png?raw=true
>         <https://github.com/pedros007/mapserver-tindex-problem/blob/master/expected.png?raw=true>
>
>         but instead I get this:
>         https://github.com/pedros007/mapserver-tindex-problem/blob/master/gray.png?raw=true
>         <https://github.com/pedros007/mapserver-tindex-problem/blob/master/gray.png?raw=true>
>
>         I have a couple of work-arounds that I'm not crazy about:
>
>         1. Add Layer masks
>         http://www.mapserver.org/development/rfc/ms-rfc-79.html
>         <http://www.mapserver.org/development/rfc/ms-rfc-79.html> to two
>         separate layers, make two WMS requests and stack the results
>         client side. I have gotten this working for two layers. However,
>         I may need to render a very large number of such layers and
>         think this wont work client-side (via OpenLayers-3).
>         2. I could burn in NODATA values (or use a mask) to gray.tif,
>         but I'd rather not modify my source data.
>
>         A version of this email & example files are in a GitHub
>         repo: https://github.com/pedros007/mapserver-tindex-problem
>         <https://github.com/pedros007/mapserver-tindex-problem>
>
>         --
>         Pete
>
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>
>
>
>
> --
> Pete
>
>
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