[Qgis-user] Limit a raster processing to a subset/region/mask area?

[Kirk Schmidt]

Good Day:

I would have a look at the masked raster and confirm the no data value 
(-999) is written to the header so that it is recognized by the 
segmentation routine. Another option would be to delete the segmented 
polygons from your result layer (if your result is a vector) or using 
the raster calculator to reset these pixels to -999.

Kirk Schmidt

On 6/13/2025 8:12 AM, celati Laurent via QGIS-User wrote:
> Hello again,
> Following to my previous own post I'm taking the liberty to do a first 
> reaction/addition. I tried clipping my multispectral SPOT6 image with 
> my BD FORET mask (clip a raster with a masking extent coverage) by 
> setting the value -999 (16-bit signed output) for the no data values 
> during clipping.This worked. But when I try segmentation with the OTB 
> GRM tool on this image, the process isn't ignored in the areas for 
> which I had assigned no data values. The tool segmented the entire 
> image. I sharean additional screenshot showing the pixel values in an 
> area of no data values. The values are indeed set on no data values 
> for the multispectral SPOT6 input image. And you can clearly see that 
> it assigned a value to my segmentation result. The process has been 
> done for no data values also :
> SS_raster_segmentation_spot6_nodatavalues.jpg
> Thanks.
>
> Le ven. 13 juin 2025 à 11:58, celati Laurent 
> <laurent.celati at gmail.com> a écrit :
>
>     Dear all,
>     I'm working in a QGIS environment (graphical interface).
>     Specifically, I'm using a QGIS modeler. Thanks to the processing
>     toolbar provides, i have access to QGIS, Grass, Saga, and Orfeo
>     toolbox tools algo. My goal is to generate a segmentation for
>     geographic area. For the segmentation, I use the OTB tool via QGIS
>     "Generic region merging":
>     https://www.orfeo-toolbox.org/CookBook-7.0/Applications/app_GenericRegionMerging.html
>
>     The complexity comes from the fact that I want to
>     contextualize/differentiate the parameters and input data required
>     for the segmentation step,within my study area based on a land use
>     layer/mask layer. So i want to define specific parameters/input
>     data for the segmentation generated on forest areas. And other
>     input data/segmentation parameters generated for non-forest areas
>     (urban, agricultural). I therefore have a vector data set, "Forest
>     DB mask," which allows me to locate/mask forest areas.
>
>     I therefore want to :
>
>      1. Generate an initial "raster segmentation" from an
>         aerial/satellite image with specific parameters for forest
>         areas (segmentation calculation only for raster pixels that
>         overlap my forest DB mask). Therefore, the calculation must
>         NOT be performed/ be ignored for pixels not covered by this
>         mask layer (agricultural, urban, etc.).
>      2. Then generate a second "raster segmentation" for the rest of
>         the study area that was ignored/skiped during the first
>         segmentation (segmentation calculation only for raster pixels
>         that do not overlap my forest DB mask: agricultural, urban, etc.).
>      3. Then, once these two raster segmentations are complete, the
>         idea is to merge them into a single raster. Then, generate a
>         polygonization (raster to vector).
>
>     This little workflow isn't really complicated. But I'm wondering
>     if it's possible to do it in a QGIS modeling/graphical interface
>     environment. I'd be interested in advice/ideas. Do I must to  clip
>     input rasters with the forest DB masking layer? With a
>     management/assignment of "no data values" to ignore the
>     segmentation processing on areas not covered by the mask layer?
>
>     I've attached below a screenshot that allows you to display an
>     input raster image (here a multispectral Spot6/7 satellite image)
>     and the mask layer (vector forest DB mask) to better visualize the
>     context.
>
>     SS_MS_masque_bd_foret.jpg
>
>     Thanks so much.
>
>
>
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-- 
Kirk Schmidt, MScF, BScF, RPF
General Manager
Nortek Resource Solutions Inc.
26 Church Road
Sutherlands River, NS
B0K 1W0
Tel (902) 922.3607
Email:kirk at nortekresources.com
Web:www.nortekresources.com
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