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

[celati Laurent]

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.

[image: SS_MS_masque_bd_foret.jpg]

Thanks so much.
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