How to resample a raster considering a minimum number of valid pixels

Question

I'm looking for a function to resample a raster that can consider a minimum number of valid pixels in order to compute the new pixel value. E.g. If I have to resample a raster of 300m/px to 1Km/px I would consider a window of 3x3 pixels to compute the average value. But I would like to set a control to be sure that at least 5 pixels inside my matrix have valid data.

I'm trying using GDAL (in PyQGIS) but neither GDAL Translate nor GDAL Warp have this option

Answer 1

It could be done with r.neighbors in grass :

1. Sum number of cells using a 3x3

2. Count number of cells without Nan/Null value using a 3x3

3. Divide first generated raster by second one

Example :

Set a random region 3x3 cells

g.region rows=3 cols=3

Initialize with random cell raster

r.random.cells output=random_cells distance=0 ncells=5

Sum cells (3x3 is default but you could use -c option to set a different cell research)

r.neighbors input=random_cells output=sums method=sum

Count cells

r.neighbors input=random_cells output=counts method=count

Change region resolution to output desired raster resampling :

g.region res=1

Divide sum by count

r.mapcalc "outresamp = sums / counts"

Optinoal : copy colors from random cells to newly resampling raster :

r.colors map=outresamp raster=random_cells

https://grass.osgeo.org/grass78/manuals/r.neighbors.html

Alternatively, you could use interpolation with r.fillnulls or r.resamp* methods

Answer 2

Thanks Sylvain for your answer.

At the end I understood that there isn't a way to solve this issue in only one step. So, I wrote this PyQGIS code to do all these steps: https://github.com/fgianoli/CopernicusGlobalLand/blob/master/CGL_resampler.py

Basically, I did a reclass of the input raster in 0-1 (where 1 are my valid values), then I did a resample using mode -so 0 is where at least 5 pixels inside my kernel aren't valid - and I have multiplied this results with the result of 3x3 resampling using average. In this way I have obtained my results.