Percentage of feature area covered by features of other layer in QGIS

Question

I'm working with QGIS 3.16 Hannover under MacOS 10.13 environment. I've got two polygon layers A and B with multiple features in each. I want to calculate, for each feature in layer A, the percentage of its area that is actually covered by any feature of layer B. I have built in the field calculator of layer A the following code, that seems to work in some cases, but definitely doesn't in others
(area(intersection($geometry, collect_geometries(overlay_intersects('B', $geometry)))))*100/area($geometry)

Any idea of what's wrong?

MrXsquared · Accepted Answer

I see two cases when this does not work:

If a feature is not covered at all (result = NULL) (which is actually correct)
If covering features of B do overlap each other

To cover both cases, you can use this expression:
 coalesce((area(intersection($geometry, buffer(collect_geometries(overlay_intersects('B', $geometry)),0))))*100/area($geometry),0)

Explanation:

for case 1) coalesce() will cover the NULL case and insert 0 if no
result is returned by the actual expression.
for case 2) buffer(...,0) will dissolve the overlapping parts of the multipolygon by a buffer of 0 (workaround since there is no actual dissolve() function) so will prevent from causing an error in the calculation

Example:

Percentage of feature area covered by features of other layer in QGIS

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