This guide describes the process of using alpha-blended decals to create broken concrete pieces and building ruins of high visual quality.
In most games destroyed walls and other buildings structures are done by simply cutting into the geometry and applying a tiling concrete texture to the broken area.
While it is performance-effective and simple to do this, the disadvantage of this technique is that you are left with a very visible and sharp seam between the two materials. It is a more work-intensive but visually better solution to use decals with alpha-blending in order to avoid these seams.
Note the sharp seam between the two materials (marked in orange).
Note the more believable transition between the two materials (marked in orange).
The normal map used for this decal:
Example for "chipped-off" pieces using alphablend decals:
The normal map used for this decal:
Here you can see the extra geometry element following the edge of your broken piece of building:
Here you can see how the UV mapping must be adjusted to create a convincing overlap of the two materials:
Use the chipped-off looking edge decals by
Illustration of these steps:
Another example:
For the best results, use the low-res geometry you used for rendering the normal map, snap them on the edge of the object you are working on, and cut its shape out of the solid block using the knife tool together with vertex snapping.
Using this method, you will also have the correct silhouette.