The atlas layer

The atlas layer is the first level of a terrain construction. It acts as a kind of basement and it's always available, even for an empty terrain:

From left to right we find:

The atlas does not produce any visible effect until a first level soil gets added to it. It's kind of invisible and only works by influencing other terrain soils.

Influencing soils

Let's start with a simple terrain made with a first level soil:

Every landscape definition parameter will result of a combination of the atlas layer with the first soil layer. In the picture above we show the combinations for the color (highlighted in orange) and the combination for the densities (highlighted in blue). Combinations occur also for the terrain elevation (height), ambient occlusion and reflectance. The combination is always like CURRENT_VALUE (OPERATOR) TOP_SOIL_VALUE. The CURRENT_VALUE results of the atlas and all soils combined up to now and the TOP_SOIL_VALUE is the set of values of the last dropped soil (so the soil on top of the stack). These two values are mixed using OPERATOR (for instance multiply, add, subtract,...)

There are default combinations that take place:

All operators and their effects are detailed here: Soil blending operators.

Let's see some practical samples on a simple terrain:

From the top left reference image: