In some cases you might want to clean up a raster layer and replace certain values with another value. A a common example is having a NDSM with <0 values that you want to replace with "0".

You can use the "layer arithmetics" algorithm, to set all values <0 to 0. This will create a new temporary layer which then can be used for the consecutive processes. Here are two approaches to tackle this.

1. Here are the settings to set values below 0 to 0 for "Layer 1" and keep all other values as they are! The output layer created is as mentioned before a temporary layer in this case called "arithmetics". The range between the minimum (yellow) and maximum (blue) value define the values that you want to re-assign. By default this is set to zero (Output value). Make sure that the "output layer type" is the same bit depth as the input.
2. There is another more elegant way to solve your issue with the "layer arithmetics" algorithm. 
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   "(Layer 1 > 0)*Layer 1" is the key here. If the value of Layer one is greater than 0 (it gets the value 1 because it is true) multiply it with Layer 1. All other values are multiplied by 0 (not greater than zero and therefore get set to zero.

Interesting related content:

• Project & Rule Set Examples: Layer Arithmetics
• eCognition Deconstructed video: Layer Arithmetics