According to examples, an apparatus may include a processor and a memory on which are stored machine-readable instructions that when executed by the processor, cause the processor to determine physical characteristics of a build layer of build material particles. The instructions may also cause the processor to determine an adjustment to forming data based on the determined physical characteristics, the forming data to be used informing a subsequent build layer. The instructions may further cause the processor to apply the determined adjustment to the forming data for use in forming the subsequent build layer, in which portions of a three-dimensional (3D) object are to be formed in the build layer and the subsequent build layer.