A three-dimensional printer , comprising: a recoater for forming a layer of build material; a first agent distributor to selectively distribute a pattern of fusing agent on a formed layer of build material; a second agent distributor to selectively distribute a pattern of detailing agent on a formed layer of build material; an energy module to apply fusing energy to a formed layer of build material; and a controller configured to: obtain data corresponding to an object to be generated; control the recoater to form a layer of build material; determine a pattern of fusing agent to be applied to the formed layer of build material to generate a layer of the object; determine a first pattern of detailing agent to be applied to the formed layer of build material to reduce or prevent caking of build material that is not to form part of the object; control the agent distributors to apply the determined patterns of fusing agent and detailing agent to the formed layer of build material formed; control the energy source to apply fusing energy to the formed layer of build material; characterised in that the controller is configured to determine, for a first layer of build material, the first pattern of detailing agent to have a first checkered pattern, and for a subsequent layer of build material, the first pattern of detailing agent to have a complementary checkered pattern. a recoater for forming a layer of build material; a first agent distributor to selectively distribute a pattern of fusing agent on a formed layer of build material; a second agent distributor to selectively distribute a pattern of detailing agent on a formed layer of build material; an energy module to apply fusing energy to a formed layer of build material; and a controller configured to: obtain data corresponding to an object to be generated; control the recoater to form a layer of build material; determine a pattern of fusing agent to be applied to the formed layer of build material to generate a layer of the object; determine a first pattern of detailing agent to be applied to the formed layer of build material to reduce or prevent caking of build material that is not to form part of the object; control the agent distributors to apply the determined patterns of fusing agent and detailing agent to the formed layer of build material formed; control the energy source to apply fusing energy to the formed layer of build material; characterised in that the controller is configured to determine, for a first layer of build material, the first pattern of detailing agent to have a first checkered pattern, and for a subsequent layer of build material, the first pattern of detailing agent to have a complementary checkered pattern. obtain data corresponding to an object to be generated; control the recoater to form a layer of build material; determine a pattern of fusing agent to be applied to the formed layer of build material to generate a layer of the object; determine a first pattern of detailing agent to be applied to the formed layer of build material to reduce or prevent caking of build material that is not to form part of the object; control the agent distributors to apply the determined patterns of fusing agent and detailing agent to the formed layer of build material formed; control the energy source to apply fusing energy to the formed layer of build material; characterised in that the controller is configured to determine, for a first layer of build material, the first pattern of detailing agent to have a first checkered pattern, and for a subsequent layer of build material, the first pattern of detailing agent to have a complementary checkered pattern. <EOS>