Generating three-dimensional objects

A method comprising:

• delivering a layer of build material;
• selectively depositing coalescing agent to a first portion of the layer at a first density;
• selectively depositing coalescing agent to a second portion of the layer at a second density lower than the first density, the second portion disposed around a boundary of the first portion;
• and
• applying energy to the layer to cause the first portion to coalesce and solidify to form a slice of a three-dimensional object, wherein presence of the coalescing agent in the second portion prevents at least some heat from flowing away from the first portion when the energy is applied.

A non-transitory computer readable storage medium including executable instructions that, when executed by a processor, cause the processor to:

• obtain data representing a three-dimensional object to be generated, the data including a first portion defining where coalescing agent is to be delivered at a first density;
• modify the data to include a second portion on which coalescing agent is to be delivered at a second density less than the first density, the second portion surrounding at least part of the first portion;
• and
• using the modified data, control at least one agent distributor to deliver the coalescing agent to the first portion at a first density and to the second portion at a second density such that the first portion is to coalesce and solidify when energy is applied, wherein the second density is insufficient to achieve full solidification in the second portion when the energy is applied, and wherein presence of the coalescing agent in the second portion prevents heat from flowing away from the first portion when the energy is applied.