New in version 3.19.

Specify relative output directory for ISPC headers provided by the target.

If the target contains ISPC source files, this specifies the directory in which the generated headers will be placed. Relative paths are treated with respect to the value of CMAKE_CURRENT_BINARY_DIR. When this property is not set, the headers will be placed a generator defined build directory. If the variable CMAKE_ISPC_HEADER_DIRECTORY is set when a target is created its value is used to initialize this property.