1. An image signal processing apparatus outputting to a projection unit an output image signal to project image light onto a projection plane of arbitrary shape, the apparatus comprising: an image signal input unit which receives a two-dimensional image signal to project image light onto a two-dimensional projection plane; an external input unit which externally receives a distortion correction table as a correspondence map between the two-dimensional projection plane and a mesh model of the projection plane of arbitrary shape, the distortion correction table being used to perform distortion correction processing for the two-dimensional image signal received by the image signal input unit; a distortion correction unit which performs distortion correction processing for each pixel of the two-dimensional image signal received by the image signal input unit with reference to the distortion correction table received by the external input unit to generate the output image signal to project the image light onto the projection plane; and an output unit which outputs the output image signal generated by the distortion correction unit to the projection unit.