1. An image signal processing apparatus outputting, to a projector, an output image signal to project image light onto a projection plane of an arbitrary shape, the image signal processing apparatus comprising: an image signal input which receives a two-dimensional image signal to project image light onto a two-dimensional projection plane; an external input 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 the 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; a distortion corrector which performs the distortion correction processing for each pixel of the two-dimensional image signal received by the image signal input with reference to the distortion correction table received by the external input to generate the output image signal to project the image light onto the projection plane; and an output which outputs the output image signal generated by the distortion corrector to the projector, wherein the distortion correction table describes at least one shift amount for each pixel of a plurality of pixels for shifting the pixel from a first point before conversion to a second point after the conversion, the distortion corrector mixes color information of the plurality of pixels before the conversion in a ratio corresponding to the shift amount of each pixel, and the distortion corrector generates the output image signal with mixed color information of the color information of the plurality of pixels after the conversion of each of the plurality of pixels.