you can use jit.submatrix to output only the area of the matrix you want to pixelate (e.g. the face), then to place back into the original matrix use usedstdim 1 dstdimstart and dstdimend attributes. search the forum and the jitter tutorials for examples. post a patch showing what you've tried for more help.

It took a while but I did it! I used usesrcdim - > reduced the dimensions - > put it back on a 320/240 matrix in the right position (same XY but with usedstdim - > and project both the new matrix with the original one Thanks a lot!