looks like a box filter with radius .5 for nearest interpolation: http://graphics.cs.cmu.edu/courses/15-463/2007_fall/Lectures/SamplingReconstruction.pdf
and a 2d tent function for bilinear: https://www.cs.toronto.edu/~guerzhoy/320/lec/upsampling.pdf
looks like a box filter with radius .5 for nearest interpolation: http://graphics.cs.cmu.edu/courses/15-463/2007_fall/Lectures/SamplingReconstruction.pdf
and a 2d tent function for bilinear: https://www.cs.toronto.edu/~guerzhoy/320/lec/upsampling.pdf