Type 0 (Sampled) Functions are described in 3.9.1 of the pdf spec and basically, its cheating: there's an n-dimensional grid of values ("samples") and the function shall return these values or something in-between.
PDFunctionType0 has two bugs:
1) it does not do any interpolation. The function interpolate() is called several times, but only adjust values between ranges etc, not to calculate the color between 2^n samples - that part is "outputValues[i] = (outputValuesPrevious[i] + outputValuesNext[i]) / 2". The spec does not tell much, only that "Interpolation is used to determine output values from the nearest surrounding values in the sample table". I have done a linear/bilinear interpolation implementation for 1D/2D inputs. I did not do an interpolation implementation for 3D and higher, because its unclear whether this is actually used. Instead, I return random values.
2) the sample bits are not collected correctly, the current code ignores the leftover bits when a row is done. The spec tells us "Successive values are adjacent in the bit stream; there is no padding at byte boundaries". Luckily, that one is easy to correct, three lines must be moved up. Alternatively, one might use the bit-io lib I mention in