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### B.2.3 Weaving collisions

This perfect weave is not possible in all cases. Let's look at another example:

@math{S=6}, @math{J=4}:

    0 *-----*-----*-----*
    1     *-----*-----*-----*
    2         *-----*-----*-----*
    3             *-----*-----*-----*
    4             ^   *-^---*-----*-----*
    5             |   ^ | *-^---*-----*-----*
                  OUCH!   ^ |   ^
                          |     |

Here we have a collision. Some lines printed in later passes overprint lines printed by earlier passes. We can see why by considering which row number is printed by a given jet number @math{j} (numbered from 0) of a given pass, @math{p}:

    @math{row(p, j) = p*J + j*S}

Because @math{J=4} and @math{S=6} have a common factor of 2, jet 2 of pass 0 prints the same row as jet 0 of pass 3:

    @math{row(0, 2) = 0*4 + 2*6 = 12}
    @math{row(3, 0) = 3*4 + 0*6 = 12}

In fact, with this particular weave pattern, jets 0 and 1 of pass @math{p+3} always overprint jets 2 and 3 of pass @math{p}. We'll represent overprinting rows by a ``^'` in our diagrams, and correct rows by ``*'`:

@math{S=6} @math{J=4}:

    0 *-----*-----*-----*
    1     *-----*-----*-----*
    2         *-----*-----*-----*
    3             ^-----^-----*-----*
    4                 ^-----^-----*-----*
    5                     ^-----^-----*-----*

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