A one-row training image turns pixel indices into one checked sum. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Read a one-row image
The row contains pixel indices 1, 2, and 3.
1+2+3
A row shadow is a line sum
The projection score is 6, because the whole row lies under the same ray.