The back edge of the painted popping-crease marking defines the crease line for the relevant measurement. The whole painted strip is not one broad legal line. The popping crease is also different from the bowling crease.
What “back edge” means
Imagine a white painted strip running across the pitch. The edge nearer the bowling crease at the same end of the pitch is its back edge, not the edge toward the opposite wicket. MCC Law 7 defines the popping crease by that edge, four feet in front of and parallel to the bowling crease. MCC Appendix A.4 and diagram C establish the orientation.
Label the reference edge before comparing positions. Do not switch to the centre or opposite edge during the same assessment.
Hypothetical ground-sketch example
Hypothetical example: A sketch shows one set of stumps and its bowling crease, then the popping-crease strip ahead. Label its edge nearer those same stumps A and its opposite edge B. A is the defining back edge. Measuring to B would use the wrong reference, even though both edges belong to the same painted strip.
The marked line also need not be assumed to show the complete legal extent across the pitch. Its painted length is not automatically the entire extent of the crease for every question.
What a photograph can and cannot establish
Hypothetical example: A cropped photograph shows only a batter’s foot beside some white paint. That image alone may not reveal which edge is the back edge, the camera angle, the relevant crease alignment, or the precise position of the foot or bat. Seeing paint in the frame is therefore not enough to settle whether a grounded bat or foot satisfies another law.
This answer identifies the crease definition only. A run-out, stumping, no-ball, or other decision would require the separate conditions of the applicable law and reliable evidence. Competition playing conditions may modify how laws are applied, so the relevant match regulations must also be checked.