A histogram can summarize how many pixels occupy each intensity bin. It does not preserve their positions in the image.
Define the counted values
An image histogram counts samples by intensity or another selected channel quantity. Its bins describe a value distribution rather than a map of pixel positions. The selected channel, value range and mask therefore belong to the histogram definition before two count arrays are compared. Primary reference: OpenCV image histogram definition and bins.
For an original inspection worksheet, retain the image coordinates and region selection alongside the distribution. Matching counts can support a limited value-frequency comparison, but the count array alone cannot show whether bright pixels form a vertical edge, a horizontal edge or disconnected spots.
Compare two fictional arrays
Consider two invented two-by-two grayscale arrays. A has rows [0,255] and [0,255], forming a vertical bright column. B has rows [0,0] and [255,255], forming a horizontal bright row. Both contain two zero-valued pixels and two pixels with value 255.
Their full discrete intensity histograms are identical, although their layouts differ. No intensity count is missing; spatial arrangement was discarded by the summary. These tiny arrays demonstrate the information boundary rather than a practical resolution or inspection acceptance threshold.
Review the intended inference
Prepare a trial with the original array, selected mask, channel and bin counts. Check whether the proposed question concerns brightness distribution or spatial geometry. If it concerns an edge location or feature arrangement, retain an appropriate spatial representation instead of using the histogram as the entire comparison.
Also record binning: broad bins can combine distinct input intensities even before spatial information is lost. This guide claims no defect-detection function or accuracy for HM equipment. Its decision is whether a frequency summary has been used to answer a question that needs pixel locations.
Customer Questions
Do the fictional arrays have equal histograms?
Yes. Each has two zeros and two values of 255.
Do equal histograms prove equal layout?
No. Their pixel positions can differ.
Does this prescribe an inspection threshold?
No. It identifies the information retained by a summary.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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