Why Does OpenCV's Point(x,y) Use (Column, Row) Instead of (Row, Column)?

Point(x,y) Representation in OpenCV

OpenCV's Point(x,y) represents the pixel location using (row,column) coordinates. However, it is often expected to use (column,row) instead.

Reason for the Difference

OpenCV uses row-major-order for matrices, similar to mathematical conventions where row index comes first. Conversely, images typically use column-major-order, where column index comes first in axis ordering.

Interpretation

To clarify the difference:

• src.at(i,j): This notation uses (row,column) ordering, where i is the row number and j is the column number.
• Point(x,y): Here, x represents the column number and y represents the row number.

Example

Consider an image with 300 rows and 200 columns. The pixel at the top-left corner has:

• Matrix coordinates: (0,0)
• Point coordinates: (0,0)

The pixel at the bottom-right corner has:

• Matrix coordinates: (299,199)
• Point coordinates: (199,299)

Conclusion

Understanding the representation of Point(x,y) in OpenCV is crucial for effectively accessing image pixels. By keeping these conventions in mind, developers can avoid confusion and ensure correct image manipulation operations.

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