# What is the meaning of the distance parameter for line control points?

After fine-tuning control points in Hugin, the distance parameter indicates the correlation of two control points in different images, and is a value between 0 and 1. However, line control points (horizontal/vertical/straight) indicate a distance that is much greater than 1.

What does the distance parameter mean for these line control points?

• Just tried to recreate your problem by stitching to shots. For line control points I get distance values of 300 to 2500, but also those for normal CPs are in the range between 0.4 and 4.4. A correlation value, as you said, (normally) couldn't get bigger than 1; mayby those show the distance (in pixels?) from where hugin expected the point, and line CPs are real pixel distances from one end of the line to the other? (really, just guessing now..)
– smow
Commented Dec 3, 2016 at 23:22

Distance, the distance in pixels between a perfect alignment and the actual alignment achieved by the optimiser. Otherwise, after selecting Fine-tune all Points from the Edit menu, this column shows the correlation between the points (0.0 indicates no correlation and 1.00 indicates 100% correlation) - Typically values over 0.8 show that the image areas around each point of the pair are very similar (an 80% correlation).

also (emphasis is mine):

Clicking Select by Distance allows you to select all the points depending on the value in the Distance column, eg:

• Enter 10 to select all the points with an optimiser error greater than ten pixels.
• Enter -5 to select all points with an optimiser error less than five pixels.
• Enter -0.8 to select all points with a fine-tune correlation less than 80%.

When Hugin gives you a number greater than 1 as distance, it is indeed expressed in pixels. When between 0 and 1, it is a correlation value.

Still, exactly what Hugin means by the distance in pixels between a perfect alignment and the actual alignment achieved by the optimiser isn't very clear, but it's probably processed from actual pixels distance between the real points and their expected corrected positions.