How can i convert diameter of irregular object in pixel to mm, without drawing line again and again.. also i am not avaliable with the true height of the image in mm...???

 conversion from pixels to mm

Answer 1

[Image Analyst]

Ask regionprops for "EquivDiameter" - the equivalent circular diameter - to get the diameter as if your blob were a circle.

Then see my attached spatial calibration demo.

[Image Analyst]

Savy's "Answer" moved here:

Sir.. i have already calculated diameter and also referred your spatial calibration demo but i want to ask is their any way to automate the process of spatial calibration ( so that i don't have to draw line again and again as their are large number of images)... thanks in advance..

[Image Analyst]

If you know the spatial calibration factor in advance, just use it. If you don't know it, then how are you going to find it out? If there is some object of known dimensions in the image, then you can find it and calibrate off that. If the factor is in the header/metadata of the image, then you can use that. If they're just random snapshots that thousands of people around the planet took, then you're going to have to figure out somehow, like by drawing lines and giving distance estimates.

[Savy Gulati]

As, mmPerPixel = distanceInMM / distance;

i don't know the distanceInMM of the image.

[Image Analyst]

Then you're out of luck. I can't tell whether that photo of the moon you're looking at is the real moon from a telescope image, or a digital photo of a poster in your room. The real distance is vastly different in both cases even though the image looks the same.