Area measurement

[Tracey Jones]

Hi,
I'm wondering if there is anyone who has advice on using image J to measure
area. We have just purchase a scion image grabber but using our own camera,
and cant seem to calibrate the programme to measure area accurately. I can
get it to measure accurately in a straight line but not to measure area
properly, which is unfortunate as that’s the reason we have it! The only
reason I can see that it would not measure a square as even on all sides to
give an accurate area is that the pixel scale is different horizontally
from vertically. I cant see any evidence of this though or any option to
change this if it is so.

Kind Regards,

Tracey Jones

[Joel Sheffield]

If you look at Image>properties, you can specify the pixel
dimensions.  I assume that this will then correct the measurements.

There is also an option in Analyze>Set Scale that asks for the pixel
aspect ratio.

Joel


Date sent:       Thu, 6 Apr 2006 19:53:39 -0400
Send reply to:   ImageJ Interest Group <[hidden email]>
From:           Tracey Jones <[hidden email]>
Subject:         Area measurement
To:             [hidden email]

> Hi,
> I'm wondering if there is anyone who has advice on using image J to
> measure area. We have just purchase a scion image grabber but using
> our own camera, and cant seem to calibrate the programme to measure
> area accurately. I can get it to measure accurately in a straight line
> but not to measure area properly, which is unfortunate as that’s the
> reason we have it! The only reason I can see that it would not measure
> a square as even on all sides to give an accurate area is that the
> pixel scale is different horizontally from vertically. I cant see any
> evidence of this though or any option to change this if it is so.
>
> Kind Regards,
>
> Tracey Jones

Joel B. Sheffield, Ph.D
Department of Biology
Temple University
Philadelphia, PA 19122
Voice: 215 204 8839
e-mail: [hidden email]

[Corentin Cras-Méneur-2]

Le 06 avr. 2006 à 18:53, Tracey Jones a écrit :

> Hi,
> I'm wondering if there is anyone who has advice on using image J to  
> measure
> area. We have just purchase a scion image grabber but using our own  
> camera,
> and cant seem to calibrate the programme to measure area  
> accurately. I can
> get it to measure accurately in a straight line but not to measure  
> area
> properly, which is unfortunate as that’s the reason we have it! The  
> only
> reason I can see that it would not measure a square as even on all  
> sides to
> give an accurate area is that the pixel scale is different  
> horizontally
> from vertically. I cant see any evidence of this though or any  
> option to
> change this if it is so.

We are using ImageJ for that here. We calibrated the acquisition and  
use thresholding to select the areas we are interested in. We can  
then simply measure the binary mask resulting from the thresholing.
Not very complicated :-)

Corentin

[Robert Baer]

Others have already mentioned Image | Set Scale and using the pixel aspect
ratio.

I do know the Scion image grabber but I presume it is grabbing a 'video
image' from a scanning camera using either NTSC standard or PAL standard.
The grabber software may let you set the size of the image you want to grab
to 320 x 240 or 640 x 480.  It probably also will let you set the size of
the 'grabbed image at 352x288 (called CIF) or 704 x 576 (called 4CIF).

You will note that the first two have a 4:3 aspect ratio (w/h=1.3333)
whereas the second two (CIF and 4CIF) have a w/h aspect aspect ratio of
1.222. You should generally choose CIF or 4CIF at capture time if you want
square pixels when you open the saved image in imgeJ.  If, however,  you
choose 320x240 or 640x480 at capture time you stretch the image in one
direction.  The pixel aspect ratio you need to enter in Image | Set Scale |
Pixel Aspect ratio is therefore 1.2222/1.33333 to re-square your pixels.

The other potential 'gottcha' that comes with frame grabbers is what is (I
believe) referred to as as raster overscanning.  Video camera's scan a wider
field than video monitors typically display.  This means that the 'true
image' extends beyond the edges of the display screen by a fixed amount.
Some video frame grabber cards have a provision to capture either the
'camera' image or the 'video' image.  This needs to be properly tuned for
you setup to assure square pixels on the saved image.

HTH,

Rob Baer


____________________________
Robert W. Baer, Ph.D.
Associate Professor
Department of Physiology
A. T. Still University of Health Science
800 W. Jefferson St.
Kirksville, MO 63501-1497 USA
----- Original Message -----
From: "Tracey Jones" <[hidden email]>
To: <[hidden email]>
Sent: Thursday, April 06, 2006 6:53 PM
Subject: Area measurement


Hi,
I'm wondering if there is anyone who has advice on using image J to measure
area. We have just purchase a scion image grabber but using our own camera,
and cant seem to calibrate the programme to measure area accurately. I can
get it to measure accurately in a straight line but not to measure area
properly, which is unfortunate as that's the reason we have it! The only
reason I can see that it would not measure a square as even on all sides to
give an accurate area is that the pixel scale is different horizontally
from vertically. I cant see any evidence of this though or any option to
change this if it is so.

Kind Regards,

Tracey Jones