IMAQ ROI line grid size

[Sable]

Hi,

The minimum ROI step size when drawing a line is one pixel (ROI Grid Size property).

Is there a way to adjust the ROI grid size for subpixel resolution step size?

- Sable

[Bob_Schor]

Pictures are made of pixels.  You cannot (easily) divide a pixel, so what will you do with sub-pixel resolution?

Note that if an image is 100 x 100 pixels, and you compute an ROI that's 2.3 x 4.7 "pixels" in size, you could have as many as 3 x 5 = 15 pixels "inside" it (assuming it isn't tilted), or as few as 2 x 4 = 8 pixels.  The problem comes when you want to visualize it -- it has to take on "integer" values, so the height will be either 2 or 3, the width either 4 or 5, a bit of a mess (but you could write your own sub-pixel code, I suppose ...).

Bob Schor

[Sable]

I want to measure the length of an item on the image. My thoughts are subpixel resolution would yield better accuracy than single pixels.

For example if the actual length of an item takes 3.5 pixels in the image, the current ROI measures 3 (or) 4 pixels because of integer values.

- Sable

[udka]

-How are you calculating the length?
-If by using simple edge detection or Caliper or Clamp will have Subpixel accuracy in detecting the edges.
-Drawing ROI cannot be taken as Subpixel values.
-If possible upload sample images and Code

[Sable]

This is a sample image.

Just use the attached vi to load the image to IMAQ.

Draw a ROI line against the ruler scale to calibrate the length to 10 cm (example) - assume the ruler is not tilted or rotated which will be the actual scenario.

Then use that length to measure an object on the same image (Note: object is not present on this image).

- Sable

[udka]

-Converting Pixels to Real world Values is known as Spatial Calibration.
-You can check examples included in LabVIEW->Find Examples->Search for Calibration.
-I Suggest you to go through Vision Basics-> http://zone.ni.com/reference/en-XX/help/370281AE-01/TOC33.htm
-As mentioned earlier you cannot draw ROI with Subpixel Accuracy(It is considered only during calculation Step)
-Please go through this link for indepth on Calibration: http://zone.ni.com/reference/en-XX/help/370281AE-01/TOC5.htm