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Help using vision assistant to identify LCD defects

I am trying to implement some image recognition on a test machine for an LCD screen, and having trouble recognising defects. Is anyone well versed in vision assistant that may be able to assist? I've attached a template image to this post. The image should display several coloured stripes, but occasionally the screen will malfunction and display completely black or white. I have used a simple color pattern matching step in VA to identify this successfully.

 

But now I need to add some more steps afterwards that can essentially 'scan' the image for any defects. These defects are usually small blemishes like a dead pixel or small crack. The end aim is to have the script return a couple of scores - one that identifies the striped pattern (and should score high), and one that identifies defects (and should score low, indicating none present). 

 

I think a big problem is the quality of the image. Despite using a good camera, the images of the LCD screens are very pixelated and I'm struggling to get any improvement. You'll see this in the template below. I've tried to add pattern matching steps that look for a defect with an ROI over the entire image, but it is picking up false 'defects' that don't actually exist. I also have the ability to change the screen from striped to a block colour, so I thought that may help, but due to the pixelisation VA is still picking up false defects.

 

If anyone is able to help, have a look at the attached images and let me know if you can think of ways to improve the quality/uniformity of the image, and to identify these defects?

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Have you ever heard of Moiré Patterns?  That is what is causing the "stripes" that you are seeing.

 

Bob Schor

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@Bob_Schor wrote:

Have you ever heard of Moiré Patterns?  That is what is causing the "stripes" that you are seeing.

 

Bob Schor


Hi Bob, I've heard of moire patterns but don't know much about them. According to Google, these happen when a partially opaque pattern with transparent gaps is overlaid on a similar pattern. I am struggling to see how this is happening in my situation? Could you explain why taking a photo of an LCD in this way cause moire patterns?

 

And does NI vision assistant have any way of getting rid of these? I have checked the manual but couldn't find anything useful.

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Your "image" is pixelated, and the camera creating an "image" of a pixelated image is also pixelated.  If you imagine a single pixel in your camera looking at the image on the LCD, it might be looking right at an LCD pixel, but one pixel over on the camera, it might be aligned 10% "off" from the next LCD pixel, hence be a little less bright, and the next pixel might be 20% off, and so on.  There are probably ways to deal with this, but that's not my area of expertise ...

 

Bob Schor

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Ways to deal with this that I know of:

 

Approach 1: Better resolution.  If you measure with high enough resolution, the effect disappears as you will have many many pixels on your camera measuring just one pixel on the LCD.

Some combination of:

* Increase the capture resolution of the camera (probably with a new better camera).

* Zoom in a lot more than you do now before imaging

 

Approach 2: Motion

If you can move the camera or the LCD in micro-steps the pattern will shift slightly each time.  You may be able to control this well enough to get good readings on all the stripes at different times.

 

I'm not a vision expert though... take this advice with that grain of salt added.

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