Digital zoom calculations

Image quality, Resolution, Light sensitivity, Signal/noise ratio, etc.
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CCTVguru
Posts: 6
Joined: 14 May 2009, 00:50

Digital zoom calculations

Post by CCTVguru »

If I have a 5MP 1/2" camera with a 20mm lens looking at an object 220m away and I want person ID, how do I calculate this.

Based on and email from Stanislav Utochkin, to get a 2X digital zoom, which to me is a 40mm effective lens, I make the camera 1.2MP.

As this is 1/4 I did some homework :
http://www.dcresource.com/forums/archiv ... -2275.html
A high megapixel is not an effective zoom replacement, but here's the maths to show why.

Let's say you have a 5 megapixel camera.

If you wanted to zoom the image in by 2x, then you would be dividing the number of effective pixels by 4, hence you'd end up with a 1.25 megapixel camera. If you wanted to zoom in by 3x, then you would be dividing the number of effective pixels by 9, hence you'd end up with a 0.56 megapixel image.

So you see, even if you want a 3x zoom, you're dividing the pixel count by nine.

I mean, a high number of pixels is good for reasonable cropping, but not for any serious zooming where you go right into a certain part of the image.
Apparently the distant object will also show any camera shake more dramatically.

Support
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Re: Digital zoom calculations

Post by Support »

Digital zoom 'cuts' and magnifies a part from a whole 5 MP image.

Vertical and horizontal image size (in pixel) should by divided by zoom factor.
For example, 2560x1920 (4.9MP) image after 2х digital zoom makes 1280x960 (1.23MP).
Total Number of pixel should by divided by zoom factor square. For 2x zoom you should divide by 4, 3x zoom -divide by 9 etc.

Digital zoom doesn't increase resolution of image in archive. However in live surveillance the whole 5 MP image can't be placed on the screen with lesser resolution without loss of resolution. But cut 1.2MP image can be placed with native 1.2MP resolution, what seems as increasing resolution on the screen.
Also cut image has less size (in bytes).

CCTVguru
Posts: 6
Joined: 14 May 2009, 00:50

Re: Digital zoom calculations

Post by CCTVguru »

Thanks for the clarification.

Makes sense.

In CCTVcad, If I created a camera of 2592 x 1944, to simulate a 2X digital zoom, would I view it in the 3D viewer and got to IMAGE PARAMETERS / PROCESSING and adjust the resolution there to 1296 x 972 ?

Also as a side note, v6.2 does not seem to give an option of where the "save JPG or BMP" are saved to.
If they are actually being saved, I can't see where.
The same under monitor gives you an option of where

Support
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Re: Digital zoom calculations

Post by Support »

CCTVguru wrote: In CCTVcad, If I created a camera of 2592 x 1944, to simulate a 2X digital zoom, would I view it in the 3D viewer and got to IMAGE PARAMETERS / PROCESSING and adjust the resolution there to 1296 x 972 ?
Yes. It is more convenient than changing camera parameters. Digital zoom is an image processing in essence. But do not forget to change lens focal length.

Here is some information about modeling resolution.

First of all, VideoCAD allows to set different numbers of pixels (Horizontal and vertical) and horizontal resolution (TVL) for each CAMERA MODEL in model database. This feature allows to model image sensor and lens resolution.

In addition you can set different numbers of pixels for each camera in the project on the Processing tab, as output image after processing might have number of pixels differed from camera model. Digital zoom is a particular case of image processing.
On the same tab you can change Sharpness-Smoothing and Compression.

Resultant resolution of 3D image is calculated and modeled on the basis of entered values. Calculated resolution applied for image in archive.

Also on the View tab you can set Horizontal number of pixels on the Screen. As number of pixel on the screen can differ from native image resolution. Vertical number of pixel is calculated using Aspect ratio.

Thus VideoCAD allows to model image resolution very precisely and flexibly, but it demands understanding from an user for effective working.
CCTVguru wrote: Also as a side note, v6.2 does not seem to give an option of where the "save JPG or BMP" are saved to.
If they are actually being saved, I can't see where.
The same under monitor gives you an option of where
I have just checked saving 3D image to BMP and JPG. It is OK.
Some old versions have simular bug, but the bug is fixed in 6.1 version.
If it is possible, see the file C:\Documents and Settings\<user name>\Application Data\CCTVCAD\VideoCAD6\Directories.ini
In the file all open/save directories are stored. See line 'image3d='. May be the line contains nonexistent or inaccessible directory.

Support
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Re: Digital zoom calculations

Post by Support »

VideoCAD 7 has a possibility of simulating digital zoom on the PiP tab of the Image parameter panel. See http://www.cctvcad.com/videocad_help/index.html?pip.htm

Image

Support
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Re: Digital zoom calculations

Post by Support »

New interesting handy tools for modeling resolution in pixels, pixel density and digital ZOOM have been added in VideoCAD10.
3D Test allows you to quickly evaluate how any 3D model from VideoCAD library can look anywhere in space through the active camera.

Image

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