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Example 38  Specifying active area size of the image sensor

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Example 38  Specifying active area size of the image sensor

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Example 38  Specifying active area size of the image sensor

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The Image sensor size in VideoCAD implies the size of the entire light sensitive area of the image sensor, which can form an image with the maximum number of pixels horizontally and vertically.

 

Active area of the image sensor in VideoCAD implies an area of the ​​image sensor on which the image is formed in a given mode of the camera. The sizes of the active area may be less than or equal to the image sensor size. Active area can differ in different modes of operation of the same camera.

 

 

Exactly the active area size and lens focal length determines the angles of view, and through them influences on the results of camera modeling. Accuracy of specifying the active area size greatly affects the accuracy of modeling the camera.

 

       The traditional method of specifying the active area size of the image sensor

 

In the past, specifying the active area size was a simple task. Each camera specification includes the image sensor format (type) in "vidicon" inches from the standard series (1/4 ", 1/3", 1/2 "..) which determines the size of image sensor. Although the translation of the format to the size is not accurate, the accuracy was acceptable for most practical purposes.

 

Native Aspect ratio of image sensors used in CCTV cameras was 4:3. When camera's output image had the same aspect ratio of 4:3, the active area occupied the entire image sensor. To get image with the Aspect ratio of 16:9 only central area across the width of the image sensor was used, and areas on the top and bottom of the image sensor was not used. Reducing the number of pixels of the output image had no affect on the active area size.

 

 

In such circumstances, to uniquely specify the active area of the image sensor it was enough to choose in the Image sensor size box the Image sensor format from a list of standard formats (1/4 ", 1/3", 1/2 "..) and choose the Aspect ratio of the output image in the Aspect ratio box (4:3, 16:9 ...).

 

 

Now this way might be used for specifying active area size of the image sensor of cameras of previous generations, or in cases when accuracy of the simulation is not important.

For modern cameras, this method is not sufficiently accurate. An error may exceed 20-30%.

 

Image sensors with non-standard sizes have appeared, with fractional inch formats that do not fall into the list of the standard formats ​​(eg, 1/2.8", 1/2.9", 1/3.2", 1/1.9" ..).
Image sensors with aspect ratio different from 4:3 have appeared.
In specifications of some cameras the sensor size is indicated more accurately in the form of diagonal length or side lengths, in millimeters.
Cameras can form images with different numbers of pixels and aspect ratios.
Some cameras in some modes use active area of the image sensor which does not touch the edges of the image sensor.

 

 

       Examples of errors during specifying size of the active area of image sensor by the traditional way

 

Example 1:  A camera has an image sensor of 1/3" with aspect ratio of 16:9, but the output image has aspect ratio of 4:3.

 

The Image sensor format (1/3") determines the length of diagonal (6mm). Aspect ratio of any image sensor in the traditional way is always assumed to be 4:3. Therefore, in camera simulation the active area with diagonal of 6 mm and aspect ratio of 4:3 will be used, ie 4,8 * 3,6mm.

 

However, the real aspect ratio of the image sensor is 16:9, so the actual size of the image sensor with diagonal of 6mm will be 2,94mm to 5,23mm.

 

At the same time, the aspect ratio of the output image (=aspect ratio of the active area) is 4:3, so that the active area is formed by ''cutting' side areas from the Image sensor of 16:9 laterally. Therefore, the actual size of the active area is 3.92 * 2,94mm.

 

Thus, the actual size of the active area image sensor, and hence the field of view will be 20% less than the calculated values.

 

Example 2: Some cameras can produce images with reduced number of pixels using the active area in the central part of the image sensor. The active area does not touch the edges of the image sensor. The size of the active area may depend on the number of pixels of the output image.

 

For example, the same image sensor SONY Exmor R IMX291LQR has format 1/2.8"(diagonal 6.46 mm) in the full HD1080 mode and 1/4.2" (diagonal 4.31 mm) in the HD720 mode. In the HD720 mode the image sensor uses only the central part of pixels.

 

If you use format 1/2.8" in the HD720 mode, the actual size of the field of view will be 33% less than calculated one.

 

 

Problem

 

For accurate camera modeling we must specify the sizes of the active area of the image sensor in a given mode of the camera.

 

Order of work

 

1. If side sizes of the active area of the image sensor in a given camera mode are known, then enter them directly.

 

       Direct specifying side sizes of active area of the image sensor

 

If you know the side sizes of active area of the image sensor in a given mode of the camera, simply enter them in the Image sensor size box separated by asterisk in the form W*H. Where W and H - width and height of the active area of the image sensor in millimeters.

For example: 4.8*3.6

Thus the size of the active area image sensor was specified and the problem is solved.

 

 

However, in most cases, camera specifications don't give the side sizes of the active area for each camera mode separately, so the program will have to calculate them. For the calculation it is necessary to set the following parameters, are known in the most cases:

 

Image sensor size (it is better to set side sizes in millimeres, but possible via the length of diagonal (format, type) and aspect ratio of the image sensor).
Aspect ratio of the output image.
Crop - a factor of decreasing active area size. The Crop is provided for cases when the active area does not touch the edges of the image sensor. In other cases Crop=1.

 

On the basis of this information, VideoCAD will calculate size of the active area of the image sensor, which will be used in camera modeling.

 

You can specify the required parameters in the Image sensor size and the Aspect ratio boxes in the Camera Geometry box, but it is more convenient to do it in the special Sensor and Lens box .

Open the Camera Geometry box, and then clicking on the Sensor and Lens button , open the Sensor and Lens box.

 

2.1.1 If the image sensor side sizes in millimeters are known, then enter them directly.

 

       Direct specifying the image sensor side sizes

 

If the camera specification contains the sensor side sizes (that is the maximum size of the active area), then enter them to the Image sensor size box separated by hash, in the form W#H. Where W and H - width and height in millimeters of the image sensor.

 

For example: For the camera WV-SFV631L the sensor sizes can be found in the string: Scanning area 5.346 mm (H) x 3.336 mm (V)

Enter to the Image sensor size box 5.346#3.336

 

 

2.1.2 When the image sensor side sizes are unknown, specify them via the length of diagonal (format, type) and aspect ratio of the image sensor.

 

       Specifying image sensor size via the length of diagonal and the aspect ratio

 

The length of diagonal in "vidicon" inches as a format (type) almost always exists in camera specification. For example, 1/2", 1/2.9". Enter the format to the Image sensor size box. In the box you can choose format from the list ​​or type any value, for example, 1/3.15", 1/2.75". If you enter a value which not found in the standard list, the Image sensor size box turns yellow and VideoCAD will calculate diagonal from the format.

 

If a specifications give diagonal length in millimeters, it is preferable to use it. The value in millimeters is usually more accurate.The diagonal length in millimeters must be typed with the letter d, for example d6.0 - the length of diagonal is 6mm.

 

For example: For the camera WV-SFV631L the image sensor format can be found in the line: Image Sensor Approx. 1/3 type;

In the specification of the image sensor SONY Exmor R IMX291LQR the length of diagonal line can be obtained from the line: Diagonal 6.46 mm (Type 1/2.8)

 

To set the size of image sensor through the format or length of diagonal you must also specify the aspect ratio of the image sensor, since Image sensors with the same diagonal, but different aspect ratios will have different sizes.

One should distinguish between the aspect ratio of the image sensor and the aspect ratio of the output image.

Aspect ratio of the output image is the ratio of sides of the image produced by the camera in the given mode. It equals to the ratio of sides of the active area of image sensor in the given mode.

The aspect ratio of the output image can be different from the aspect ratio of image sensor. A camera with the same image sensor in different modes can produce images with different aspect ratios.

 

 

You can learn the aspect ratio of image sensor by several ways:

 

1. From the camera specification.

 

For example: In the specification of the camera WV-SFV631L sensor sizes can be found from the string: Scanning area 5.346 mm (H) x 3.336 mm (V)

Aspect ratio of the image sensor: 5.346 / 3.336 = 1.60 = 16:10

 

2. You can learn the aspect ratio of the image sensor visually, if you remove the camera lens. To determine the aspect ratio of the image sensor, photograph it and calculate the ratio of the number of pixels in the image of the image sensor vertically and horizontally.

 

 

3. It is possible to calculate a ratio of the maximum number of horizontal pixels to the maximum number of vertical pixels in all possible modes of the camera.

 

For example, a camera can produce images with the following number of pixels:

1920 x 1080, 1280 x 720, 640 x 360, 320 x 180, 160 x 90

1600 x 1200, 1280 x 960, 800 x 600, 640 x 480, 400 x 300, 320 x 240, 160 x120

 

The maximum number of horizontal pixels - 1920, the maximum number of vertical pixels - 1200. Expected aspect ratio of the image sensor 1920/1200=1,6=16:10

 

Enter the obtained values of the diagonal (format) and the aspect ratio of the image sensor to the Image sensor size box in the Camera Geometry separated by space character. If the aspect ratio of the image sensor is 4:3, it can be omitted.

For example:

1/3"

1/3" 16:9

d6.0 16:9

 

 

Or enter the diagonal (format) and the aspect ratio of the image sensor separately to the Image sensor>Size and Image sensor>Aspect ratio in the Sensor and Lens box.

 

2.2 Then you need to specify the aspect ratio of the output image.

 

       Specifying aspect ratio of the output image

 

The Aspect ratio of the output image equals to the aspect ratio of the active area of the image sensor. It equals to the ratio of number of pixels of the output image horizontally and vertically in the given camera mode. The aspect ratio of the output image can be different from the aspect ratio of image sensor. A camera with the same image sensor in different modes can produce images with different aspect ratios.

 

For examples for the following number of pixels:

 

1920 / 1080=1.78=16:9. In the mode 1920 x 1080 the aspect ratio of the output image = 16:9

1600 / 1200=1.33=4:3.  In the mode 1600 x 1200 the aspect ratio of the output image = 4:3

 

Enter the Aspect ratio of the output image to the Aspect ratio box in the Camera Geometry.

 

 

It is convenient to use the Image sensor calculator, which can be invoked by the Calculator button in the Sensor and Lens box.

 

 

2.3 If it is possible that the active area does not touch the edges of the image sensor, you must set the crop factor.

 

       Specifying the Crop factor

 

Crop is a factor of decreasing active area size provided for case when the active area does not touch the edges of the image sensor. In other cases Crop=1.

 

For example, the same image sensor has format 1/2.8"(diagonal 6.46 mm) in the full HD1080 mode and 1/4.2" (diagonal 4.31 mm) in the HD720 mode. In the HD720 mode the image sensor uses only the central part of pixels.

 

 

When setting the crop factor one should be guided by the rules:

 

1. If, in a given mode, the camera uses the maximum number of pixels of at least one dimension (horizontal and (or) vertical), it is used the whole width or height of the image sensor, then the Crop factor equals one.

 

For example, a camera can produce images with the following number of pixels:

1920 x 1080, 1280 x 720, 640 x 360, 320 x 180, 160 x 90

1600 x 1200, 1280 x 960, 800 x 600, 640 x 480, 400 x 300, 320 x 240, 160 x120

 

The maximum number of horizontal pixels - 1920, the maximum number of vertical pixels - 1200. The modes with maximum numbers of pixels: 1920 x 1080 and 1600 x 1200.

 

2. If in the given camera mode not the maximum number of pixels is used, it is necessary to get know how the camera produces images with less than the maximum number of pixels. The 3 ways are possible:

 

1. By resizing the image with the maximum number of pixels of at least one of the direction.

2. Using only the central part of the image sensor, do not touch sides of image sensor.

3. Simultaneous use of 1 and 2 ways. The intermediate image is produced using the center area of ​​the image sensor, and then the resulting image is resized.

 

In the first case, the size of the active area of the image sensor is not dependent on the number of pixels of the output image, therefore the Crop = 1.

In the second and third cases, the size of the active area of the image sensor decreases with the number of pixels of the output image.

 

First of all it is necessary to search the specification information whether the image sensor size (format) or angles of view depend on the number of pixels of the output image. If such information is not available, it is likely that the Crop = 1. But if there is a suspicion that the camera reduces the size of the active area, you can check the camera practically:

 

1. Display the image from the camera in the mode with the maximum number of pixels of the output image.

2. Switch to the mode with reduced number of pixels and note whether the size of the field of view is reduced.

 

If the size of the field of view is not reduced or it is reduced only in one direction, then the Crop = 1.

If the size of the field of view is reduced in both directions in proportion of reduction in the number of pixels of the output image, it is necessary to separately calculate the horizontal and vertical ratio of the number of pixels in the given camera mode to the maximum number of pixels in appropriate direction. Crop equals to the minimum of these ratios.

 

For example, if the camera has modes of 1920 x 1080 and 1280 x 720, and the image in 1280 x 720 mode is formed by a central area of the image sensor, then the Crop = 720/1080 = 1920/1280, or 0.67.

 

If the size of the field of view is reduced in both directions less than the reduction of the number of pixels of the output image, it is necessary to separately calculate the horizontal and vertical ratios of the field of view size in the given mode of the camera to the size  the field of view in the mode with the maximum number of pixels. Crop equals to the minimum of these relations.

 

As you can see, in the most cases the Crop = 1. But on those rare occasions when the Crop <> 1 ignoring this fact can result to significant error. To calculate the crop factor it is convenient to use the Image sensor calculator that can be invoked by clicking on the Calculator  button in the Sensor and Lens box.

 

Enter the Crop  to the Image sensor's active area> Crop box in the Sensor and Lens box. You can enter a number with decimal point or ratio separated by slash /.

 

For example:

0.67

720/1080

 

 

You can enter the Crop to the Aspect ratio box in the Camera Geometry separated by space after the aspect ratio of the output image.

 

For example: 16:9 0.67

 

 

 

 

As a result of specifying all parameters, on the image in the Sensor and lens box the aspect ratios, relative position and size of the active area on the image sensor will be schematically displayed.

 

Examples of image sensor images of real cameras:

 

Aspect ratio of Image sensor

4:3

16:9

4:3

16:9

16:10

16:10

16:9

(1920*1080)

Aspect ratio of Active area of Image sensor

4:3

16:9

16:9

4:3

16:9

4:3

16:9

(1280*720)

Crop

1

1

1

1

1

1

720/1080=0.67

Image

 

In the Image sensor's active area> Hor. Vertical Diag. boxes the calculated sizes of the active area of the image sensor will be displayed.

In the Lens> Calculated angles> Hor. Vertical. Diag. the calculated view angles will be displayed.

 

The sizes of the active area may be adjusted or typed new values.

 

After setting all parameters for recording the result to the active camera, click Save.

 

3. After specifying all parameters it is recommended to verify correctness of the set active area size using the obtained angles of view.

 

       Verifying correctness of the set active area size using camera view angles

 

Sometimes camera specifications include view angles with a lens with known focal length. Considering these angles as a result of practical measurements, it is possible to verify correctness of the set active area size by comparing the angles from specification with the angles calculated by VideoCAD in the Sensor and Lens box>Lens> Calculated angles> Hor. Vertical. Diag. The comparison may confirm the correctness of the obtained size of the active area of the image sensor or show an error. If an error will be detected, please carefully check all parameters you specified.

 

When verifying it is necessary to bear in mind the following:

You need to know which angle is given in the specification of the camera (horizontal, vertical or diagonal) and compare it with the same calculated angle.
You need to know in what mode of the camera the angle is obtained and simulate this particular mode.
When the lens focal length is less than 6mm possible discrepancy between the calculated angles and angles from the specification because of lens distortion.
The difference between the calculated and specification's values of angles within 5% is normal, due to measurement error, error of specifying the size of image sensor and lens focal length.

 

You can measure the view angles practically.

 

See also: About Lens distortion.