Learn more about Video and Image Processing Blockset

2-D Variance (Obsolete) - Compute variance of each input matrix

Library

Statistics

Description

Note The 2-D Variance block is obsolete. It may be removed in a future version of the Video and Image Processing Blockset software. Use the replacement block Variance.

The 2-D Variance block computes the variance of each M-by-N input matrix or of a sequence of inputs over time. Use the Running variance check box to choose between the block's basic and running operation.

Port	Input/Output	Supported Data Types	Complex Values Supported
Input / I	Vector or matrix of intensity values	Double-precision floating point Single-precision floating point Fixed point 8-, 16-, and 32-bit signed integer 8-, 16-, and 32-bit unsigned integer	Yes
Rst	Signal that triggers a reset event	Double-precision floating point Single-precision floating point Boolean 8-, 16-, and 32-bit signed integer 8-, 16-, and 32-bit unsigned integer	No
ROI	Rectangle — [r c height width] Lines — [r1 c1 r2 c2], where r1 and c1 are the row and column coordinates of the beginning of the line and r2 and c2 are the row and column coordinates of the end of the line. Binary mask — Binary image matrix that enables you to specify which pixels to highlight.	Rectangles and lines Double-precision floating point Single-precision floating point Boolean 8-, 16-, and 32-bit signed integer 8-, 16-, and 32-bit unsigned integer Binary mask Boolean	No
Label	Matrix where pixels equal to 0 represent the background, pixels equal to 1 represent the first object, pixels equal to 2 represent the second object, and so on.	8-, 16-, and 32-bit unsigned integer	No
Label Numbers	Vector containing the label numbers for the regions for which the block will compute the statistics.	8-, 16-, and 32-bit unsigned integer	No
Output/Out	Without ROI processing — Variance of each M-by-N input matrix or the variance for each element in a sequence of M-by-N inputs. With ROI processing — Vector of separate statistical values for each ROI or a scalar value that represents the statistical value for all specified ROIs.	Same as Input port	Yes
Flag	Boolean value that indicates whether the ROI is within the image bounds or the label number is within the label matrix.	Boolean	No

Length-M 1-D vector inputs are treated as M-by-1 column vectors.

Basic Operation

If you clear the Running variance check box, the block outputs the variance of each M-by-N input matrix. A scalar input generates a zero-valued output.

For purely real or purely imaginary inputs, the variance of a M-by-N matrix is the square of the standard deviation:

For complex inputs, the variance is given by the following equation:

When the input values are double-precision floating point, the equivalent MATLAB code is var(u(:)), where u is the input.

Running Operation

If you select the Running variance check box, the block computes the variance of a sequence of M-by-N inputs.

For example, suppose A is the first input to the block and B is the second and current input to the block, where

and

The block computes the variance,

and outputs

For the next input, the block computes the variance for each element of the first three inputs, and so on.

Resetting the Running Variance

The block resets the running variance whenever a reset event is detected at the optional Rst port. The reset signal rate must be a positive integer multiple of the rate of the data signal input.

You specify the reset event using the Reset port parameter:

None — Disables the Rst port
Rising edge — Triggers a reset operation when the Rst input does one of the following:
- Rises from a negative value to a positive value or 0
- Rises from 0 to a positive value, where the rise is not a continuation of a rise from a negative value to 0 (see the following figure)
Falling edge — Triggers a reset operation when the Rst input does one of the following:
- Falls from a positive value to a negative value or 0
- Falls from 0 to a negative value, where the fall is not a continuation of a fall from a positive value to 0 (see the following figure)

Either edge — Triggers a reset operation when the Rst input is a Rising edge or Falling edge (as described previously)

Non-zero sample — Triggers a reset operation at each sample time that the Rst input is not 0

Note When running simulations in the Simulink MultiTasking mode, reset signals have a one-sample latency. Therefore, when the block detects a reset event, there is a one-sample delay at the reset port rate before the block applies the reset. For more information on latency and the Simulink tasking modes, seeConfiguration Parameters Dialog Box in the Simulink documentation.

ROI Processing

To calculate the statistical value within a particular region of each image, select the Enable ROI processing check box. This option is not available when the block is in running mode.

Use the ROI type parameter to specify whether the ROI is a binary mask, label matrix, rectangle, or line.

A binary mask is a binary image that enables you to specify which pixels to highlight, or select.
In a label matrix, pixels equal to 0 represent the background, pixels equal to 1 represent the first object, pixels equal to 2 represent the second object, and so on. When the ROI type parameter is set to Label matrix, the Label and Label Numbers ports appear on the block. Use the Label Numbers port to specify the objects in the label matrix for which the block calculates statistics. The input to this port must be a vector of scalar values that correspond to the labeled regions in the label matrix.
For more information about the format of the input to the ROI port when the ROI is a rectangle or a line, see the Draw Shapes reference page.

Note For rectangular ROIs, use the ROI portion to process parameter to specify whether to calculate the statistical value for the entire ROI or just the ROI perimeter.

Use the Output parameter to specify the block output. The block can output separate statistical values for each ROI or the statistical value for all specified ROIs. This parameter is not available if, for the ROI type parameter, you select Binary mask.

If, for the ROI type parameter you select Rectangles or Lines, the Output flag indicating if ROI is within image bounds check box appears in the dialog box. If you select this check box, the Flag port appears on the block. The following tables describe the Flag port output based on the block parameters.

Output = Individual Statistics for Each ROI

Flag Port Output	Description
0	ROI is completely outside the input image.
1	ROI is completely or partially inside the input image.

Output = Single Statistic for All ROIs

Flag Port Output	Description
0	All ROIs are completely outside the input image.
1	At least one ROI is completely or partially inside the input image.

If the ROI is partially outside the image, the block only computes the statistical values for the portion of the ROI that is within the image.

If, for the ROI type parameter you select Label matrix, the Output flag indicating if input label numbers are valid check box appears in the dialog box. If you select this check box, the Flag port appears on the block. The following tables describe the Flag port output based on the block parameters.

Output = Individual Statistics for Each ROI

Flag Port Output	Description
0	Label number is not in the label matrix.
1	Label number is in the label matrix.

Output = Single Statistic for All ROIs

Flag Port Output	Description
0	None of the label numbers are in the label matrix.
1	At least one of the label numbers is in the label matrix.

Fixed-Point Data Types

The following diagram shows the data types used in the 2-D Variance block for fixed-point signals.

The results of the magnitude squared calculations in the preceding diagram are in the product output data type. You can set the accumulator, product output, and output data types in the dialog box.

Dialog Box

The Main pane of the 2-D Variance dialog box appears as shown in the following figure.

Running variance: Select this check box to enable the block's running operation.
Reset port: Determines the reset event that causes the block to reset the running variance. The reset signal rate must be a positive integer multiple of the rate of the data signal input. This parameter is visible only if you select the Running variance check box.
Enable ROI processing: Select this check box to calculate the statistical value within a particular region of each image. This parameter is not available when the block is in running mode.
ROI type: Specify the type of ROI to use. Your choices are Rectangles, Lines, Label matrix, or Binary mask.
ROI portion to process: Specify whether to calculate the statistical value for the entire ROI or just the ROI perimeter. This parameter is only visible if, for the ROI type parameter, you specify Rectangles.
Output: Specify the block output. The block can output a vector of separate statistical values for each ROI or a scalar value that represents the statistical value for all the specified ROIs. This parameter is not available if, for the ROI type parameter, you select Binary mask.
Output flag indicating if ROI is within image bounds: If you select this check box, the Flag port appears on the block. For a description of the Flag port output, see the tables in ROI Processing. This parameter is visible if, for the ROI type parameter, you select Rectangles or Lines.
Output flag indicating if label numbers are valid: If you select this check box, the Flag port appears on the block. For a description of the Flag port output, see the tables in ROI Processing. This parameter is visible if, for the ROI type parameter, you select Label matrix.

The Fixed-point pane of the 2-D Variance dialog box appears as shown in the following figure:

Rounding mode

Select the rounding mode for fixed-point operations.

Overflow mode

Select the overflow mode for fixed-point operations.

Note Refer to Fixed-Point Data Types for more information on how the product output, accumulator, and output data types are used in this block.

Input-squared product

Use this parameter to specify how to designate the input-squared product word and fraction lengths:

When you select Same as input, these characteristics match those of the input to the block.
When you select Binary point scaling, you can enter the word length and the fraction length of the input-squared product, in bits.
When you select Slope and bias scaling, you can enter the word length, in bits, and the slope of the input-squared product. This block requires power-of-two slope and a bias of 0.

Input-sum-squared product

Use this parameter to specify how to designate the input-sum-squared product word and fraction lengths:

When you select Same as input-squared product, these characteristics match those of the input-squared product.
When you select Binary point scaling, you can enter the word length and the fraction length of the input-sum-squared product, in bits.
When you select Slope and bias scaling, you can enter the word length, in bits, and the slope of the input-sum-squared product. This block requires power-of-two slope and a bias of 0.

Note To compute the required fixed-point settings for this parameter, pick your brightest image and sum all the pixel values across the image. Then, square the value. Specify a word length and fraction length so that the resulting value fits within the Input-sum-squared product data type without overflow. This specification might require picking a large scaling factor (LSB weight , L>1), or, in other words, setting a negative fraction length.

Accumulator

Use this parameter to specify the accumulator word and fraction lengths resulting from a complex-complex multiplication in the block:

When you select Same as input-squared product, these characteristics match those of the input-squared product.
When you select Same as input, these characteristics match those of the input to the block.
When you select Binary point scaling, you can enter the word length and the fraction length of the accumulator, in bits.
When you select Slope and bias scaling, you can enter the word length, in bits, and the slope of the accumulator. This block requires power-of-two slope and a bias of 0.

Output

Choose how to specify the output word length and fraction length:

When you select Same as input, these characteristics match those of the input to the block.
When you select Binary point scaling, you can enter the word length and the fraction length of the output, in bits.
When you select Slope and bias scaling, you can enter the word length, in bits, and the slope of the output. This block requires power-of-two slope and a bias of 0.

Lock scaling against changes by the autoscaling tool

Select this parameter to prevent any fixed-point scaling you specify in this block mask from being overridden by the autoscaling tool in the Fixed-Point Tool. For more information, see fxptdlg, a reference page on the Fixed-Point Tool in the Simulink documentation.

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