Custom Convolution

Detailed Description

Convolves the input with the client supplied convolution matrix.

The client can supply a vx_int16 typed convolution matrix \( C_{m,n} \). Outputs will be in the VX_DF_IMAGE_S16 format unless a VX_DF_IMAGE_U8 image is explicitly provided. If values would have been out of range of U8 for VX_DF_IMAGE_U8, the values are clamped to 0 or 255.

\begin{eqnarray} k_0 &=& \frac{m}{2} \\ l_0 &=& \frac{n}{2} \\ sum &=& \sum_{k=0,l=0}^{k=m-1,l=n-1} input(x+k_0-k, y+l_0-l) C_{k,l} \end{eqnarray}

Note

The above equation for this function is different than an equivalent operation suggested by the OpenCV Filter2D function.

This translates into the C declaration:

    // A horizontal Scharr gradient operator with different scale.
    vx_int16 gx[3][3] = {
        {  3, 0, -3},
        { 10, 0,-10},
        {  3, 0, -3},
    };
    vx_uint32 scale = 9;
    vx_convolution scharr_x = vxCreateConvolution(context, 3, 3);
    vxAccessConvolutionCoefficients(scharr_x, NULL);
    vxCommitConvolutionCoefficients(scharr_x, (vx_int16*)gx);
    vxSetConvolutionAttribute(scharr_x, VX_CONVOLUTION_ATTRIBUTE_SCALE, &scale, sizeof(scale));

For VX_DF_IMAGE_U8 output, an additional step is taken:

\[ output(x,y) = \begin{cases} \cr 0 & \text{if } sum < 0 \cr 255 & \text{if } sum / scale > 255 \cr sum / scale & \text{otherwise} \end{cases} \]

For VX_DF_IMAGE_S16 output, the summation is simply set to the output

\[ output(x,y) = sum / scale \]

The overflow policy used is VX_CONVERT_POLICY_SATURATE.

Functions
vx_node	vxConvolveNode (vx_graph graph, vx_image input, vx_convolution conv, vx_image output)
	[Graph] Creates a custom convolution node. More...
vx_status	vxuConvolve (vx_context context, vx_image input, vx_convolution matrix, vx_image output)
	[Immediate] Computes a convolution on the input image with the supplied matrix. More...

Function Documentation

vx_node vxConvolveNode	(	vx_graph	graph,
		vx_image	input,
		vx_convolution	conv,
		vx_image	output
	)

[Graph] Creates a custom convolution node.

Parameters

[in]	graph	The reference to the graph.
[in]	input	The input image in VX_DF_IMAGE_U8 format.
[in]	conv	The vx_int16 convolution matrix.
[out]	output	The output image in VX_DF_IMAGE_S16 format.

Returns

vx_node.

Return values

0	Node could not be created.
*	Node handle.

vx_status vxuConvolve	(	vx_context	context,
		vx_image	input,
		vx_convolution	matrix,
		vx_image	output
	)

[Immediate] Computes a convolution on the input image with the supplied matrix.

Parameters

[in]	context	The reference to the overall context.
[in]	input	The input image in VX_DF_IMAGE_U8 format.
[in]	matrix	The convolution matrix.
[out]	output	The output image in VX_DF_IMAGE_S16 format.

Returns

A vx_status_e enumeration.

Return values

VX_SUCCESS	Success
*	An error occurred. See vx_status_e.