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# Generate Code to Detect Edges on Images

This example shows how to generate a standalone C library from MATLAB® code that implements a simple Sobel filter that performs edge detection on images. The example also shows how to generate and test a MEX function in MATLAB prior to generating C code to verify that the MATLAB code is suitable for code generation.

### About the `sobel` Function

The `sobel.m` function takes an image (represented as a double matrix) and a threshold value and returns an image with the edges detected (based on the threshold value).

`type sobel`
```% edgeImage = sobel(originalImage, threshold) % Sobel edge detection. Given a normalized image (with double values) % return an image where the edges are detected w.r.t. threshold value. function edgeImage = sobel(originalImage, threshold) %#codegen assert(all(size(originalImage) <= [1024 1024])); assert(isa(originalImage, 'double')); assert(isa(threshold, 'double')); k = [1 2 1; 0 0 0; -1 -2 -1]; H = conv2(double(originalImage),k, 'same'); V = conv2(double(originalImage),k','same'); E = sqrt(H.*H + V.*V); edgeImage = uint8((E > threshold) * 255); ```

### Generate the MEX Function

Generate a MEX function using the `codegen` command.

`codegen sobel`
```Code generation successful. ```

Before generating C code, you should first test the MEX function in MATLAB to ensure that it is functionally equivalent to the original MATLAB code and that no run-time errors occur. By default, `codegen` generates a MEX function named `sobel_mex` in the current folder. This allows you to test the MATLAB code and MEX function and compare the results.

### Read in the Original Image

Use the standard `imread` command.

```im = imread('hello.jpg'); image(im);```

### Convert Image to a Grayscale Version

Convert the color image (shown above) to an equivalent grayscale image with normalized values (0.0 for black, 1.0 for white).

`gray = (0.2989 * double(im(:,:,1)) + 0.5870 * double(im(:,:,2)) + 0.1140 * double(im(:,:,3)))/255;`

### Run the MEX Function (The Sobel Filter)

Pass the normalized image and a threshold value.

`edgeIm = sobel_mex(gray, 0.7);`

### Display the Result

```im3 = repmat(edgeIm, [1 1 3]); image(im3);```

### Generate Standalone C Code

`codegen -config coder.config('lib') sobel`
```Code generation successful. ```

Using `codegen` with the `-config coder.config('lib')` option produces a standalone C library. By default, the code generated for the library is in the folder `codegen/lib/sobel/`.

### Inspect the Generated Function

`type codegen/lib/sobel/sobel.c`
```/* * File: sobel.c * * MATLAB Coder version : 5.6 * C/C++ source code generated on : 03-Mar-2023 04:58:50 */ /* Include Files */ #include "sobel.h" #include "conv2AXPYSameCMP.h" #include "sobel_data.h" #include "sobel_emxutil.h" #include "sobel_initialize.h" #include "sobel_types.h" #include <math.h> /* Function Declarations */ static void binary_expand_op(emxArray_real_T *in1, const emxArray_real_T *in2); /* Function Definitions */ /* * Arguments : emxArray_real_T *in1 * const emxArray_real_T *in2 * Return Type : void */ static void binary_expand_op(emxArray_real_T *in1, const emxArray_real_T *in2) { emxArray_real_T *b_in1; const double *in2_data; double *b_in1_data; double *in1_data; int aux_0_1; int aux_1_1; int b_loop_ub; int i; int i1; int loop_ub; int stride_0_0; int stride_0_1; int stride_1_0; int stride_1_1; in2_data = in2->data; in1_data = in1->data; emxInit_real_T(&b_in1, 2); if (in2->size[0] == 1) { loop_ub = in1->size[0]; } else { loop_ub = in2->size[0]; } i = b_in1->size[0] * b_in1->size[1]; b_in1->size[0] = loop_ub; if (in2->size[1] == 1) { b_loop_ub = in1->size[1]; } else { b_loop_ub = in2->size[1]; } b_in1->size[1] = b_loop_ub; emxEnsureCapacity_real_T(b_in1, i); b_in1_data = b_in1->data; stride_0_0 = (in1->size[0] != 1); stride_0_1 = (in1->size[1] != 1); stride_1_0 = (in2->size[0] != 1); stride_1_1 = (in2->size[1] != 1); aux_0_1 = 0; aux_1_1 = 0; for (i = 0; i < b_loop_ub; i++) { for (i1 = 0; i1 < loop_ub; i1++) { double b_in1_tmp; double in1_tmp; in1_tmp = in1_data[i1 * stride_0_0 + in1->size[0] * aux_0_1]; b_in1_tmp = in2_data[i1 * stride_1_0 + in2->size[0] * aux_1_1]; b_in1_data[i1 + b_in1->size[0] * i] = in1_tmp * in1_tmp + b_in1_tmp * b_in1_tmp; } aux_1_1 += stride_1_1; aux_0_1 += stride_0_1; } i = in1->size[0] * in1->size[1]; in1->size[0] = b_in1->size[0]; in1->size[1] = b_in1->size[1]; emxEnsureCapacity_real_T(in1, i); in1_data = in1->data; loop_ub = b_in1->size[1]; for (i = 0; i < loop_ub; i++) { b_loop_ub = b_in1->size[0]; for (i1 = 0; i1 < b_loop_ub; i1++) { in1_data[i1 + in1->size[0] * i] = b_in1_data[i1 + b_in1->size[0] * i]; } } emxFree_real_T(&b_in1); } /* * Arguments : const emxArray_real_T *originalImage * double threshold * emxArray_uint8_T *edgeImage * Return Type : void */ void sobel(const emxArray_real_T *originalImage, double threshold, emxArray_uint8_T *edgeImage) { emxArray_real_T *H; emxArray_real_T *V; double *H_data; double *V_data; int k; int loop_ub; unsigned char *edgeImage_data; if (!isInitialized_sobel) { sobel_initialize(); } /* edgeImage = sobel(originalImage, threshold) */ /* Sobel edge detection. Given a normalized image (with double values) */ /* return an image where the edges are detected w.r.t. threshold value. */ emxInit_real_T(&H, 2); conv2AXPYSameCMP(originalImage, H); H_data = H->data; emxInit_real_T(&V, 2); b_conv2AXPYSameCMP(originalImage, V); V_data = V->data; if ((H->size[0] == V->size[0]) && (H->size[1] == V->size[1])) { loop_ub = H->size[0] * H->size[1]; for (k = 0; k < loop_ub; k++) { H_data[k] = H_data[k] * H_data[k] + V_data[k] * V_data[k]; } } else { binary_expand_op(H, V); H_data = H->data; } emxFree_real_T(&V); loop_ub = H->size[0] * H->size[1]; for (k = 0; k < loop_ub; k++) { H_data[k] = sqrt(H_data[k]); } k = edgeImage->size[0] * edgeImage->size[1]; edgeImage->size[0] = H->size[0]; edgeImage->size[1] = H->size[1]; emxEnsureCapacity_uint8_T(edgeImage, k); edgeImage_data = edgeImage->data; for (k = 0; k < loop_ub; k++) { edgeImage_data[k] = (unsigned char)((unsigned int)(H_data[k] > threshold) * 255U); } emxFree_real_T(&H); } /* * File trailer for sobel.c * * [EOF] */ ```