Andrew Seaford

In this example we threshold the image based on the position of the track bar. Then find contours on the image an display the contours as white lines.

#include "stdafx.h"
#include "cv.h"
#include "highgui.h"

// global variables
IplImage* input = NULL;
IplImage* gray = NULL;
int threshold = 100;
CvMemStorage* storage = NULL;

/** trackbar event
 * @param trackbar position
 */
void on_trackbar(int)
{
	
	if (storage == NULL)
	{
		// create storage
		gray = cvCreateImage(cvGetSize(input), 8,1);
		storage = cvCreateMemStorage(0);
	}
	else
	{
		// clear storage
		cvClearMemStorage(storage);
	}

	// convert to gray scale and then threshold
	cvCvtColor(input, gray, CV_BGR2GRAY);
	cvThreshold(gray,gray,threshold,255,CV_THRESH_BINARY);

	// find the edges
	CvSeq* edges = 0;
	cvFindContours(gray, storage, &edges);
	cvZero(gray);
	if (edges)
	{
		// display the edges as whiet lines
		cvDrawContours(gray, edges, cvScalarAll(255),cvScalarAll(255),100);
	}
	cvShowImage("Input", gray);
}

/** main function
 * @param argc arguments
 * @param argv argument values
 * @return exit code
 */
int _tmain(int argc, _TCHAR* argv[])
{
	// open and display input image  
    input = cvLoadImage("test.jpg");  
    cvNamedWindow("Input", CV_WINDOW_AUTOSIZE);  
    cvShowImage("Input", input); 

	// create trackbar callback
	cvCreateTrackbar("Threshold", "Input", &threshold, 255, on_trackbar);
	on_trackbar(0);

	// wait for user
	cvWaitKey(0);

	// garbage collection	
	cvReleaseImage(&input);
	cvDestroyWindow("Input");
	return 0;
}

The input image

Threshold value 100

Threshold value 150

Threshold value 200

The cvThreshold function allows us to reject pixels above or below a set value while keeping the others. In this example the input image is separated into the RGB channels. Then we preform a threshold on the red channel, with a maximum value of 100. The result of this is that all the light areas of the image are removed.

#include "stdafx.h"
#include "cv.h"
#include "highgui.h"

int _tmain(int argc, _TCHAR* argv[])
{
	// open and display input image  
    IplImage* input = cvLoadImage("test.jpg");  
    cvNamedWindow("Input", CV_WINDOW_AUTOSIZE);  
    cvShowImage("Input", input); 

	// create the output images
	IplImage* channel_r = cvCreateImage(cvSize(input->width, input->height), input->depth, 1);
	IplImage* channel_g = cvCreateImage(cvSize(input->width, input->height), input->depth, 1);
	IplImage* channel_b = cvCreateImage(cvSize(input->width, input->height), input->depth, 1);

	// split the image
	cvSplit(input, channel_r, channel_g, channel_b, NULL);
	
	// display image
	cvNamedWindow("Channel R", CV_WINDOW_AUTOSIZE);
	cvShowImage("Channel R", channel_r);
	cvSaveImage("channel_r.png", channel_r);

	// threshold red channel
	IplImage* output = cvCreateImage(cvSize(channel_r->width, channel_r->height), channel_r->depth, 1);
	cvThreshold(channel_r, output, 100,100, CV_THRESH_TRUNC);

	// display the output image
	cvNamedWindow("Output", CV_WINDOW_AUTOSIZE);
	cvShowImage("Output", output);
	cvSaveImage("output.png", output);

	// wait for user
	cvWaitKey(0);

	// garbage collection	
	cvReleaseImage(&input);
	cvDestroyWindow("Input");
	cvReleaseImage(&channel_r);
	cvDestroyWindow("Channel R");
	cvReleaseImage(&output);
	cvDestroyWindow("Output");
	return 0;
}

Input Image

The red channel, before threshold

The red channel, after threshold

To use the flood fill, first a seed point is selected, then all neighbouring pixels of a similar colour are converted to a uniform colour. In this example the seed point is at 200, 200 (shown by a blue circle). The neighbouring pixels are then flood filled with a red colour.

#include "stdafx.h"
#include "cv.h"
#include "highgui.h"

int _tmain(int argc, _TCHAR* argv[])
{
	// load the input image
	IplImage* img = cvLoadImage("test.jpg");

	// define the seed point
	CvPoint seedPoint = cvPoint(200,200);

	// flood fill with red
	cvFloodFill(img, seedPoint, CV_RGB(255,0,0), CV_RGB(8,90,60), CV_RGB(10,100,70),NULL,4,NULL);
	
	// draw a blue circle at the seed point
	cvCircle(img, seedPoint, 3, CV_RGB(0,0,255), 3, 8);

	// show the output
	cvNamedWindow("Output", CV_WINDOW_AUTOSIZE);  
    cvShowImage("Output", img); 

	// wait for user
	cvWaitKey(0);

	// save image
	cvSaveImage("output.jpg",img);

	// garbage collection		
	cvReleaseImage(&img);
	cvDestroyWindow("Output");
	return 0;
}

Input image

The flood filled output image

To separate a multi channel image into the three component RGB channels, we can use the cvSplit function. The example below opens a RGB image and then using the cvSplit function creates three output images.

#include "stdafx.h"
#include "cv.h"
#include "highgui.h"

int _tmain(int argc, _TCHAR* argv[])
{
	// open and display input image  
    IplImage* input = cvLoadImage("test.jpg");  
    cvNamedWindow("Input", CV_WINDOW_AUTOSIZE);  
    cvShowImage("Input", input); 

	// create the output images
	IplImage* output_r = cvCreateImage(cvSize(input->width, input->height), input->depth, 1);
	IplImage* output_g = cvCreateImage(cvSize(input->width, input->height), input->depth, 1);
	IplImage* output_b = cvCreateImage(cvSize(input->width, input->height), input->depth, 1);

	// split the image
	cvSplit(input, output_r, output_g, output_b,NULL);
	
	// display image
	cvNamedWindow("Output R", CV_WINDOW_AUTOSIZE);
	cvShowImage("Output R", output_r);

	cvNamedWindow("Output G", CV_WINDOW_AUTOSIZE);
	cvShowImage("Output G", output_g);

	cvNamedWindow("Output B", CV_WINDOW_AUTOSIZE);
	cvShowImage("Output B", output_b);

	// wait for user
	cvWaitKey(0);

	// garbage collection	
	cvReleaseImage(&input);
	cvDestroyWindow("Input");
	cvReleaseImage(&output_r);
	cvDestroyWindow("Output R");
	cvReleaseImage(&output_g);
	cvDestroyWindow("Output G");
	cvReleaseImage(&output_b);
	cvDestroyWindow("Output B");
	return 0;
}

Input Image

Blue Channel

Green Channel

Red Channel

Here is the Hello World example code for OpenCV. This simple example creates a image called output, then the text “Hello World” is added to the image.

#include "stdafx.h"
#include "cv.h"
#include "highgui.h"

int _tmain(int argc, _TCHAR* argv[])
{
	// create image
	IplImage* output = cvCreateImage(cvSize(400, 200), 8, 3);
	
	// create font and add text to the image
	CvFont font;
    cvInitFont(&font, CV_FONT_HERSHEY_SIMPLEX, 1,1,0,1,8);
	cvPutText(output, "Hello World", cvPoint(100,100), &font, cvScalar(255,255,0));
	
	// display image
	cvNamedWindow("Output", CV_WINDOW_AUTOSIZE);
	cvShowImage("Output", output);

	// wait for user
	cvWaitKey(0);

	// garbage collection	
	cvReleaseImage(&output);
	cvDestroyWindow("Output");
	
	return 0;
}

Hello World output image