Simple Image Processing

This is a discussion on Simple Image Processing within the C++ Programming forums, part of the General Programming Boards category; Hello, I am trying to do some very simple image processing, using OpenCV to access data from an image. What ...

  1. #1
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    Simple Image Processing

    Hello,

    I am trying to do some very simple image processing, using OpenCV to access data from an image. What I want to do is check whether the "B" value of each RGB pixel is greater than 100 - and if so, set that "B" value to 255.

    Here is my code:

    Code:
    IplImage* image = cvLoadImage("C:\\myPicture.bmp"); // Pointer to OpenCV IplImage structure
    data = (char*)image -> imageData; // char buffer of image data
    height = originalImage -> height; // height of image
    width = originalImage -> width; // width of image
    step = originalImage -> widthStep; // size in bytes of one line in image
    
    for (int i = 0; i < height; i ++)
    	{
    		for (int j = 0; j < width; j ++)
    		{
    			if ((int)data[i * step + j * 3] > 100)
    			{
    				data[i * step + j * 3] = 255;
                                                     }
    		}
    	}
    The problem is, the line which sets the "B" component to zero is never reached. In fact, using a breakpoint in debugging mode, I noticed that the value of data[i * step + j * 3] is always 0...

    I have checked that the image is displaying correctly by displaying the image - so it is loading into the buffer ok.

    I am able to do SOME image processing, such as inversion, where I simply say :

    Code:
    data[i * step + j * 3] = 255 - data[i * step + j * 3];
    Also, if I assign a value to an element of the char array, that works too, such as making the entire image dark grey by saying :

    Code:
    data[i * step + j * 3] = 50;
    BUT, when I actually try to compare the value in the char array to a number, such as in thresholding :

    Code:
    if (data[i * step + j * 3] > 100)
    {
    // do something
    }
    It doesn't work. I have tried checking the values of the array, for example :

    Code:
    int x = data[i * step + j * 3];
    std :: cout << x;
    ...and it always tells me that the value of x is 0, no matter which element of the image buffer I am accessing.

    It seems like it is something to do with converting between char and int...any ideas??

    Thanks

  2. #2
    Captain Crash brewbuck's Avatar
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    Portland, OR
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    So, what's the difference between image and originalImage?

    I see nothing immediately wrong.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  3. #3
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    Ooops..."yeh originalImage" should just be "image". They are the same.

    Can't work out why I can assign values to the data, but I cannot compare the values in an if statement.

    If you say that it all looks fine, then I guess there is something going on at a deeper level I will have to look at...

  4. #4
    Captain Crash brewbuck's Avatar
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    Location
    Portland, OR
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    7,158
    Hmm. You may have a signedness issue.

    Since the data pointer is char, any value greater than 127 will be treated as negative. When that casts to int, it's still negative, so your > 100 comparison will fail.

    That still doesn't explain why everything in the array appears to be 0, but it's an issue. Try using an unsigned char * to point at the image data, instead of a char *.

    Also, are you treating the pixel format correctly? Is this truly 24-bit RGB, or could there be a pad byte to make each pixel 32 bits wide?
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  5. #5
    Guest Sebastiani's Avatar
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    Waterloo, Texas
    Posts
    5,600
    Well, assuming that each rgb value is packed into 3 consecutive bytes, wouldn't your blue values be indexed by:

    Code:
    data[ i * step * 3 + j * 3 + 2 ]
    Incidentally, you could make your life a lot easier if you just treated the buffer as an array of RGB's, instead of chars...

    Code:
    #pragma pack(push, 1)
    struct rgb
    {
    	unsigned char red, green, blue;
    };
    #pragma pack(pop)
    
    
    IplImage* image = cvLoadImage("C:\\myPicture.bmp"); // Pointer to OpenCV IplImage structure
    rgb* data = (rgb*)image -> imageData; // char buffer of image data
    height = originalImage -> height; // height of image
    width = originalImage -> width; // width of image
    step = originalImage -> widthStep; // size in bytes of one line in image
    
    for (int i = 0; i < height; i ++)
    {
    	for (int j = 0; j < width; j ++)
    	{
    		if (data[i * step + j].blue > 100)
    		{
    			data[i * step + j].blue = 255;
    		}
    	}
    }
    Code:
    if( numeric_limits< byte >::digits != bits_per_byte )
        error( "program requires bits_per_byte-bit bytes" );
    24bbs.cpp

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