Question

So I’ve successfully access pixel data in a frame using the c++ frame access wrapper on the opencv webpage

template<class Frame> 
class Frame_Data {
        IplImage *imgp;
        public:
        Frame_Data (IplImage *img=0) {imgp = img;}
        ~Frame_Data () {imgp = 0;}
        void operator=(IplImage *img) {imgp=img;}
        inline Frame* operator[] (int rowIndex) {
                return ((Frame*)(imgp->imageData + rowIndex*imgp->widthStep));
        }
};

typedef struct {
        unsigned char b,g,r;
} RgbPixel;
typedef struct {
        float b,g,r;
} RgbPixelFloat;

typedef Frame_Data<RgbPixel> RgbImage;

Im then using 2 for loops to go through the frame pixel array such as:

for (int i = ymin; i < ymax; i++) 
{
    for (int j = xmin; j < xmax; j++) 
    {
        int r = image[i][j].r;
        int g = image[i][j].g;
        int b = image[i][j].b;

So lets say I want to throw in an IF statement to check pixel data colors. I’ve seen some websites list them as stuff like

image[i][j].r=0xFF;

or if g < 0x20

Im not used to the hex looking values, i tried to look them up but can’t find any refernece, im used to cvscalars, so what do these mean? Like what does 0x20 stand for? or what about 0xFF?

thanks

Answer 1

The range from 0x00 ... 0xFF that you are seeing is one byte which can hold a value between 0 and 255 which is how pixel color data is stored, generally in 3 or 4 bytes consisting of Red, Blue, Green and optionally Alpha.

The CvScalar is just a convenience container of 1, 2, 3 or 4 doubles which can be used to hold these values in a slightly different form.

For example:

cv.RGB(1.0, 0.5, 0.3) sets the red component of the color to 1.0 or 100%, the green component to 0.5 or 50% and the blue component to 0.3 or 30%. When the actual color structure is created each of these components will be made up of exactly one byte so this is analagous to setting the

R (red component) to 1.0 * 0xFF = 0xFF

G (green component) to 0.5 * 0xFF = 0x7F

B (blue component) to 0.3 * 0xFF = 0x26

The alpha is automatically set to 1.0 or 0xFF