OpenCV RGB values in hex? - c++

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

Answer 2

Hexidecimal is just another representation of a number (base 16).
It's not too hard to get used to, you just need to learn how to convert to and from regular base 10 numbers.
Open up your favourite windows/mac calculator, switch to Hex mode, and type in FF. (the 0x prefix just tells the code that it's it's hexidecimal number)
Switch to Dec[imal] and the number will change to 255.
Type 32 in, in Decimal mode, then click hex, you'll see the number change to 20 (or 0x20 as it is in your code)
Now you can go from hexidecimal to decimal, you can go from decimal to scalar quite easily; Just convert the range;
float Scalar = static_cast<float>( Decimal ) / 255.f; // 255 being the largest value for your byte-colour
Enjoy Hex! You'll find it a very useful, neat and important way of looking at data.