c++opencvimage-processing

Opencv create new image using cv::Mat


I'm new to OpenCV and I'm trying on some sample codes.

In one code,

Mat gr(row1, col1, CV_8UC1, scalar(0));
int x = gr.at<uchar> (row, col);

And in another one,

Mat grHistrogram(301, 260, CV_8UC1, Scalar(0,0,0));
line(grHistrogram, pt1, pt2, Scalar(255,255,255), 1, 8, 0);

Now my question is if I used scalar(0) instead of scalar(0,0,0) in second code, The code doesn't work. My question is

  1. Why this happening since both create a Mat image structure.
  2. What is the purpose of const cv:Scalar &_s.

I search the documentaion from OpenCV site (opencv.pdf, opencv2refman.pdf) and Oreilly's OpenCV book. But couldn't find an explained answer.

I think I'm using the Mat(int _rows, int _cols, int _type, const cv:Scalar &_s) struct.


Solution

  • First, you need the following information to create the image:

    1. Width: 301 pixels
    2. Height: 260 pixels
    3. Each pixel value (intensity) is 0 ~ 255: an 8-bit unsigned integer
    4. Supports all RGB colors: 3 channels
    5. Initial color: black = (B, G, R) = (0, 0, 0)

    You can create the Image using cv::Mat:

    Mat grHistogram(260, 301, CV_8UC3, Scalar(0, 0, 0));
    

    The 8U means the 8-bit Usigned integer, C3 means 3 Channels for RGB color, and Scalar(0, 0, 0) is the initial value for each pixel. Similarly,

    line(grHistrogram,pt1,pt2,Scalar(255,255,255),1,8,0);
    

    is to draw a line on grHistogram from point pt1 to point pt2. The color of line is white (255, 255, 255) with 1-pixel thickness, 8-connected line, and 0-shift.

    Sometimes you don't need a RGB-color image, but a simple grayscale image. That is, use one channel instead of three. The type can be changed to CV_8UC1 and you only need to specify the intensity for one channel, Scalar(0) for example.

    Back to your problem,

    Why this happening since, both create a Mat image structure?

    Because you need to specify the type of the Mat. Is it a color image CV_8UC3 or a grayscale image CV_8UC1? They are different. Your program may not work as you think if you use Scalar(255) on a CV_8UC3 image.

    What is the purpose of const cv:Scalar &_s ?

    cv::Scalar is use to specify the intensity value for each pixel. For example, Scalar(255, 0, 0) is blue and Scalar(0, 0, 0) is black if type is CV_8UC3. Or Scalar(0) is black if it's a CV_8UC1 grayscale image. Avoid mixing them together.