cv::Mat convert channel to float[]

C++
1

Hi,

I’m familiar with OpenCV, but I never had to analyse how the cv::Mat container works.

I’m currently writing a ROS node to receive images over the network and feed them to an algorithm that my colleague has developed. His library is developed in C and uses a custom struct to contain the image data.

typedef struct {
    int nb_row, nb_col, zoom;
    float light;
    float *r, *g, *b;   // 0.0 to 1.0 r, g, and b
} t_image_rgb;

I’ve figured out that I’ll need to multiply the pixel values by a factor of 1./255 to convert from int 0-255 to float 0-1.

How can I access a single cv::Mat channel and convert it to an array? I could use two for loops and iterate using img.at() but I’m wondering if there’s a better way?

2

img.at(y,x) is quite slow operation. It is much faster to use pointer access img.ptr(y).

Normally the images are continuous (unless you are working on a part of the image), so the data is at img.ptr(0) over rowscolschannels*bytes/value.

As the color planes are interleaved (so in memory the values are arranged like B G R B G R ...), you must separate them first.

So there are two ways:

  • Split and copy
Mat channels[3];
split(image,channels);
b = (float*)malloc(nb_row*nb_col*sizeof(float)); //allocate the blue channel
memcpy(b,channels[0],nb_row*nb_col*sizeof(float)); //copy channel[0] to the blue channel
...
  • Pixel by pixel loop
float *bp,*rp,*gp; //pointers to the planes
//allocate the r,g,b buffers
bp=b;rp=r;gp=g;
for(int y=0; y<nb_rows;y++) {
    Vec3f *line=img.ptr(y);
    for(int x=0;x<nb_cols;x++){
        *bp=*line[0];*gp=*line[1];*rp=*line[2];
        bp++;gp++;rp++;line++;
    }
}

In this case, you can even do the normalization to 0…1 in the inner loop.

2 Likes
3

that would be: extractChannel()

however, given your C struct, it rather looks like you want “seperated colour planes” (not a single channel, but all 3 of them)

proposed solution is quite simple:;

// step 1: get seperated float planes in [0..1]
Mat floatimg;
src.convertTo(floatimg,  CV_32F, 1.0/255);
vector<Mat> planes;
split(floatimg, planes);

// step 2: assign to your C struct
t_image_rgb rgb;
rgb.b = planes[0].ptr<float>(0); // note: bgr vs rgb
rgb.g = planes[1].ptr<float>(0);
rgb.r = planes[2].ptr<float>(0);
1 Like
4

I tried the solution you proposed. I’m getting a compilation error.

At step 2 when assigning to the C struct. It looks like the convertTo() output isn’t a Mat containing float values.

error: cannot convert uchar* {aka unsigned char*}’ to ‘float*’ in assignment  rgb.b = planes[0].ptr(0);

Oh, I just realised that you’ve edited your answer! :smile:

1 Like