how to load data from cv::Mat

[bonne658]

I tried to use stbi_load_from_memory, but got NULL

int height  = image.cols;
  int width   = image.rows;
  int channel = image.channels();

  if(!image.isContinuous()){
  	printf("clone!\n");
  	image=image.clone();
  }
  unsigned char * imageData = (unsigned char *)image.data;
  size_t img_length =  image.total() * image.elemSize();
  unsigned char* image_data = stbi_load_from_memory(imageData, img_length, &width, &height, &channel, 3);

I also tried code which was found in https://github.com/nihui/opencv-mobile/blob/master/highgui/src/highgui.cpp:

Mat buf = _buf.getMat();

    if (!buf.isContinuous())
    {
        buf = buf.clone();
    }

    size_t buf_size = buf.cols * buf.rows * buf.elemSize();

    int w;
    int h;
    int c;
    unsigned char* pixeldata = stbi_load_from_memory((const unsigned char*)buf.data, buf_size, &w, &h, &c, desired_channels);

I found stbi__load_main run to :

return stbi__errpuc("unknown image type", "Image not of any known type, or corrupt");

[rygorous]

stb_image is used to load image files. PNG, JPG, BMP etc.

A cv::Mat is already an array of pixels in memory, which is the result of what stb_image would do, not the input to it. There's no loading step involved. You can just access the bytes in the cv::Mat directly (using ptr or other methods).

Either way, you seem to have a OpenCV question, not a stb_image question, so you'll have better odds of getting an answer there.

[bonne658]

Thanks for your answer !
Is the code which I showed in question in https://github.com/nihui/opencv-mobile/blob/master/highgui/src/highgui.cpp wrong?

[rygorous]

No, that code is fine, since in that case buf is also the bytes contained in an image file, not pixels.

[bonne658]

Thank you. Finally, I use cv::Mat directly.