Learning OpenCV

Loading an Image

Before we can display an image in our window, we'll need to know how to load an image from disk. The function for this is cvLoadImage():

IplImage* cvLoadImage(
    const char* filename,
    int         iscolor = CV_LOAD_IMAGE_COLOR
);

When opening an image, cvLoadImage() does not look at the file extension. Instead, cvLoadImage() analyzes the first few bytes of the file (aka its signature or "magic sequence") and determines the appropriate codec using that. The second argument iscolor can be set to one of several values. By default, images are loaded as three-channel images with 8 bits per channel; the optional flag CV_LOAD_IMAGE_ANYDEPTH can be added to allow loading of non-8-bit images. By default, the number of channels will be three because the iscolor flag has the default value of CV_LOAD_IMAGE_COLOR. This means that, regardless of the number of channels in the image file, the image will be converted to three channels if needed. The alternatives to CV_LOAD_IMAGE_COLOR are CV_LOAD_IMAGE_GRAYSCALE and CV_LOAD_IMAGE_ANYCOLOR. Just as CV_LOAD_IMAGE_COLOR forces any image into a three-channel image, CV_LOAD_IMAGE_GRAYSCALE automatically converts any image into a single-channel image. CV_LOAD_IMAGE_ANYCOLOR will simply load the image as it is stored in the file. Thus, to load a 16-bit color image you would use CV_LOAD_IMAGE_COLOR | CV_LOAD_IMAGE_ANYDEPTH. If you want both the color and depth to be loaded exactly "as is", you could instead use the all-purpose flag CV_LOAD_IMAGE_UNCHANGED ...