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. Note that cvLoadImage() does not signal a runtime error when it fails to load an image; it simply returns a null pointer.

The obvious complementary function to cvLoadImage() is cvSaveImage(), which takes two arguments:

int cvSaveImage(
  const char*   filename,
  const CvArr*  image
);

The first argument gives the filename, whose extension is used to determine the format in which the file will be stored. The second argument is the name of the image to be stored. Recall that CvArr is kind of a C-style way of creating something equivalent to a base-class in an object-oriented language; wherever you see CvArr*, you can use an IplImage*. The cvSaveImage() function will store only 8-bit single- or three-channel images for most file formats. Newer back ends for flexible image formats like PNG, TIFF or JPEG2000 allow storing 16-bit or even float formats and some allow four-channel images (BGR plus alpha) as well. The return value will be 1 if the save was successful and should be 0 if the save was not.^[41]

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^[41] The reason we say "should" is that, in some OS environments, it is possible to issue save commands that will actually cause the operating system to throw an exception. Normally, however, a zero value will be returned to indicate failure.