OpenCV - The IMREAD_XXX Flag

OpenCV supports various types of images such as colored, binary, grayscale, etc. Using the imread() method and predefined fields of the Imgcodecs class, you can read a given image as another type.

The flags parameter of imread() method (IMREAD_XXX)

In the earlier chapters, we have seen the syntax of imread() method of the Imgcodecs class. It accepts a string argument representing the location of the image that is to be read.

imread(filename)

The imread() method has another syntax.

imread(filename, int flags)

This syntax accepts two parameters −

filename − It accepts an argument (filename), a variable of the String type representing the path of the file that is to be read.
flags − An integer value representing a predefined flag value. For each value, this reads the given image as a specific type (gray scale color etc.)

Following is the table listing various fields provided in the Imgproc class as values for this parameter.

S.No	Fields and Description
1	IMREAD_COLOR If the flag is set to this value, the loaded image will be converted to a 3-channel BGR (Blue Green Red) color image.
2	IMREAD_GRAYSCALE If the flag is set to this value, the loaded image will be converted to a single-channel grayscale image.
3	IMREAD_LOAD_GDAL If the flag is set to this value, you can load the image using the gdal driver.
4	IMREAD_ANYCOLOR If the flag is set to this value, the image is read in any possible color format.
5	IMREAD_REDUCED_COLOR_2 IMREAD_REDUCED_COLOR_4 IMREAD_REDUCED_COLOR_8 If the flag is set to this value, the image is read as three-channel BGR, and the size of the image is reduced to ½, ¼^th or ⅛^th of the original size of the image with respect to the field used.
6	IMREAD_REDUCED_GRAYSCALE_2 IMREAD_REDUCED_GRAYSCALE_4 IMREAD_REDUCED_GRAYSCALE_8 If the flag is set to this value, the image is read as a single-channel grayscale image, and the size of the image is reduced to ½, ¼^th or ⅛^th of the original size of the image with respect to the field used.
7	IMREAD_UNCHANGED If the flag is set to this value, the loaded image is returned as it is.

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