OpenCV Lesson 1: Basic I/O scripts

I decided to document what I learn about OpenCV as my days going.
This is day 1: input/output scripts.

Let's dive right into it.
We have input fuction: cv2.imread(filename, flags)
and output function: cv2.imwrite(filename, img, params)

These function is so basic and simple as Hello world! for OpenCV.
So the difference between opencv 2 and opencv 3 is that in opencv 3x, they name flags in a manner that relate to the function to which it refers to. So in cv2.imread(), flags would be something like cv2.IMREAD_

For example: we need to read an image directly to grayscale in the main folder of the project (which is the same folder of the running code):

---

import cv2

image_grayscale = cv2.imread("sample.jpg", cv2.IMREAD_GRAYSCALE)

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And this is the table about all the flags for cv2.imread() in opencv 3.3:

Flags	Content
IMREAD_UNCHANGED	If set, return the loaded image as is (with alpha channel, otherwise it gets cropped).
IMREAD_GRAYSCALE	If set, always convert image to the single channel grayscale image.
IMREAD_COLOR	If set, always convert image to the 3 channel BGR color image.
IMREAD_ANYDEPTH	If set, return 16-bit/32-bit image when the input has the corresponding depth, otherwise convert it to 8-bit.
IMREAD_ANYCOLOR	If set, the image is read in any possible color format.
IMREAD_LOAD_GDAL	If set, use the gdal driver for loading the image.
IMREAD_REDUCED_GRAYSCALE_2	If set, always convert image to the single channel grayscale image and the image size reduced 1/2
IMREAD_REDUCED_COLOR_2	If set, always convert image to the 3 channel BGR color image and the image size reduced 1/2.
IMREAD_REDUCED_GRAYSCALE_4	If set, always convert image to the single channel grayscale image and the image size reduced 1/4.
IMREAD_REDUCED_COLOR_4	If set, always convert image to the 3 channel BGR color image and the image size reduced 1/4.
IMREAD_REDUCED_GRAYSCALE_8	If set, always convert image to the single channel grayscale image and the image size reduced 1/8.
IMREAD_REDUCED_COLOR_8	If set, always convert image to the 3 channel BGR color image and the image size reduced 1/8.
IMREAD_IGNORE_ORIENTATION	If set, do not rotate the image according to EXIF's orientation flag.

And this table is for cv2.imwrite() params:

Params	Content
IMWRITE_JPEG_QUALITY	For JPEG, it can be a quality from 0 to 100 (the higher is the better). Default value is 95.
IMWRITE_JPEG_PROGRESSIVE	Enable JPEG features, 0 or 1, default is False.
IMWRITE_JPEG_OPTIMIZE	Enable JPEG features, 0 or 1, default is False.
IMWRITE_JPEG_RST_INTERVAL	JPEG restart interval, 0 - 65535, default is 0 - no restart.
IMWRITE_JPEG_LUMA_QUALITY	Separate luma quality level, 0 - 100, default is 0 - don't use.
IMWRITE_JPEG_CHROMA_QUALITY	Separate chroma quality level, 0 - 100, default is 0 - don't use.
IMWRITE_PNG_COMPRESSION	For PNG, it can be the compression level from 0 to 9. A higher value means a smaller size and longer compression time. If specified, strategy is changed to IMWRITE_PNG_STRATEGY_DEFAULT (Z_DEFAULT_STRATEGY). Default value is 1 (best speed setting).
IMWRITE_PNG_STRATEGY	One of cv::ImwritePNGFlags, default is IMWRITE_PNG_STRATEGY_RLE.
IMWRITE_PNG_BILEVEL	Binary level PNG, 0 or 1, default is 0.
IMWRITE_PXM_BINARY	For PPM, PGM, or PBM, it can be a binary format flag, 0 or 1. Default value is 1.
IMWRITE_WEBP_QUALITY	For WEBP, it can be a quality from 1 to 100 (the higher is the better). By default (without any parameter) and for quality above 100 the lossless compression is used.
IMWRITE_PAM_TUPLETYPE	For PAM, sets the TUPLETYPE field to the corresponding string value that is defined for the format.

Source: opencv 3.3.0 documents