OpenCV2的学习就从imread 和 imwrite函数开始

加载图片Loads an image from a file.

C++:Mat imread(const string& filename, int flags=1 )

Python:cv2.imread(filename[, flags]) → retval

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

C:CvMat* cvLoadImageM(const char* filename, int iscolor=CV_LOAD_IMAGE_COLOR )

Python:cv.LoadImage(filename, iscolor=CV_LOAD_IMAGE_COLOR) → None

Python:cv.LoadImageM(filename, iscolor=CV_LOAD_IMAGE_COLOR) → None
imread 函数第二各参数默认情况下读取彩色图片。

Parameters:

filename – Name of file to be loaded. flags – Flags specifying the color type of a loaded image: CV_LOAD_IMAGE_ANYDEPTH - If set, return 16-bit/32-bit image when the input has the corresponding depth, otherwise convert it to 8-bit. CV_LOAD_IMAGE_COLOR - If set, always convert image to the color one CV_LOAD_IMAGE_GRAYSCALE - If set, always convert image to the grayscale one CV_LOAD_IMAGE_UNCHANGED -1 原始图像 CV_LOAD_IMAGE_GRAYSCALE 0 灰度图像 CV_LOAD_IMAGE_COLOR 1 彩色 CV_LOAD_IMAGE_ANYDEPTH 2 任何彩度 CV_LOAD_IMAGE_ANYCOLOR 4 任何彩色 >0 Return a 3-channel color image. Note In the current implementation the alpha channel, if any, is stripped from the output image. Use negative value if you need the alpha channel. =0 Return a grayscale image. <0 Return the loaded image as is (with alpha channel).

The function imread loads an image from the specified file and returns it. If the image cannot be read (because of missing file, improper permissions, unsupported or invalid format), the function returns an empty matrix ( Mat::data==NULL ). 读图失败会返回空，而不是报错

Currently, the following file formats are supported:

• Windows bitmaps - *.bmp, *.dib (always supported)
• JPEG files - *.jpeg, *.jpg, *.jpe (see the Notes section)
• JPEG 2000 files - *.jp2 (see the Notes section)
• Portable Network Graphics - *.png (see the Notes section)
• Portable image format - *.pbm, *.pgm, *.ppm (always supported)
• Sun rasters - *.sr, *.ras (always supported)
• TIFF files - *.tiff, *.tif (see the Notes section)

imwrite

Saves an image to a specified file.

C++:bool imwrite(const string& filename, InputArray img, const vector<int>& params=vector<int>() )

Python:cv2.imwrite(filename, img[, params]) → retval

C:int cvSaveImage(const char* filename, const CvArr* image, const int* params=0 )

Python:cv.SaveImage(filename, image) → None

Parameters:

filename – Name of the file. image – Image to be saved. params – Format-specific save parameters encoded as pairs paramId_1, paramValue_1, paramId_2, paramValue_2, ... . The following parameters are currently supported: For JPEG, it can be a quality ( CV_IMWRITE_JPEG_QUALITY ) from 0 to 100 (the higher is the better). Default value is 95. For PNG, it can be the compression level ( CV_IMWRITE_PNG_COMPRESSION ) from 0 to 9. A higher value means a smaller size and longer compression time. Default value is 3. For PPM, PGM, or PBM, it can be a binary format flag ( CV_IMWRITE_PXM_BINARY ), 0 or 1. Default value is 1.

The function imwrite saves the image to the specified file. The image format is chosen based on the filename extension (see imread()for the list of extensions). Only 8-bit (or 16-bit unsigned (CV_16U) in case of PNG, JPEG 2000, and TIFF) single-channel or 3-channel (with ‘BGR’ channel order) images can be saved using this function. If the format, depth or channel order is different, use Mat::convertTo() , and cvtColor() to convert it before saving. Or, use the universal FileStorage I/O functions to save the image to XML or YAML format.

It is possible to store PNG images with an alpha channel using this function. To do this, create 8-bit (or 16-bit) 4-channel image BGRA, where the alpha channel goes last. Fully transparent pixels should have alpha set to 0, fully opaque pixels should have alpha set to 255/65535. The sample below shows how to create such a BGRA image and store to PNG file. It also demonstrates how to set custom compression parameters

#include <vector>
#include <stdio.h>
#include <opencv2/opencv.hpp>using namespace cv;
using namespace std;void createAlphaMat(Mat &mat)
{CV_Assert(mat.channels() == 4);for (int i = 0; i < mat.rows; ++i) {for (int j = 0; j < mat.cols; ++j) {Vec4b& bgra = mat.at<Vec4b>(i, j);bgra[0] = UCHAR_MAX; // Bluebgra[1] = saturate_cast<uchar>((float (mat.cols - j)) / ((float)mat.cols) * UCHAR_MAX); // Greenbgra[2] = saturate_cast<uchar>((float (mat.rows - i)) / ((float)mat.rows) * UCHAR_MAX); // Redbgra[3] = saturate_cast<uchar>(0.5 * (bgra[1] + bgra[2])); // Alpha}}
}int main(int argv, char **argc)
{// Create mat with alpha channelMat mat(480, 640, CV_8UC4);createAlphaMat(mat);vector<int> compression_params;compression_params.push_back(CV_IMWRITE_PNG_COMPRESSION);compression_params.push_back(9);try {imwrite("alpha.png", mat, compression_params);}catch (runtime_error& ex) {fprintf(stderr, "Exception converting image to PNG format: %s\n", ex.what());return 1;}fprintf(stdout, "Saved PNG file with alpha data.\n");return 0;
}