[opencv] generate transparent PNG image

This article explains how to use opencv Of cv::imwrite() Function generates transparency / Opaque PNG Images ：

1. PNG A brief introduction to the image

2. cv::imwrite() Function introduction

3. Sample code

1. PNG A brief introduction to the image

PNG yes 20 century 90 A replacement developed in the s GIF and TIFF Image file storage format , Portable image file format suitable for network transmission .

• PNG Images have streaming readability , Allows continuous reading and writing of image data , Therefore, it is suitable for network communication .
• PNG The image can be approximated and displayed step by step , This feature enables the image to be displayed on the terminal while transmitting the image file on the communication link , After displaying the whole outline, gradually display the details of the image .
• PNG Images can be transparent , Some parts of the image can not be displayed , Used to create some characteristic images .
• PNG Image is independent of computer hardware and software environment , Using lossless compression , You can store multiple images in one file .

So many features above , What is used in this article is PNG Transparency .

PNG File structure

PNG Image format consists of file signature and data block .

PNG The signature of the document is 8 byte , Used to identify PNG Format , be-all PNG There is a file signature at the beginning of the image file . this 8 Bytes are ：89H50H 4EH 47H0DH 0AH1AH0AH

• 89H： Used to detect whether the transmission system supports 8 Bit character encoding , It is used to reduce the text file being incorrectly recognized as PNG Opportunities for documentation ;
• 50H 4EH 47H：PNG Each letter corresponds to ASCII, Recognize that it is PNG file ;
• 0DH 0AH：DOS Line breaks in style , be used for DOS-Unix Line break conversion of data ;
• 1AH： stay DOS At the command line , The end of file character used to block the display of files ;
• 0AH：Unix Line breaks in style , be used for Unix-DOS Line feed conversion ;

Use the file header flag to determine the file type (mythsman.com) Lists the header flags of more file formats .

PNG Data blocks define two types of data blocks , One is called critical data block , Is a required data block , The other is called auxiliary data block , Is an optional data block . Each data block contains a length 、 Block type 、 Block data 、 Cyclic redundancy detection has four parts .

For more detailed information, please refer to ：PNG File format details - maxiongying - Blog Garden (cnblogs.com)

2. cv::imwrite() Function introduction

cv::imwrite() The function is Opencv Functions provided to save pictures , The function prototype ：

bool cv::imwrite(
    const string& filename, 
    cv::InputArray image, 
    const vector<int>& params =vector<int>()); 

Parameters params Is the parameter code saved for a specific format , yes STL vector type , The specific content is ： A series of parameters ID And the parameter value corresponding to this parameter . For this article , preservation PNG Format , So parameters params It can be defined as ：

vector<int> compression_params;
compression_params.push_back(CV_IMWRITE_PNG_COMPRESSION);
compression_params.push_back(9);

CV_IMWRITE_PNG_COMPRESSION Means compression PNG;9 Indicates the strength of compression , A higher value means more compression , The scope is 0~9, The default is 3.

3.  Sample code

#include <iostream>
#include <opencv.hpp>

using namespace std;
using namespace cv;

int main()
{
	//  Create with alpha( transparent ) The tunnel Mat
	cv::Mat pngAlphaMat(480, 640, CV_8UC4);
	cv::Mat pngMat(480, 640, CV_8UC3);

	for (int i = 0; i < pngAlphaMat.rows; i++)
	{
		for (int j = 0; j < pngAlphaMat.cols; j++)
		{
			Vec4b &rgba = pngAlphaMat.at<Vec4b>(i, j);
			Vec3b &rgb = pngMat.at<Vec3b>(i, j);

			rgba[0] = UCHAR_MAX;
			rgba[1] = saturate_cast<uchar>((float(pngAlphaMat.cols - j) / ((float)pngAlphaMat.cols))*UCHAR_MAX);
			rgba[2] = saturate_cast<uchar>((float(pngAlphaMat.rows - i) / ((float)pngAlphaMat.rows))*UCHAR_MAX);
			rgba[3] = saturate_cast<uchar>(0.5 * (rgba[1] + rgba[2]));

			rgb[0] = saturate_cast<uchar>((float(pngMat.cols - j) / ((float)pngMat.cols))*UCHAR_MAX);
			rgb[1] = saturate_cast<uchar>((float(pngMat.rows - i) / ((float)pngMat.rows))*UCHAR_MAX);
			rgb[2] = saturate_cast<uchar>(0.5 * (rgb[1] + rgb[2]));

		}
	}

	vector<int> compression_params;
	compression_params.push_back(CV_IMWRITE_PNG_COMPRESSION);
	compression_params.push_back(9);
	cv::imwrite(" transparent Alpha chart .png", pngAlphaMat, compression_params);
	cv::imwrite(" Opaque chart .png", pngMat, compression_params);

	cv::imshow(" transparent PNG chart ", pngAlphaMat);
	cv::imshow(" Opaque PNG chart ", pngMat);

	cv::waitKey(0);

	return 0;
}

Saved pictures ：