OpenCV-创建PNG图像

Question

As part of my project I wanted to send stream of images using websockets from embedded machine to client application and display them in img tag to achieve streaming.

Firstly I tried to send raw RGB data (752*480*3 - something about 1MB) but in the end I got some problems with encoding image to png in javascript based on my RGB image so I wanted to try to encode my data to PNG firstly and then sent it using websockets.

The thing is, I am having some problems with encoding my data to PNG using OpenCV library that is already used in the project.

Firstly, some code:

websocketBrokerStructure.matrix = cvEncodeImage(0, websocketBrokerStructure.bgrImageToSend, 0);
websocketBrokerStructure.imageDataLeft = websocketBrokerStructure.matrix->rows * websocketBrokerStructure.matrix->cols *  websocketBrokerStructure.matrix->step;
websocketBrokerStructure.imageDataSent = 0;

but I am getting strange error during execution of the second line:

terminate called after throwing an instance of 'std::logic_error'
what():  basic_string::_S_construct NULL not valid

and I am a bit confused why I am getting this error from my code.

Also I am wondering if I understand it right: after invoking cvEncodeImage (where bgrImage is IplImage* with 3 channels - BGR) I just need to iterate through data member of my CvMat to get all of the png encoded data?

Answer 1

The cvEncodeImage function takes as its first parameter the extension of the image you want to encode. You are passing 0 , which is the same thing as NULL . That's why you are getting the message NULL not valid .

You should probably use this:

websocketBrokerStructure.matrix = cvEncodeImage(".png", websocketBrokerStructure.bgrImageToSend, 0);

You can check out the documentation of cvEncodeImage here .

You can check out some examples of cvEncodeImage , or its C++ brother imencode here: encode_decode_test.cpp . They also show some parameters you can pass to cvEncodeImage in case you want to adjust them.