opencv: single pixal assignment confusion - c++

Question

I am recently working with OpenCV and C++ for a project and I found a weird thing:
when I try to access single pixal value in IplImage and assign other value, it could run properly but the result is that it can only operate part of the whole image.
relevant code:
IplImage* output_frame = cvCreateImage(size, IPL_DEPTH_8U, 1);
IplImage* current_frame = cvCreateImage(size, IPL_DEPTH_8U, 1);
while((current_frame = cvQueryFrame(video_gray)) != 0 )
{
for (int row=0;row<height;row++)
{
uchar* ptr_current_frame = (uchar *)(current_frame->imageData+current_frame->widthStep*row);
uchar* ptr_output_frame = (uchar *)(output_frame->imageData+output_frame->widthStep*row);
for (int cols=0;cols<width;cols++)
{
//other codes...
ptr_output_frame[cols]=ptr_current_frame[cols];
}
}
}
The result is the left part of the image was copied to the output_frame. And when I run the following code:
IplImage* output_frame = cvCreateImage(size, IPL_DEPTH_8U, 1);
IplImage* current_frame = cvCreateImage(size, IPL_DEPTH_8U, 1);
while((current_frame = cvQueryFrame(video_gray)) != 0 )
{
for (int row=0;row<height;row++)
{
uchar* ptr_current_frame = (uchar *)current_frame->imageData+current_frame->width*row;
uchar* ptr_output_frame = (uchar *)output_frame->imageData+output_frame->width*row;
for (int cols=0;cols<width;cols++)
{
//other codes...
ptr_output_frame[cols]=ptr_current_frame[cols];
}
}
}
I got the upside part of the image in the output_frame.
I cannot get the the whole image copy in output_frame either way. Could anybody help me with this? Thanks a lot!
[updates] 05/16/2015
I have found out that output_frame->widthStep is different from current->widthStep after current_frame is executed in the following code:
current_frame = cvQueryFrame(video_gray);
It makes sense why the first part of the code wouldnt work. But I still dont know why the second part of the code doesnt work.
I now have my whole codes updated here and hope you can help me make this right. I am really appreciate your help.
CvCapture* video_gray = cvCreateFileCapture("test_gray.avi");
const double fps = cvGetCaptureProperty(video_gray, CV_CAP_PROP_FPS);
const int width = (int)cvGetCaptureProperty(video_gray, CV_CAP_PROP_FRAME_WIDTH);
const int height = (int)cvGetCaptureProperty(video_gray, CV_CAP_PROP_FRAME_HEIGHT);
const CvSize size = cvSize(width, height);
IplImage* current_frame = cvCreateImage(size, IPL_DEPTH_8U, 1);
IplImage* output_frame=cvCreateImage(size, IPL_DEPTH_8U, 1);
int flag = 0;
cvNamedWindow("test",1);
cvNamedWindow("test2",1);
while((current_frame = cvQueryFrame(video_gray)) != 0 )
{
cout<<flag++<<endl;
if(flag<500) continue;
for (int row=0;row<height;row++)
{
uchar* ptr_current_frame = (uchar *)(current_frame->imageData+current_frame->widthStep*row);
uchar* ptr_output_frame = (uchar *)(output_frame->imageData+output_frame->widthStep*row);
for (int cols=0;cols<width;cols++)
{
ptr_output_frame[cols]= ptr_current_frame[cols];
}
}
cvShowImage("test",output_frame);
cvShowImage("test2",current_frame);
cvWaitKey(10);
}

Answer 1

You don't handle the number of channels...
please try
IplImage* output_frame = cvCreateImage(size, IPL_DEPTH_8U, 1);
IplImage* current_frame = cvCreateImage(size, IPL_DEPTH_8U, 1);
while((current_frame = cvQueryFrame(video_gray)) != 0 )
{
for (int row=0;row<height;row++)
{
uchar* ptr_current_frame = (uchar *)(current_frame->imageData+current_frame->widthStep*row);
uchar* ptr_output_frame = (uchar *)(output_frame->imageData+output_frame->widthStep*row);
for (int cols=0;cols<width;cols++)
{
//other codes...
// here insead take care of the channels too. Only write ne channel to the output
ptr_output_frame[cols]=ptr_current_frame[cols*current_frame->nChannels];
}
}
}
but you should really try to switch to C++ API: try something like this:
cv::VideoCapture video = cv::VideoCapture("test_gray.avi");
int width = ...
int height = ...
cv::Mat image_captured;
cv::Mat image_gray; // if wanted
cv::Mat image_output1 = cv::Mat(height, width, CV_8UC1);
cv::Mat image_output2 = cv::Mat(height, width, CV_8UC1);
while(video.read(image_captured))
{
// if you want to convert the image to
if(image_captured.channels() > 1)
{
cv::cvtColor(image_captured, image_gray, CV_BGR2GRAY);
}
else
{
image_gray = image_captured;
}
[...]
for(int j=0; j<height; ++j)
{
for(int i=0; i<width; ++i)
{
image_output1.at<uchar>(j,i) = image_gray.at<uchar>(j,i); // read from single channel image
image_output1.at<uchar>(j,i) = image_captured.at<cv::Vec3b>(j,i)[0]; // only read the first channel of a multi-channel image
}
}
}
cv::imshow("output1", image_output1);
cv::imshow("output2", image_output2);
cv::imshow("input", image_capured);
cv::waitKey(0);
}
much easier to use, and you can still make it more efficient by using row-pointers in each iteration etc