Question

i am using images that are 2048 x 500 and when I use cvShowImage, I only see half the image. This is not a big deal because the interesting part is on the top half of the image. Now, when I use the mouseHandler to get the x,y coordinates of my clicks, I noticed that the coordinate for y (the dimension that doesnt fit in the screen) is wrong.

It seems OpenCV think this is the whole image and recalibrates the coordinate system although we are only effectively showing half the image.

I would need to know how to do 2 things:
– display a resized image that would fit in the screen

• get the proper coordinate.

Did anybody encounter similar problems?

Thanks!

Update: it seems the y coordinate is divided by 2 of what it is supposed to be

code:

EXPORT void click_rect(uchar * the_img, int size_x, int size_y, int * points)
{
CvSize size;
size.height = size_y ;
size.width = size_x;

IplImage * img;
img = cvCreateImageHeader(size, IPL_DEPTH_8U, 1);
img->imageData = (char *)the_img;
img->imageDataOrigin = img->imageData;

img1 = cvCreateImage(cvSize((int)((size.width)) , (int)((size.height)) ),IPL_DEPTH_8U, 1);

cvNamedWindow("mainWin",CV_WINDOW_AUTOSIZE); 
cvMoveWindow("mainWin", 100, 100);

cvSetMouseCallback( "mainWin", mouseHandler_rect, NULL );

cvShowImage("mainWin", img1 );

//// wait for a key
cvWaitKey(0);

points[0] = x_1;
points[1] = x_2;
points[2] = y_1;
points[3] = y_2;

//// release the image
cvDestroyWindow("mainWin");
cvReleaseImage(&img1 );
cvReleaseImage(&img);
}

Answer 1

I am having the same problem with OpenCV 2.1 using it with Windows and mingw compiler. It took me forever to find out what was wrong. As you describe it, cvSetMouseCallback gets too large y coordinates. This is apparently due to the image and the cvNamedWindow it is shown in being bigger than my screen resolution; thus I cannot see the bottom of the image.

As a solution I resize the images to a fixed size, such that they fit on the screen (in this case with resolution 800×600, which can be any other values:

// g_input_image, g_output_image and g_resized_image are global IplImage* pointers.

int img_w = cvGetSize(g_input_image).width;
int img_h = cvGetSize(g_input_image).height;

// If the height/width ratio is greater than 6/8 resize height to 600.
if (img_h > (img_w*6)/8) {
    g_resized_image = cvCreateImage(cvSize((img_w*600)/img_h, 600), 8, 3);
}
// else adjust width to 800.
else {
    g_resized_image = cvCreateImage(cvSize(800, (img_h*800)/img_w), 8, 3);
}

cvResize(g_output_image, g_resized_image);

Not a perfect solution, but works for me…

Cheers,
Linus