OpenCV : the difference between mask and ROI

Question

I tried to set a ROI with angle in an image. At first, I thought that using a mask would give me the same result as setting the ROI in my IplImage structure. Then, I would just use cvResize the same way I did when I used ROI, but this time for a mask.

However, this is obviously not this easy, because of the angle.

Is there any way to copy the inside of any rectangle, at any angle, into a new IplImage, which would be of the size of this very rectangle?

CvSeq* approximatedContour = cvApproxPoly(currentContour,
                                          sizeof(CvContour),
                                          0,
                                          CV_POLY_APPROX_DP,
                                          8);

// Circonscrire le polygone trouve dans un rectangle
etiquetteBox = cvMinAreaRect2(approximatedContour);

CvPoint2D32f boxPoints[4];
CvPoint2D32f* c1 = (&cvPoint2D32f(0,0),
                    &cvPoint2D32f(200,0),
                    &cvPoint2D32f(0,200),
                    &cvPoint2D32f(200,200));

CvMat* mmat = cvCreateMat(3,3,CV_32FC1);

cvBoxPoints(etiquetteBox, boxPoints);

IplImage* mask = cvCreateImage(cvSize(in->width,in->height), IPL_DEPTH_8U, 1);
IplImage* ROIimg = cvCreateImage(cvSize(in->width,in->height), IPL_DEPTH_8U, 1);
drawBox(mask,etiquetteBox,target_color[3]);

cvAnd(thresImg,mask,ROIimg,mask);
if(voirSeuillage)
    cvCvtColor(ROIimg,in,CV_GRAY2BGR); //ROIimg is OK here!

mmat = cvGetPerspectiveTransform(boxPoints,c1,mmat);
cvWarpPerspective(ROIimg,thresImgResized,mmat); // here I get a full black image!

Doing this, as kindly suggested by Banthar, I get a full black image instead of what is delimited by boxPoints in ROIimg, what's wrong with this code?

After applying answer:

Here is what I do now :

double angle = 0.;
// TODO adaptive angle compensation
if(abs(etiquetteBox.angle) > 30)
    angle = etiquetteBox.angle + 270.;
else
    angle = etiquetteBox.angle - 270.;

CvPoint2D32f boxPoints[4];
CvPoint2D32f c1[] = {cvPoint2D32f(0,0),
                     cvPoint2D32f(20,0),
                     cvPoint2D32f(20,20),
                     cvPoint2D32f(0,20)};

CvMat* mmat = cvCreateMat(3,3,CV_32FC1);
cvBoxPoints(etiquetteBox, boxPoints);
Point center = Point(10,10);

//warp the image to fit the polygon into the 20x20 image
mmat = cvGetPerspectiveTransform(boxPoints,c1,mmat);
cvWarpPerspective(thresImg,thresImgResized,mmat);
//rotate the image because the inconsistent angle of etiquetteBox
// from a frame to the next ...
//it would be very cool to find a way to fix this...
CvMat rot_mat = getRotationMatrix2D( center, angle,1.0);
cvWarpAffine(thresImgResized,rotatedIm,&rot_mat);
It is still not quite what I want, because the object is rotating into the 20x20 rotatedImg; in thresImgResized, after cvWarpPerspective, the object is well segmented, BUT it is reversed because the inconsistency of the angle of etiquetteBox (-0 degrees in a frame, -90 in the next, depending on how I hold the object to be detected), which I get this way:

cvFindContours(dilImage,
               contoursStorage,
               &contours,
               sizeof(CvContour),
               CV_RETR_LIST,
               CV_CHAIN_APPROX_TC89_KCOS);

// Trouver des polygones
CvSeq* currentContour = contours;
while(currentContour != 0 && !etiquette)
{
    CvSeq* approximatedContour = cvApproxPoly(currentContour,
                                              sizeof(CvContour),
                                              0,
                                              CV_POLY_APPROX_DP,
                                              9);

    // Circonscrire le polygone trouve dans un rectangle
    etiquetteBox = cvMinAreaRect2(approximatedContour);

I don't know how to fix this, but, at least, it is better than setting my IplImage ROI, because I compensate the etiquetteBox's angle switch from -0 to -90 degrees in consecutive frames.

Answer 1

You are using the wrong brackets in definition of c1. Try this:

    CvPoint2D32f c1[] = {
                        cvPoint2D32f(0,200),
                        cvPoint2D32f(0,0),
                        cvPoint2D32f(200,0),
                        cvPoint2D32f(200,200),
    };