How to crop a triangle - c++

Question

I am working on some "Face Normalization" Project.
What I did till now is:
Face detection
Facial Landmark detection (68)
Split the face is a few triangles by connecting the several landmarks (Delaunay Triangulation -->AAM)
Create some 3D Model of a generic face (consists of 68 (same as Landmarks) Points) in 3D and also did some Delaunay Triangulation
Now what i need to do now:
I know all the Landmark coordinates and all the 3D coordinates so i want to crop each triangle in 2D and put it on its right place on the 3D generic model to generate a 3D Model of the detected face.
Question:
1.)Does anyone know a way to crop a single Triangle by knowing all three coords?
2.)And what kind of transformation do i have to use to "copy" the cropped triangle on its right place on the generic 3D model?
I am programming in c++ and took dlib and openCV for the facial landmark detection and on the 3D side I am working with openGL
EDIT:
Maybe it is better to "see" the problem. This is what i have already
and now i just want to crop all these triangles separately. So how can I crop a triangle (when i know all 3 coords) from a picture and safe it in another window?

Answer 1

In order to crop a triangle, we need to use warpaffine method.
http://docs.opencv.org/2.4/doc/tutorials/imgproc/imgtrans/warp_affine/warp_affine.html
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
using namespace cv;
using namespace std;
/// Global variables
char* source_window = "Source image";
char* warp_window = "Warp";
char* warp_rotate_window = "Warp + Rotate";
/** #function main */
int main( int argc, char** argv )
{
Point2f srcTri[3];
Point2f dstTri[3];
Mat rot_mat( 2, 3, CV_32FC1 );
Mat warp_mat( 2, 3, CV_32FC1 );
Mat src, warp_dst, warp_rotate_dst;
/// Load the image
src = imread( argv[1], 1 );
/// Set the dst image the same type and size as src
warp_dst = Mat::zeros( src.rows, src.cols, src.type() );
/// Set your 3 points to calculate the Affine Transform
srcTri[0] = Point2f( 0,0 );
srcTri[1] = Point2f( src.cols - 1, 0 );
srcTri[2] = Point2f( 0, src.rows - 1 );
dstTri[0] = Point2f( src.cols*0.0, src.rows*0.33 );
dstTri[1] = Point2f( src.cols*0.85, src.rows*0.25 );
dstTri[2] = Point2f( src.cols*0.15, src.rows*0.7 );
/// Get the Affine Transform
warp_mat = getAffineTransform( srcTri, dstTri );
/// Apply the Affine Transform just found to the src image
warpAffine( src, warp_dst, warp_mat, warp_dst.size() );
/** Rotating the image after Warp */
/// Compute a rotation matrix with respect to the center of the image
Point center = Point( warp_dst.cols/2, warp_dst.rows/2 );
double angle = -50.0;
double scale = 0.6;
/// Get the rotation matrix with the specifications above
rot_mat = getRotationMatrix2D( center, angle, scale );
/// Rotate the warped image
warpAffine( warp_dst, warp_rotate_dst, rot_mat, warp_dst.size() );
/// Show what you got
namedWindow( source_window, CV_WINDOW_AUTOSIZE );
imshow( source_window, src );
namedWindow( warp_window, CV_WINDOW_AUTOSIZE );
imshow( warp_window, warp_dst );
namedWindow( warp_rotate_window, CV_WINDOW_AUTOSIZE );
imshow( warp_rotate_window, warp_rotate_dst );
/// Wait until user exits the program
waitKey(0);
return 0;
}