marker.cpp

/*
 * Copyright (c) 2012, 2016, Yutaka Tsutano
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
 * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
 * OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 * PERFORMANCE OF THIS SOFTWARE.
 */

#include "marker.h"
#include <iostream>


int decode_marker(CvMat *mark_mat, marker_rotation_t &rotation)
{
	/*
	An explanation of cvmGet(mark_mat, i, j) below, throughout this file:
	
	mark_mat is the 6x6 matrix of pixels for each glyph. Some are black ('< 0.5' in the code below), and some are white ('> 0.5' in the code below).
	
	Position i,j (e.g., '2' and '3' in 'cvmGet(mark_mat, 2, 3)') represents a Y-coordinate (counting from top) and an X-coordinate (counting from left). The glyphs are each a 6x6 grid/matrix (two outer rows, with a 4x4 matrix in the center showing the glyph's unique mark). Thus, position (1,1) = top left of outer border, (1,6) = top right of outer border, etc.
	*/
	
    // Make sure that the outermost cells are black (< 0.5 = black, > 0.5 = white).
    // (The glyphs' outside edges are a 6x1 matrix, so 6x6 coordinates are available overall. This iterates over the perimeter.)
    for (int i = 0; i < 6; i++) {
        if (cvmGet(mark_mat, i, 0) > 0.5
                    || cvmGet(mark_mat, i, 5) > 0.5
                    || cvmGet(mark_mat, 0, i) > 0.5
                    || cvmGet(mark_mat, 5, i) > 0.5) {
            return -1;
        }
    }

    // Make sure that the next cells of the outermost cells are white.
    // This skips all corners (1,1, 1,4, 4,4, and 4,1 (listing them clockwise), since they'll be checked below for orientation.
    if (cvmGet(mark_mat, 2, 1) < 0.5
                || cvmGet(mark_mat, 3, 1) < 0.5
                || cvmGet(mark_mat, 1, 2) < 0.5
                || cvmGet(mark_mat, 1, 3) < 0.5
                || cvmGet(mark_mat, 2, 4) < 0.5
                || cvmGet(mark_mat, 3, 4) < 0.5
                || cvmGet(mark_mat, 4, 2) < 0.5
                || cvmGet(mark_mat, 4, 3) < 0.5) {
        return -1;
    }

    // Make sure that the number of corner markers is exactly one. Also
    // detect the orientation.
    int numberOfCornerMarkers = 0;
    if (cvmGet(mark_mat, 1, 1) < 0.5) {
        numberOfCornerMarkers++; // Used below to check that at least one of these conditions (and only one, lest the number be greater than 1) has been met.
        rotation = MARKER_ROT_0_DEG;
    }
    if (cvmGet(mark_mat, 1, 4) < 0.5) {
        numberOfCornerMarkers++;
        rotation = MARKER_ROT_90_DEG;
    }
    if (cvmGet(mark_mat, 4, 4) < 0.5) {
        numberOfCornerMarkers++;
        rotation = MARKER_ROT_180_DEG;
    }
    if (cvmGet(mark_mat, 4, 1) < 0.5) {
        numberOfCornerMarkers++;
        rotation = MARKER_ROT_270_DEG;
    }
    if (numberOfCornerMarkers != 1) {
        return -1;
    }

    /*
	An explanation of the marker identification process below:

	'<<' is a bitshift operator (i.e., go from the binary number 001 to the binary number 010 (i.e., 2)). In effect, it multiplies any number by 2 to the power of the bitshift (e.g., 1 << 8 is equivalent to 1*2^8).

	Each case statement (i.e., each rotation) contains the same four 'if' statements (rotated by 90 degrees more than the last case for each rotation -- if you draw a 4x4 matrix and write the bitshift values (0, 1, 2, and 3) on the coordinates for each of the four rotation cases and compare them to each other, you'll see that they all follow the same (rotated) pattern).

	Within each rotation case, each of the if statements looks at a particular coordinate of the matrix and returns true if that coordinate is black rather than white, and makes a bitshift (starting at 0, following the 'int id = 0' line below).

	The bitshift results are additive (not multiplicative) over the if statements: '|' is the bitwise OR operator, meaning that it considers all bits in the list that are true. Thus, for the '5' glyph, which meets the first, second, and fourth conditions (since those three pixels are black), the math would be (1*2^3) + 1*2^2 + 0 + 1*2^0 = 13 (showing the math for the first, second, third, and fourth condition, respectively). The 13th element in the ID array (counting from 0) is 5.
    */

    int id = 0;
    switch (rotation) {
    case MARKER_ROT_0_DEG:
        id = ((cvmGet(mark_mat, 2, 2) < 0.5) << 3)
                | ((cvmGet(mark_mat, 2, 3) < 0.5) << 2)
                | ((cvmGet(mark_mat, 3, 2) < 0.5) << 1)
                | ((cvmGet(mark_mat, 3, 3) < 0.5) << 0);
        break;
    case MARKER_ROT_90_DEG:
        id = ((cvmGet(mark_mat, 2, 2) < 0.5) << 1)
                | ((cvmGet(mark_mat, 2, 3) < 0.5) << 3)
                | ((cvmGet(mark_mat, 3, 2) < 0.5) << 0)
                | ((cvmGet(mark_mat, 3, 3) < 0.5) << 2);
        break;
    case MARKER_ROT_180_DEG:
        id = ((cvmGet(mark_mat, 2, 2) < 0.5) << 0)
                | ((cvmGet(mark_mat, 2, 3) < 0.5) << 1)
                | ((cvmGet(mark_mat, 3, 2) < 0.5) << 2)
                | ((cvmGet(mark_mat, 3, 3) < 0.5) << 3);
        break;
    case MARKER_ROT_270_DEG:
        id = ((cvmGet(mark_mat, 2, 2) < 0.5) << 2)
                | ((cvmGet(mark_mat, 2, 3) < 0.5) << 0)
                | ((cvmGet(mark_mat, 3, 2) < 0.5) << 3)
                | ((cvmGet(mark_mat, 3, 3) < 0.5) << 1);
        break;
    }

    // Now determine the id using a (0-indexed) table.
    static const unsigned char id_table[] = {
        8, 2, 4, 15, 6, 13, 11, 1,
        0, 10, 12, 7, 14, 5, 3, 9
    };
    id = id_table[id];
    
    // Uncomment the line below for debugging text.
    //std::cout << "Detected marker ID is " << id << ".\n";
    
    return id;
}

int analyze_marker(const IplImage *src_img, CvSeq *poly, CvPoint2D32f *points)
{
    // Make sure the shape is square and convex.
    if (poly->total != 4
            || !cvCheckContourConvexity(poly)
            || cvContourArea(poly) < 360.0) {
        return -1;
    }

    for (int i = 0; i < 4; i++) {
        points[i] = cvPointTo32f(*CV_GET_SEQ_ELEM(CvPoint, poly, i));
    }

    // Refine to sub-pixel accuracy.
    cvFindCornerSubPix(src_img, points, 4, cvSize(3, 3), cvSize(-1, -1),
             cvTermCriteria (CV_TERMCRIT_ITER | CV_TERMCRIT_EPS, 20, 0.03));

    double a[] = {
        points[0].x, points[0].y,
        points[1].x, points[1].y,
        points[2].x, points[2].y,
        points[3].x, points[3].y
    };
    CvMat src_points = cvMat(poly->total, 2, CV_64FC1, a);

    static const int MARK_WIDTH = 6*3;
    static const int MARK_HEIGHT = 6*3;
    static const int MARK_TEMP_WIDTH = MARK_WIDTH * 10;
    static const int MARK_TEMP_HEIGHT = MARK_HEIGHT * 10;
    IplImage *mark_temp_img = cvCreateImage(
            cvSize(MARK_TEMP_WIDTH, MARK_TEMP_HEIGHT), IPL_DEPTH_8U, 1);
    double b[] = {
        0, 0,
        0, MARK_TEMP_HEIGHT,
        MARK_TEMP_WIDTH, MARK_TEMP_HEIGHT,
        MARK_TEMP_WIDTH, 0,
    };
    CvMat mark_points = cvMat(poly->total, 2, CV_64FC1, b);

    // Compute homography matrix.
    CvMat *h = cvCreateMat(3, 3, CV_64FC1);
    cvFindHomography(&src_points, &mark_points, h);

    // Transform perspective.
    cvWarpPerspective(src_img, mark_temp_img, h);

    IplImage *mark_img = cvCreateImage(
            cvSize(MARK_WIDTH, MARK_HEIGHT), IPL_DEPTH_8U, 1);
    cvResize(mark_temp_img, mark_img, CV_INTER_AREA);
    int threshold = cvAvg(mark_img).val[0];
    cvThreshold(mark_img, mark_img, threshold, 255, CV_THRESH_BINARY);
    for (int i = 0; i < MARK_WIDTH; i++) {
        for (int j = 0; j < MARK_HEIGHT; j++) {
            if ((i % 3) != 1 || (j % 3) != 1) {
                cvSetReal2D(mark_img, i, j, threshold);
            }
        }
    }

    // Create marker matrix.
    CvMat *mark_mat = cvCreateMat(6, 6, CV_64FC1);
    for (int i = 0; i < 6; i++) {
        for (int j = 0; j < 6; j++) {
            cvmSet(mark_mat, i, j, (cvGetReal2D(mark_img, i*3+1, j*3+1) > 128));
        }
    }

    // Decode the marker ID.
    marker_rotation_t rotation;
    int marker_id = decode_marker(mark_mat, rotation);

    // Based on rotation, correct the points array so that it starts from
    // the corner with the rotation dot.
    CvPoint2D32f temp[4];
    for (int i = 0; i < 4; i++) {
        temp[i] = points[i];
    }
    for (int i = 0; i < 4; i++) {
        points[i] = temp[(i + rotation) % 4];
    }

    // Show decoded marker image for debugging.
    //*
    if (marker_id != -1) {
        cvResize(mark_img, mark_temp_img, CV_INTER_AREA);
        cvShowImage("Window", mark_temp_img);
    }
    //*/

    // Clean up.
    cvReleaseMat(&h);
    cvReleaseMat(&mark_mat);
    cvReleaseImage(&mark_img);
    cvReleaseImage(&mark_temp_img);

    return marker_id;
}