matlab/+gtsam/cylinderSampleProjectionStereo.m
function [visiblePoints] = cylinderSampleProjectionStereo(K, pose, imageSize, cylinders)

import gtsam.*

%% memory allocation
cylinderNum = length(cylinders);

visiblePoints.data = cell(1);
visiblePoints.Z = cell(1);
visiblePoints.cylinderIdx = cell(1);
visiblePoints.overallIdx = cell(1);

%% check visiblity of points on each cylinder
pointCloudIndex = 0;
visiblePointIdx = 1;
for i = 1:cylinderNum
    
    pointNum = length(cylinders{i}.Points);

    % to check point visibility        
    for j = 1:pointNum

        pointCloudIndex  = pointCloudIndex + 1;
                
        % For Cheirality Exception
        sampledPoint3 = cylinders{i}.Points{j};
        sampledPoint3local = pose.transformTo(sampledPoint3);
        if sampledPoint3local(3) < 0
            continue; 
        end
        
        % measurements 
        Z.du = K.fx() * K.baseline() / sampledPoint3local(3);  
        Z.uL = K.fx() * sampledPoint3local(1) / sampledPoint3local(3) + K.px();
        Z.uR = Z.uL + Z.du;
        Z.v = K.fy() / sampledPoint3local(3) + K.py();

        % ignore points not visible in the scene
        if Z.uL < 0 || Z.uL >= imageSize(1) || ...
                Z.uR < 0 || Z.uR >= imageSize(1) || ...
                Z.v < 0 || Z.v >= imageSize(2) 
            continue;       
        end            

        % too small disparity may call indeterminant system exception
        if Z.du < 0.6
            continue;
        end
        
        % ignore points occluded
        % use a simple math hack to check occlusion:
        %   1. All points in front of cylinders' surfaces are visible
        %   2. For points behind the cylinders' surfaces, the cylinder
        visible = true;
        for k = 1:cylinderNum

            rayCameraToPoint = sampledPoint3 - pose.translation();
            rayCameraToCylinder = cylinders{k}.centroid - pose.translation();
            rayCylinderToPoint = sampledPoint3 - cylinders{k}.centroid;

            % Condition 1: all points in front of the cylinders'
            % surfaces are visible
            if dot(rayCylinderToPoint, rayCameraToCylinder) < 0
               continue;
            else 
                projectedRay = dot(rayCameraToCylinder, rayCameraToPoint) / norm(rayCameraToCylinder);
                if projectedRay > 0
                    %rayCylinderToProjected = rayCameraToCylinder - norm(projectedRay) / norm(rayCameraToPoint) * rayCameraToPoint;
                    if rayCylinderToPoint(1) > cylinders{k}.radius && ...
                            rayCylinderToPoint(2) > cylinders{k}.radius
                       continue;
                    else
                        visible = false;
                        break;
                    end
                end
            end
            
        end
        
        if visible
            visiblePoints.data{visiblePointIdx} = sampledPoint3;
            visiblePoints.Z{visiblePointIdx} = Z;
            visiblePoints.cylinderIdx{visiblePointIdx} = i;
            visiblePoints.overallIdx{visiblePointIdx} = pointCloudIndex;
            visiblePointIdx = visiblePointIdx + 1;
        end
        
    end

end

end
