Set object position relative to other object - c++

Question

So I try to learn about game development, and I want to make my character can move its elbow. the elbow looks like this, it consist of 2 sprites, arm1 and arm2. Arm1 can rotate base on its origin, and arm2 should locate at the tip of arm1(about 60 px from arm1 origin). But I don't know how to put arm2 at the correct position like in the img. I try to use polar coordinate because I know the angle and arm distance
lines[0].position=Vector2f(arm1.getPosition().x,arm1.getPosition().y);
lines[0].color=Color::Blue;
armPos.x=arm1.getPosition().x+(d*cos(AngleToRad(arm1.getRotation()-toleransi) ));
armPos.y=arm1.getPosition().y+(d*sin(AngleToRad(arm1.getRotation()-toleransi)));
lines[1].position=armPos;
lines[1].color=Color::Blue;
cir.setPosition(armPos);
arm1.setPosition(mc.getPosition().x+10,mc.getPosition().y-50);
arm2.setPosition(arm1.getPosition().x,mc.getPosition().y-10);
, but that doesn't work. I use circle and line just for debug.
The full code looks like this:
#include <SFML/Graphics.hpp>
#include <math.h>
#include <iostream>
#include <vector>
#include "Player.h"
#include "Particle.h"
using namespace sf;
float AngleToRad(float a)
{
return (a/360.0f)*3.14159265359;
}
int main()
{
RenderWindow window(VideoMode(1000,640), "Small Life");
//////////////Setup////////////
//mc//
Texture idle_texture;
idle_texture.loadFromFile("image/idle.png");
IntRect player_rect(264,0,264,264);
Sprite mc(idle_texture,player_rect);
mc.setOrigin(132,264);
Player player(&idle_texture, Vector2u(4,1),0.3f);
mc.setPosition(0,300);
mc.setScale(0.7,0.7);
//arm//
Texture arm1_texture;
arm1_texture.loadFromFile("image/arm1.png");
Sprite arm1(arm1_texture);
Texture arm2_texture;
arm2_texture.loadFromFile("image/arm2.png");
Sprite arm2(arm2_texture);
arm1.setOrigin(70,158);
arm2.setOrigin(79,158);
arm1.setScale(0.5,0.5);
arm2.setScale(0.7,0.7);
//blood//
Texture blood_texture;
blood_texture.loadFromFile("image/blood.png");
CircleShape cir(10);
cir.setOrigin(5,5);
VertexArray lines(LinesStrip,2);
cir.setFillColor(Color::Red);
float deltaTime=0.0f;
Clock clock;
Clock particle_time;
float speed=0.2f;
std::vector<Sprite>bloodVec;
std::cout<<sin(1.5708)<<" "<<cos(AngleToRad(180))<<" "<< AngleToRad(180)<<" "<<" "<<asin(1)<<" "<<acos(1)<<std::endl;
while (window.isOpen())
{
Event event;
deltaTime=clock.restart().asSeconds();
while (window.pollEvent(event))
{
if (event.type == Event::Closed)
window.close();
}
if(Keyboard::isKeyPressed(Keyboard::W)) mc.move(0,-speed);
if(Keyboard::isKeyPressed(Keyboard::S)) mc.move(0,speed);
if(Keyboard::isKeyPressed(Keyboard::A)) mc.move(-speed,0);
if(Keyboard::isKeyPressed(Keyboard::D)) mc.move(speed,0);
//blood particle//
if(particle_time.getElapsedTime().asSeconds()>1.5f)
{
IntRect blRect(0,0,200,200);
Sprite b_blood(blood_texture,blRect);
ParticleConstDrop(b_blood,mc.getPosition());
bloodVec.push_back(b_blood);
particle_time.restart();
}
int bloodCount=bloodVec.size();
for(int i=0;i<bloodCount;i++)
{
window.draw(bloodVec[i]);
}
Vector2f armPos(arm1.getPosition());
float d=30.0f;
float toleransi=90;
lines[0].position=Vector2f(arm1.getPosition().x,arm1.getPosition().y);
lines[0].color=Color::Blue;
armPos.x=arm1.getPosition().x+(d*cos(AngleToRad(arm1.getRotation()-toleransi) ));
armPos.y=arm1.getPosition().y+(d*sin(AngleToRad(arm1.getRotation()-toleransi)));
lines[1].position=armPos;
lines[1].color=Color::Blue;
cir.setPosition(armPos);
arm1.setPosition(mc.getPosition().x+10,mc.getPosition().y-50);
arm2.setPosition(arm1.getPosition().x,mc.getPosition().y-10);
arm1.setRotation(110);//arm1.getRotation()+0.1
player.Update(0,deltaTime);
mc.setTextureRect(player.plRect);
window.draw(arm1);
//window.draw(arm2);
window.draw(mc);
window.draw(cir);
window.draw(lines);
window.display();
window.clear(Color(255,255,255));
}
return 0;
}
Can anyone please tell me what wrong with my code, or is there another way to implement this?

Answer 1

Relative positions are achieved by transform composition (matrix multiplication). You can try to do it manually, but SFML already implements it, and even better: it uses it behind sf::Sprite::draw.
So let's see: arm2 must have a position relative to arm1, so how do we do that?
Set the origin of arm2 where the elbow joint is in arm2 local coordinates.
Set the position of arm2 where the elbow joint is in arm1 local coordinates.
Pass the arm1 transform to the sf::RenderStates each time you draw arm2. The transform multiplication will be performed underneath.
// Do this once
arm2.setOrigin(elbow_x_in_arm2, elbow_y_in_arm2);
arm2.setPosition(elbow_x_in_arm1, elbow_y_in_arm1);
// But this, each time you draw them
window.draw(arm1);
window.draw(arm2, sf::RenderStates(arm1.getTransform()));
Result:
Whenever you move, rotate or scale the arm1, the arm2 will remain attached. Also, if you rotate arm2, it will rotate around the elbow.
Important!
The transform of arm2 will represent the local transformation, so even though it's drawn in the correct position, the data does not contain the global position/rotation/scale. If you wanted to , for example, check for collisions in arm2, you should take this into account:
// don't use this to get the bounding box
sf::FloatRect boundingBoxBad = arm2.getGlobalBounds(); // WRONG: now they are not global
// use this:
sf::Transform tr1 = arm1.getTransform();
sf::Transform tr2 = arm2.getTransform();
sf::FloatRect boundingbBoxGood = tr2.transformRect(tr1.transformRect(arm2.getLocalBounds()));