ParticleGenerator.cpp

#include "ParticleGenerator.h"

ParticleGenerator::ParticleGenerator(Shader shader, Texture texture, unsigned int amount)
	: shader(shader), texture(texture), amount(amount)
{
	this->init();
}

void ParticleGenerator::Update(float dt, GameObj& obj, unsigned int newParticles, glm::vec2 offset)
{
	// Add new particles
	for (unsigned int i = 0; i < newParticles; ++i)
	{
		int unusedParticle = this->firstUnusedParticle();
		this->respawnParticle(this->particles[unusedParticle], obj, offset);
	}
	
	// Update all exisiting particles
	for (unsigned int i = 0; i < this->amount; ++i)
	{
		Particle& p = this->particles[i];
		p.Life -= dt; // Reduce life of particle
		// If the life of the particle is still alive
		if (p.Life > 0.0f)
		{
			p.Position -= p.Velocity * dt; // Reduce velocity
			p.Color.a -= dt * 2.5; // Reduce alpha
		}
	}
}

void ParticleGenerator::Draw()
{
	// Use additive blending to give a glow effect
	glBlendFunc(GL_SRC_ALPHA, GL_ONE);
	this->shader.Use();
	for (Particle particle : this->particles)
	{
		if (particle.Life > 0.0f)
		{
			this->shader.SetVector2f("offset", particle.Position);
			this->shader.SetVector4f("color", particle.Color);
			this->texture.Bind();
			glBindVertexArray(this->VAO);
			glDrawArrays(GL_TRIANGLES, 0, 6);
			glBindVertexArray(0);
		}
	}
	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
}

void ParticleGenerator::init()
{
	unsigned int VBO;
	float particle_quad[] = {
	 0.0f, 1.0f, 0.0f, 1.0f,
	 1.0f, 0.0f, 1.0f, 0.0f,
	 0.0f, 0.0f, 0.0f, 0.0f,

	 0.0f, 1.0f, 0.0f, 1.0f,
	 1.0f, 1.0f, 1.0f, 1.0f,
	 1.0f, 0.0f, 1.0f, 0.0f
	};
	glGenVertexArrays(1, &this->VAO);
	glGenBuffers(1, &VBO);
	glBindVertexArray(this->VAO);
	// Fill mesh buffer
	glBindBuffer(GL_ARRAY_BUFFER, VBO);
	glBufferData(GL_ARRAY_BUFFER, sizeof(particle_quad), particle_quad, GL_STATIC_DRAW);
	// Set mesh attributes
	glEnableVertexAttribArray(0);
	glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 4 * sizeof(float), (void*)0);
	glBindVertexArray(0);

	// Create this amount default particle instances
	for (unsigned int i = 0; i < this->amount; ++i)
		this->particles.push_back(Particle());
}

unsigned int lastUsedParticle = 0;
unsigned int ParticleGenerator::firstUnusedParticle()
{
	// Search from last used particle.
	for (unsigned int i = lastUsedParticle; i < this->amount; ++i) {
		if (particles[i].Life <= 0.0f) {
			lastUsedParticle = i;
			return i;
		}
	}
	// Else linear search
	for (unsigned int i = 0; i < lastUsedParticle; ++i) {
		if (particles[i].Life <= 0.0f) {
			lastUsedParticle = i;
			return i;
		}
	}
	// Else nothing else worked, we just use the first particle again
	lastUsedParticle = 0;
	return 0;
}

void ParticleGenerator::respawnParticle(Particle& particle, GameObj& object, glm::vec2 offset)
{
	float random = ((rand() & 100) - 50) / 10.0f;
	float rColor = 0.5f + ((rand() % 100) / 100.0f);
	particle.Position = object.position + random + offset;
	particle.Color = glm::vec4(rColor, rColor, rColor, 1.0f);
	particle.Life = 1.0f;
	particle.Velocity = object.velocity * 0.1f;
}