How to apply shader to each shapes at their location? - c++

Question

I am playing with SFML and wrote a minimal program that draws a line behind the cursor.
I would like to make this line glow so I wrote a minimal shader that I apply to each line vertex. However, the maximum number of uniforms is about 4096. Instead, I would like to "run" my shader at each vertex position. Is there a better way other than using a global fragment and passing each position as uniforms?
#include <SFML/Graphics.hpp>
#include <boost/circular_buffer.hpp>
#include <iostream>
int main()
{
const float winW = 1000;
const float winH = 1000;
const int tail_length = 100;
sf::RenderWindow window(sf::VideoMode(winW, winH), "SFML");
sf::VertexArray lines(sf::LinesStrip, tail_length);
boost::circular_buffer<sf::Vector2i> tail(tail_length);
sf::Clock clk;
clk.restart();
sf::Vector2i old_mouse_pos = sf::Vector2i(0, 0);
// std::fill(tail.begin(), tail.end(), sf::Mouse::getPosition(window));
sf::Texture tex;
tex.create(winW, winH);
sf::Sprite spr(tex);
sf::Shader shader;
shader.loadFromFile("glow.glsl", sf::Shader::Fragment);
while (window.isOpen())
{
sf::Event event;
while (window.pollEvent(event))
{
if (event.type == sf::Event::Closed)
window.close();
}
shader.setUniform("resolution", sf::Vector2f(40, 40));
sf::Vector2i mousePos = sf::Mouse::getPosition(window);
if (old_mouse_pos != mousePos)
tail.push_back(mousePos);
old_mouse_pos = sf::Vector2i(mousePos.x, mousePos.y);
sf::Vector2f v[100];
for (int i = 0; i < tail.size(); i++)
{
v[i] = sf::Vector2f(tail[i].x, tail[i].y- 2* winH / 2);
lines[i].position = sf::Vector2f(tail[i].x, tail[i].y);
lines[i].color = sf::Color(255, 0, 0, i * 255 / tail_length);
}
shader.setUniformArray("tail", v, tail.size());
window.clear();
window.draw(spr, &shader);
window.draw(lines);
window.display();
}
}
Here the shader:
uniform vec2 resolution;
uniform vec2 tail[100];
void main(void) {
vec2 p = gl_FragCoord.xy / resolution.xy;
float d = 0.0;
for (int i = 0; i < 100; i++) {
vec2 u = tail[i];
vec2 q;
q.x = p.x - u.x / resolution.x;
q.y = p.y + u.y / resolution.y;
float dist = pow(1.0/length(q) * 0.1, 1.5);
d += dist;
}
vec3 col = d * vec3(1.0, 0.5, 0.25);
gl_FragColor = vec4(1.0 - exp( -col ), 1.0);
}