updated-radar/shader_experiments/sweep_line/main.cpp

/*
 * MIT License
 *
 * Copyright (c) 2026 Mark Allyn
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

// Author: Mark Allyn
//
// sweep_line/main.cpp
//
// A green vertical line sweeps left to right across a black screen at
// 0.001 clip-space units per step (60 steps/sec).  Behind the line, a
// phosphor-style trail fades from red (right behind the line) to black
// (at the left edge when the line reaches the right edge).  Then repeats.
//
// A fullscreen quad is drawn every frame; the fragment shader computes
// line position, trail color, and fade entirely from u_anim_time.

#include <glad/glad.h>
#include <GLFW/glfw3.h>

#include <cstdio>
#include <cstdlib>

// ---------------------------------------------------------------------------
// Vertex shader
//
// Covers the screen with a fullscreen quad.  v_pos carries the clip-space
// x,y of each fragment to the fragment shader.
// ---------------------------------------------------------------------------
static const char* vertex_shader_src = R"glsl(
#version 330 core

layout(location = 0) in vec2 a_pos;

out vec2 v_pos;

void main()
{
    v_pos       = a_pos;
    gl_Position = vec4(a_pos, 0.0, 1.0);
}
)glsl";

// ---------------------------------------------------------------------------
// Fragment shader
//
// sweep_x travels from -1.0 to +1.0 (2.0 clip-space units) at
// 0.001 units/step × 60 steps/sec = 0.06 units/sec → one full sweep in ~33 s.
//
// For each fragment at clip-space x = px:
//   px ≈ sweep_x          → green (current line)
//   px < sweep_x           → red trail: intensity = 1 - dist/2  (full red at
//                             line, black when dist == 2.0, i.e. when sweep_x
//                             is at +1.0 and px is at -1.0)
//   px > sweep_x           → black
// ---------------------------------------------------------------------------
static const char* fragment_shader_src = R"glsl(
#version 330 core

uniform float u_anim_time;

in  vec2 v_pos;
out vec4 frag_color;

const float STEPS_PER_SEC  = 60.0;
const float STEP_SIZE      = 0.001;
const float SWEEP_RANGE    = 2.0;     // clip space: -1.0 to +1.0
const float LINE_HALF_W    = 0.004;   // half-width of green line in clip space

void main()
{
    float px = v_pos.x;

    float step_f  = floor(u_anim_time * STEPS_PER_SEC);
    float travel  = mod(step_f * STEP_SIZE, SWEEP_RANGE);
    float sweep_x = -1.0 + travel;

    // Current sweep line: green
    if (abs(px - sweep_x) <= LINE_HALF_W)
    {
        frag_color = vec4(0.0, 1.0, 0.0, 1.0);
        return;
    }

    // Phosphor trail behind the line: red fading to black
    if (px < sweep_x)
    {
        float dist      = sweep_x - px;                         // 0 at line edge, up to 2.0
        float intensity = clamp(1.0 - dist / SWEEP_RANGE, 0.0, 1.0);
        frag_color = vec4(intensity, 0.0, 0.0, 1.0);
        return;
    }

    // Ahead of the sweep line: black
    frag_color = vec4(0.0, 0.0, 0.0, 1.0);
}
)glsl";

// ---------------------------------------------------------------------------
// compile_shader
// ---------------------------------------------------------------------------
static GLuint compile_shader(GLenum type, const char* src)
{
    GLuint shader = glCreateShader(type);
    glShaderSource(shader, 1, &src, nullptr);
    glCompileShader(shader);

    GLint ok = 0;
    glGetShaderiv(shader, GL_COMPILE_STATUS, &ok);
    if (!ok)
    {
        char log[512];
        glGetShaderInfoLog(shader, sizeof(log), nullptr, log);
        std::fprintf(stderr, "Shader compile error:\n%s\n", log);
    }
    return shader;
}

// ---------------------------------------------------------------------------
// build_program
// ---------------------------------------------------------------------------
static GLuint build_program(const char* vert_src, const char* frag_src)
{
    GLuint vert = compile_shader(GL_VERTEX_SHADER,   vert_src);
    GLuint frag = compile_shader(GL_FRAGMENT_SHADER, frag_src);

    GLuint program = glCreateProgram();
    glAttachShader(program, vert);
    glAttachShader(program, frag);
    glLinkProgram(program);

    GLint ok = 0;
    glGetProgramiv(program, GL_LINK_STATUS, &ok);
    if (!ok)
    {
        char log[512];
        glGetProgramInfoLog(program, sizeof(log), nullptr, log);
        std::fprintf(stderr, "Program link error:\n%s\n", log);
    }

    glDeleteShader(vert);
    glDeleteShader(frag);
    return program;
}

// ---------------------------------------------------------------------------
// main
// ---------------------------------------------------------------------------
int main()
{
    if (!glfwInit())
    {
        std::fprintf(stderr, "glfwInit failed\n");
        return EXIT_FAILURE;
    }

    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    GLFWwindow* window = glfwCreateWindow(800, 600, "Sweep Line", nullptr, nullptr);
    if (!window)
    {
        std::fprintf(stderr, "glfwCreateWindow failed\n");
        glfwTerminate();
        return EXIT_FAILURE;
    }

    glfwMakeContextCurrent(window);

    if (!gladLoadGLLoader(reinterpret_cast<GLADloadproc>(glfwGetProcAddress)))
    {
        std::fprintf(stderr, "GLAD init failed\n");
        return EXIT_FAILURE;
    }

    GLuint shader_program = build_program(vertex_shader_src, fragment_shader_src);
    GLint  loc_anim_time  = glGetUniformLocation(shader_program, "u_anim_time");

    /* Fullscreen quad as a triangle strip:
       (-1,-1) → (1,-1) → (-1,1) → (1,1)  */
    float quad_verts[] = {
        -1.0f, -1.0f,
         1.0f, -1.0f,
        -1.0f,  1.0f,
         1.0f,  1.0f,
    };

    GLuint vao = 0;
    GLuint vbo = 0;
    glGenVertexArrays(1, &vao);
    glGenBuffers(1, &vbo);

    glBindVertexArray(vao);
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glBufferData(GL_ARRAY_BUFFER, sizeof(quad_verts), quad_verts, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE,
                          2 * static_cast<int>(sizeof(float)),
                          reinterpret_cast<void*>(0));
    glEnableVertexAttribArray(0);
    glBindVertexArray(0);

    double anim_start = glfwGetTime();

    while (!glfwWindowShouldClose(window))
    {
        if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
            glfwSetWindowShouldClose(window, GLFW_TRUE);

        float anim_time = static_cast<float>(glfwGetTime() - anim_start);

        glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        glUseProgram(shader_program);
        glUniform1f(loc_anim_time, anim_time);
        glBindVertexArray(vao);
        glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
        glBindVertexArray(0);

        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    glDeleteVertexArrays(1, &vao);
    glDeleteBuffers(1, &vbo);
    glDeleteProgram(shader_program);

    glfwDestroyWindow(window);
    glfwTerminate();
    return EXIT_SUCCESS;
}