added example opengl shader

shader_example/flake.nix

@@ -0,0 +1,57 @@
+{
+  description = "Simple OpenGL dev/build flake";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }:
+    let
+      system = "x86_64-linux";
+      pkgs = import nixpkgs { inherit system; };
+    in
+    {
+      devShells.${system}.default = pkgs.mkShell {
+        buildInputs = with pkgs; [
+          gcc
+          glfw
+          glew
+        ];
+
+        shellHook = ''
+          echo "Run:"
+          echo 'g++ shader.cpp -o test -lglfw -lGLEW -lGL && ./test'
+        '';
+      };
+
+      packages.${system}.default = pkgs.stdenv.mkDerivation {
+        pname = "shader";
+        version = "0.1";
+
+        src = ./.;
+
+        buildInputs = with pkgs; [
+          glfw
+          glew
+        ];
+
+        nativeBuildInputs = with pkgs; [
+          gcc
+        ];
+
+        buildPhase = ''
+          g++ shader.cpp -o shader -lglfw -lGLEW -lGL
+        '';
+
+        installPhase = ''
+          mkdir -p $out/bin
+          cp shader $out/bin/
+        '';
+      };
+
+      apps.${system}.default = {
+        type = "app";
+        program = "${self.packages.${system}.default}/bin/shader";
+      };
+    };
+}

shader_example/motion.frag

@@ -0,0 +1,17 @@
+#version 330 core
+precision mediump float;
+
+in vec2 uv;
+
+uniform sampler2D texA;
+uniform sampler2D texB;
+
+out vec4 outColor;
+
+void main()
+{
+    outColor = abs(
+        texture(texA, uv) -
+        texture(texB, uv)
+    );
+}

shader_example/shader.cpp

@@ -0,0 +1,147 @@
+#include <GL/glew.h>
+#include <GLFW/glfw3.h>
+
+#define STB_IMAGE_IMPLEMENTATION
+#include "stb_image.h"
+
+#define STB_IMAGE_WRITE_IMPLEMENTATION
+#include "stb_image_write.h"
+
+#include <fstream>
+#include <vector>
+#include <iostream>
+
+std::string loadFile(const char* path)
+{
+    std::ifstream f(path);
+    return std::string((std::istreambuf_iterator<char>(f)),
+                       std::istreambuf_iterator<char>());
+}
+
+GLuint compile(GLenum t, const std::string& src)
+{
+    GLuint s = glCreateShader(t);
+    const char* c = src.c_str();
+    glShaderSource(s, 1, &c, NULL);
+    glCompileShader(s);
+    return s;
+}
+
+int main()
+{
+    glfwInit();
+    glfwWindowHint(GLFW_VISIBLE, GLFW_FALSE);
+
+    GLFWwindow* win = glfwCreateWindow(1,1,"",NULL,NULL);
+    glfwMakeContextCurrent(win);
+    glewInit();
+
+    // ---------------- LOAD IMAGES ----------------
+    int w,h,c;
+    unsigned char* img1 = stbi_load("frame1.jpg",&w,&h,&c,4);
+    unsigned char* img2 = stbi_load("frame2.jpg",&w,&h,&c,4);
+
+    if(!img1 || !img2)
+    {
+        std::cout << "image load failed\n";
+        return 0;
+    }
+
+    // ---------------- TEXTURES ----------------
+    GLuint texA, texB;
+    glGenTextures(1,&texA);
+    glGenTextures(1,&texB);
+
+    auto upload = [&](GLuint t, unsigned char* d)
+    {
+        glBindTexture(GL_TEXTURE_2D,t);
+        glTexImage2D(GL_TEXTURE_2D,0,GL_RGBA,w,h,0,GL_RGBA,GL_UNSIGNED_BYTE,d);
+
+        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);
+        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
+        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP_TO_EDGE);
+        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP_TO_EDGE);
+    };
+
+    upload(texA,img1);
+    upload(texB,img2);
+
+    // ---------------- SHADER ----------------
+    std::string frag = loadFile("motion.frag");
+
+    const char* vs =
+        "#version 330 core\n"
+        "layout(location=0) in vec2 pos;\n"
+        "out vec2 uv;\n"
+        "void main(){\n"
+        "uv = pos*0.5+0.5;\n"
+        "gl_Position = vec4(pos,0,1);\n"
+        "}";
+
+    GLuint v = compile(GL_VERTEX_SHADER,vs);
+    GLuint f = compile(GL_FRAGMENT_SHADER,frag);
+
+    GLuint prog = glCreateProgram();
+    glAttachShader(prog,v);
+    glAttachShader(prog,f);
+    glLinkProgram(prog);
+
+    // ---------------- QUAD (FIXED) ----------------
+    GLuint vao,vbo;
+
+    float quad[] = {
+        -1,-1,
+         1,-1,
+        -1, 1,
+         1, 1
+    };
+
+    glGenVertexArrays(1,&vao);
+    glBindVertexArray(vao);
+
+    glGenBuffers(1,&vbo);
+    glBindBuffer(GL_ARRAY_BUFFER,vbo);
+    glBufferData(GL_ARRAY_BUFFER,sizeof(quad),quad,GL_STATIC_DRAW);
+
+    glVertexAttribPointer(0,2,GL_FLOAT,GL_FALSE,0,0);
+    glEnableVertexAttribArray(0);
+
+    // ---------------- FBO ----------------
+    GLuint fbo,outTex;
+    glGenFramebuffers(1,&fbo);
+    glGenTextures(1,&outTex);
+
+    glBindTexture(GL_TEXTURE_2D,outTex);
+    glTexImage2D(GL_TEXTURE_2D,0,GL_RGBA,w,h,0,GL_RGBA,GL_UNSIGNED_BYTE,NULL);
+
+    glBindFramebuffer(GL_FRAMEBUFFER,fbo);
+    glFramebufferTexture2D(GL_FRAMEBUFFER,GL_COLOR_ATTACHMENT0,GL_TEXTURE_2D,outTex,0);
+
+    if(glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
+        std::cout << "FBO broken\n";
+
+    // ---------------- RENDER ----------------
+    glUseProgram(prog);
+
+    glActiveTexture(GL_TEXTURE0);
+    glBindTexture(GL_TEXTURE_2D,texA);
+    glUniform1i(glGetUniformLocation(prog,"texA"),0);
+
+    glActiveTexture(GL_TEXTURE1);
+    glBindTexture(GL_TEXTURE_2D,texB);
+    glUniform1i(glGetUniformLocation(prog,"texB"),1);
+
+    glViewport(0,0,w,h);
+    glBindFramebuffer(GL_FRAMEBUFFER,fbo);
+
+    glBindVertexArray(vao);
+    glDrawArrays(GL_TRIANGLE_STRIP,0,4);
+
+    // ---------------- READ BACK ----------------
+    std::vector<unsigned char> out(w*h*4);
+    glReadPixels(0,0,w,h,GL_RGBA,GL_UNSIGNED_BYTE,out.data());
+
+    stbi_write_jpg("frameO.jpg",w,h,4,out.data(),90);
+
+    std::cout << "done\n";
+}