程式語言 - Simple DirectMedia Layer (SDL) - OpenGL ES 2.0 - C/C++ - Use Texture

參考資訊：
https://wiki.libsdl.org/FrontPage
https://kaprikawn.wordpress.com/2018/01/07/learning-to-learn-opengl-es-2-0-part-01/
http://www.subdude-site.com/WebPages_Local/RefInfo/Computer/Linux/LinuxGuidesOfOthers/linuxProgrammingGuides/pdfs/MobileDeviceProgramming/MOBogl_OpenGL_ES_2-0_ProgrammingGuide_2009_457pgs.pdf

main.c

#include <stdio.h>
#include <stdlib.h>
#include <GLES2/gl2.h>
#include <SDL2/SDL.h>

const char *vShaderSrc =
    "attribute vec4 a_position;   \n"
    "attribute vec2 a_texCoord;   \n"
    "varying vec2 v_texCoord;     \n"
    "void main()                  \n"
    "{                            \n"
    "   gl_Position = a_position; \n"
    "   v_texCoord = a_texCoord;  \n"
    "}                            \n";

const char *fShaderSrc =
    "#ifdef GL_ES                                         \n"
    "precision mediump float;                             \n"
    "#endif                                               \n"
    "varying vec2 v_texCoord;                             \n"
    "uniform sampler2D s_texture;                         \n"
    "void main()                                          \n"
    "{                                                    \n"
    "    gl_FragColor = texture2D(s_texture, v_texCoord); \n"
    "}                                                    \n";

int main(int argc, char **argv)
{
    int w = 320;
    int h = 240;

    SDL_Window *window = NULL;
    SDL_GLContext context = {0};

    SDL_Init(SDL_INIT_VIDEO);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_ES);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 2);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 0);

    window = SDL_CreateWindow("main", 0, 0, w, h, SDL_WINDOW_OPENGL);
    context = SDL_GL_CreateContext(window);

    GLuint vShader = 0;
    GLuint fShader = 0;
    GLuint pObject = 0;

    vShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vShader, 1, &vShaderSrc, NULL);
    glCompileShader(vShader);

    fShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fShader, 1, &fShaderSrc, NULL);
    glCompileShader(fShader);

    pObject = glCreateProgram();
    glAttachShader(pObject, vShader);
    glAttachShader(pObject, fShader);
    glLinkProgram(pObject);
    glUseProgram(pObject);

    GLint positionLoc = glGetAttribLocation(pObject, "a_position");
    GLint texCoordLoc = glGetAttribLocation(pObject, "a_texCoord");
    GLint samplerLoc = glGetUniformLocation(pObject, "s_texture");
    
    GLuint textureId = 0;
    GLubyte pixels[320 * 240 * 3] = {0};

    int x = 0, y = 0;
    for (y = 0; y < 240; y++) {
        for (x = 0; x < 320; x++) {
            switch (y / 80) {
            case 0:
                pixels[(y * 320 * 3) + (x * 3) + 0] = 0xff;
                pixels[(y * 320 * 3) + (x * 3) + 1] = 0x00;
                pixels[(y * 320 * 3) + (x * 3) + 2] = 0x00;
                break;
            case 1:
                pixels[(y * 320 * 3) + (x * 3) + 0] = 0x00;
                pixels[(y * 320 * 3) + (x * 3) + 1] = 0xff;
                pixels[(y * 320 * 3) + (x * 3) + 2] = 0x00;
                break;
            case 2:
                pixels[(y * 320 * 3) + (x * 3) + 0] = 0x00;
                pixels[(y * 320 * 3) + (x * 3) + 1] = 0x00;
                pixels[(y * 320 * 3) + (x * 3) + 2] = 0xff;
                break;
            }
        }
    }

    glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
    glGenTextures(1, &textureId);
    glBindTexture(GL_TEXTURE_2D, textureId);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 320, 240, 0, GL_RGB, GL_UNSIGNED_BYTE, pixels);
    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);
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

    GLfloat vVertices[] = {
       -1.0f,  1.0f, 0.0f, 0.0f,  0.0f,
       -1.0f, -1.0f, 0.0f, 0.0f,  1.0f,
        1.0f, -1.0f, 0.0f, 1.0f,  1.0f,
        1.0f,  1.0f, 0.0f, 1.0f,  0.0f
    };
    GLushort indices[] = {0, 1, 2, 0, 2, 3};

    glViewport(0, 0, w, h);
    glClear(GL_COLOR_BUFFER_BIT);
    glVertexAttribPointer(positionLoc, 3, GL_FLOAT, GL_FALSE, 5 * sizeof(GLfloat), vVertices);
    glVertexAttribPointer(texCoordLoc, 2, GL_FLOAT, GL_FALSE, 5 * sizeof(GLfloat), &vVertices[3]);
    glEnableVertexAttribArray(positionLoc);
    glEnableVertexAttribArray(texCoordLoc);
    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, textureId);
    glUniform1i(samplerLoc, 0);
    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, indices);
    SDL_GL_SwapWindow(window);
    SDL_Delay(3000);

    glDeleteShader(vShader);
    glDeleteShader(fShader);
    SDL_DestroyWindow(window);
    SDL_GL_DeleteContext(context);
    SDL_Quit();
    return 0;
}

編譯、執行

$ gcc main.c -lSDL2 -lGLESv2 -o main
$ ./main