如何在 SDL2 中显示 OpenGL 纹理

Question

In a SDL2 window i've drawn a rectangle on which i plan to display an image. The problem i have now is that the window is completely black. It doesnt display neither the colored rectangle nor the image that was supposed to be laid over the rectangle.

#define SDL_MAIN_HANDLED
#include <GL\glew.h>
#include <GL\gl.h>
#include <SDL2\SDL.h>
#include "stb_image.h"
#include <cstdio>

const GLchar *vertexSource =
        R"glsl(
    #version 150 core
    in vec2 position;
    in vec3 color;
    in vec2 texcoord;
    out vec3 Color;
    out vec2 Texcoord;
    void main()
    {
        Color = color;
        Texcoord = texcoord;
        gl_Position = vec4(position, 0.0, 1.0);
    }
)glsl";
const GLchar *fragmentSource =
        R"glsl(
    #version 150 core
    in vec3 Color;
    in vec2 Texcoord;
    out vec4 outColor;
    uniform sampler2D tex;
    void main()
    {
        outColor = texture(tex, Texcoord) * vec4(Color, 1.0);
    }
)glsl";

int main()
{
    if (SDL_Init(SDL_INIT_VIDEO) < 0)
    {
        std::puts("Could not initialize SDL2!");
        exit(1);
    }

    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 2);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK,
            SDL_GL_CONTEXT_PROFILE_CORE);

    SDL_Window *mainWindow = SDL_CreateWindow("Main Window",
            SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 800, 600,
            SDL_WINDOW_OPENGL);

    if (nullptr == mainWindow)
    {
        std::puts("Could not create window!");
        SDL_Quit();
        exit(1);
    }

    if (!SDL_GL_CreateContext(mainWindow))
    {
        std::puts("Could not create context!");
        SDL_Quit();
        exit(1);
    }

    glewExperimental = GL_TRUE;
    if (glewInit() != GLEW_OK)
    {
        std::puts("Could not initialize GLEW!");
        SDL_Quit();
        exit(1);
    }

    GLuint vao;
    glGenVertexArrays(1, &vao);
    glBindVertexArray(vao);

    GLuint vbo;
    glGenBuffers(1, &vbo);

    GLfloat vertices[] = {
            -0.5f, 0.5f, 1.0f, 0.0f, 0.0f, 0.0f, 0.0f,
            0.5f, 0.5f, 0.0f, 1.0f, 0.0f, 1.0f, 0.0f,
            0.5f, -0.5f, 0.0f, 0.0f, 1.0f, 1.0f, 1.0f,
            -0.5f, -0.5f, 0.5f, 0.5f, 0.5f, 0.0f, 1.0f
    };

    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    GLuint ebo;
    glGenBuffers(1, &ebo);

    GLuint elements[] =
    { 0, 1, 2, 2, 3, 0 };

    glBindBuffer(GL_ARRAY_BUFFER, ebo);
    glBufferData(GL_ARRAY_BUFFER, sizeof(elements), elements, GL_STATIC_DRAW);

    GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader, 1, &vertexSource, 0);
    glCompileShader(vertexShader);

    GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader, 1, &fragmentSource, 0);
    glCompileShader(fragmentShader);

    GLuint shaderProgram = glCreateProgram();
    glAttachShader(shaderProgram, vertexShader);
    glAttachShader(shaderProgram, fragmentShader);
    glBindFragDataLocation(shaderProgram, 0, "outColor");
    glLinkProgram(shaderProgram);
    glUseProgram(shaderProgram);

    GLint posAttrib = glGetAttribLocation(shaderProgram, "position");
    glEnableVertexAttribArray(posAttrib);
    glVertexAttribPointer(posAttrib, 2, GL_FLOAT, GL_FALSE, 7 * sizeof(float),
            0);

    GLint colAttrib = glGetAttribLocation(shaderProgram, "color");
    glEnableVertexAttribArray(colAttrib);
    glVertexAttribPointer(colAttrib, 3, GL_FLOAT, GL_FALSE, 7 * sizeof(float),
            (void*) (2 * sizeof(GLfloat)));

    GLint texAttrib = glGetAttribLocation(shaderProgram, "texcoord");
    glEnableVertexAttribArray(texAttrib);
    glVertexAttribPointer(texAttrib, 2, GL_FLOAT, GL_FALSE, 7 * sizeof(float),
            (void*) (5 * sizeof(GLfloat)));

    GLuint texture;
    glGenTextures(1, &texture);
    glBindTexture(GL_TEXTURE_2D, texture);

    int width, height, noOfChannels;
    unsigned char *imageData = stbi_load("path",
            &width, &height, &noOfChannels, 0);
    if (imageData)
    {
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB,
        GL_UNSIGNED_BYTE, imageData);
        stbi_image_free(imageData);
    }
    else
    {
        std::puts("Could not load image");
        SDL_Quit();
        exit(1);
    }

    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

    SDL_Event event;
    bool running = true;

    while (running)
    {
        while (SDL_PollEvent(&event) != 0)
        {
            if (event.type == SDL_QUIT)
            {
                running = false;
            }
        }
        glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

        SDL_GL_SwapWindow(mainWindow);

    }

    glDeleteTextures(1, &texture);
    glDeleteProgram(shaderProgram);
    glDeleteShader(vertexShader);
    glDeleteShader(fragmentShader);

    glDeleteBuffers(1, &vbo);
    glDeleteBuffers(1, &ebo);

    glDeleteVertexArrays(1, &vao);

    SDL_Quit();
    return 0;
}

Is is something about the main loop that doesn't properly display the image? My understanding is that for every loop i'm supposed to clear the screen, draw the shape and then load the back buffer that has my image.

Answer 1

glDrawElements is intended for use with an Index buffer ( GL_ELEMENT_ARRAY_BUFFER ). You have accidentally specified 2 Vertex Buffers ( GL_ARRAY_BUFFER ), but no index buffer:

glBindBuffer(GL_ARRAY_BUFFER, ebo);
glBufferData(GL_ARRAY_BUFFER, sizeof(elements), elements, GL_STATIC_DRAW);

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(elements), elements, GL_STATIC_DRAW);