OpenGL基本使用

//GLSL腳本程式識別符號
static GLuint program;
//OpenGL頂點識別符號
GLuint vao;
//OpenGL buffer識別符號
GLuint vbo;
//OpenGL 紋理識別符號
GLuint tex;

static const GLfloat quad_data[] =
{
		//OpenGL繪制坐標點初始值
        0.0f, 0.0f,
        0.0f, 0.0f,
        0.0f, 0.0f,
        0.0f, 0.0f,
		//OpenGL紋理坐標點初始值
        0.0f, 0.0f,
        1.0f, 0.0f,
        1.0f, 1.0f,
        0.0f, 1.0f
};

頂點著色器腳本，主要是確定文字的繪制坐標，以及紋理坐標位置

#version 400 core

layout (location = 0) in vec2 in_position;
layout (location = 1) in vec2 in_tex_coord;

out vec2 tex_coord;

void main(void)
{
    gl_Position = vec4(in_position, 0.0f, 1.0);
    tex_coord = in_tex_coord;
}

片段著色器腳本，定義一個紋理器，輸出對應坐標的紋理顏色資訊

#version 450 core

in vec2 tex_coord;

layout (location = 0) out vec4 color;

uniform sampler2D tex;

void main(void)
{
    color = texture(tex, tex_coord);
}

編譯GLSL腳本

/* Create and compile a shader */
/* type指示編譯的是頂點腳本，還是片段腳本 */
/* src是對應腳本的字串 */
static GLuint
create_shader (int         type,
               const char *src)
{
    GLuint shader;
    int status;
	//創建一個著色器
    shader = glCreateShader (type);
    //將腳本與著色器系結
    glShaderSource (shader, 1, &src, NULL);
    //編譯GLSL腳本
    glCompileShader (shader);
	//獲取編譯資訊
    glGetShaderiv (shader, GL_COMPILE_STATUS, &status);
    if (status == GL_FALSE)
    {
        int log_len;
        char *buffer;
		//編譯失敗，獲取失敗log
        glGetShaderiv (shader, GL_INFO_LOG_LENGTH, &log_len);

        buffer = g_malloc (log_len + 1);
        glGetShaderInfoLog (shader, log_len, NULL, buffer);

        g_warning ("Compile failure in %s shader:\n%s",
                   type == GL_VERTEX_SHADER ? "vertex" : "fragment",
                   buffer);

        g_free (buffer);

        glDeleteShader (shader);

        return 0;
    }
	//回傳著色器編譯結果
    return shader;
}

鏈接著色器

/* Initialize the shaders and link them into a program */
/* vertex_path：頂點著色器腳本路徑 */
/* fragment_path：片段著色器腳本路徑 */
/* program_out：生成的著色器程式 */
static void
init_shaders (const char *vertex_path,
              const char *fragment_path,
              GLuint *program_out)
{
    GLuint vertex, fragment;
    GLuint _program = 0;
    int status;
    GBytes *source;

	//獲取頂點著色器檔案索引
    source = g_resources_lookup_data (vertex_path, 0, NULL);
    //獲取頂點著色器腳本檔案內容，并編譯著色器程式
    vertex = create_shader (GL_VERTEX_SHADER, g_bytes_get_data (source, NULL));
    g_bytes_unref (source);

    if (vertex == 0)
    {
        *program_out = 0;
        return;
    }

	//獲取片段著色器檔案索引
    source = g_resources_lookup_data (fragment_path, 0, NULL);
    //獲取片段著色器腳本檔案內容，并編譯著色器程式
    fragment = create_shader (GL_FRAGMENT_SHADER, g_bytes_get_data (source, NULL));
    g_bytes_unref (source);

    if (fragment == 0)
    {
        glDeleteShader (vertex);
        *program_out = 0;
        return;
    }

	//創建一個著色器程式
    _program = glCreateProgram ();
    //將頂點著色器與新創建的著色器程式系結起來
    glAttachShader (_program, vertex);
    //將片段著色器與新創建的著色器程式系結起來
    glAttachShader (_program, fragment);

	//鏈接著色器程式
    glLinkProgram (_program);

	//獲取鏈接資訊
    glGetProgramiv (_program, GL_LINK_STATUS, &status);
    if (status == GL_FALSE)
    {
        int log_len;
        char *buffer;
		//獲取鏈接錯誤資訊
        glGetProgramiv (_program, GL_INFO_LOG_LENGTH, &log_len);

        buffer = g_malloc (log_len + 1);
        glGetProgramInfoLog (_program, log_len, NULL, buffer);

        g_warning ("Linking failure:\n%s", buffer);

        g_free (buffer);

        glDeleteProgram (_program);
        _program = 0;

        goto out;
    }

    glDetachShader (_program, vertex);
    glDetachShader (_program, fragment);

    out:
    glDeleteShader (vertex);
    glDeleteShader (fragment);

	//回傳著色器程式
    if (program_out != NULL)
        *program_out = _program;
}

創建頂點物件、快取物件和紋理物件，并設定頂點引數和紋理引數資訊

static void
realize (GtkWidget *widget)
{
	......
	
	//創建一個快取buffer
    glGenBuffers(1, &vbo);
    //將vbo系結到GL_ARRAY_BUFFER
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    //初始化快取資料
    glBufferData(GL_ARRAY_BUFFER, sizeof(quad_data), quad_data, GL_STATIC_DRAW);

	//生成并系結一個頂點陣列物件
    glGenVertexArrays(1, &vao);
    glBindVertexArray(vao);
	//設定頂點陣列物件引數
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, 0);
    glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 0, (void*)(8 * sizeof(float)));

    glEnableVertexAttribArray(0);
    glEnableVertexAttribArray(1);

	//生成并系結一個紋理物件
    glGenTextures(1, &tex);
    glBindTexture(GL_TEXTURE_2D, tex);
	//設定紋理引數
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
    glTexEnvi(GL_TEXTURE_2D, GL_TEXTURE_ENV_MODE, GL_REPLACE);
    glGenerateMipmap(GL_TEXTURE_2D);

	......
}

vao與vbo的關系如下圖👇

①定義并系結vbo，初始化vbo系結的buffer資料
②定義并系結vao，設定vao中的pointer0 和 pointer1屬性，其實就是設定如何獲取vbo系結的buffer中的資料
③開啟pointer0 和 pointer1屬性

static gboolean
render (GtkGLArea    *area,
        GdkGLContext *context)
{
    ......

    /* Clear the viewport */
    glClearColor(0.11765f, 0.11765f, 0.11765f, 1.0f);
    glClearDepth(1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glDisable(GL_CULL_FACE);
    glUseProgram(program);

    glBindTexture(GL_TEXTURE_2D,tex);
    glBindVertexArray(vao);
    glEnable(GL_BLEND);
    //設定像素演算法
    glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);

    ......
}

下一篇文章會介紹OpenGL、FreeType2與GTK4結合使用

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