原文：http://wiki.lwjgl.org/wiki/The_Quad_with_DrawElements

Introduction 介紹

之前教程裏用glDrawArrays畫方形，缺點是你必須指定所有三角形的每一個頂點。一個方形其實只有四個頂點，但是分成兩個三角形就變成了六個！用glDrawElements的話，就可以在定義頂點時把重複的點去掉了。取而代之的是，我們必須通過指定序號告訴OpenGL每個三角形用的是哪三個頂點。接下來教程開始。

Do you remember the following… 知識回顧

之前的DrawArrays教程將作爲本教程的基礎，也就是讀本篇前你得先了解什麼是AVO和VBO並且瞭解怎樣設置它們。你還需要知道VAO有許多屬性列表（默認是16個），我們可以將VBO連在上面。VBO裏含有數據，還需要綁定和解綁對象（比如VBO就是一個對象，頂點緩衝對象）。

Defining our vertices 定義頂點

定義僅含有方形四頂點的頂點數組。順序無所謂，因爲之後要具體在另一個對象裏指定順序。

// Vertices, the order is not important.
float[] vertices = {
        -0.5f, 0.5f, 0f,    // Left top         ID: 0
        -0.5f, -0.5f, 0f,   // Left bottom      ID: 1
        0.5f, -0.5f, 0f,    // Right bottom     ID: 2
        0.5f, 0.5f, 0f  // Right left       ID: 3
};
// Sending data to OpenGL requires the usage of (flipped) byte buffers
FloatBuffer verticesBuffer = BufferUtils.createFloatBuffer(vertices.length);
verticesBuffer.put(vertices);
verticesBuffer.flip();

如你所見，只定義了四個頂點，就是方形的四個頂點。數組數目由0開始到3結束，每個頂點有三個值，代表位置。現在在我們的腦海裏可以給它們定一個序號（見代碼註釋），如果直接將四點給OpenGL，它將不知所措。所以我們還需要給OpenGL一個指引，讓它用四點畫兩個三角形，因此我們還需要建另一個VBO。

Adding indices 增加序號

方形依舊是由兩個三角形組成的，在新的VBO裏，我們用序號來定義之前頂點的順序。如下：

// OpenGL expects to draw vertices in counter clockwise order by default
byte[] indices = {
        // Left bottom triangle
        0, 1, 2,
        // Right top triangle
        2, 3, 0
};
indicesCount = indices.length;
ByteBuffer indicesBuffer = BufferUtils.createByteBuffer(indicesCount);
indicesBuffer.put(indices);
indicesBuffer.flip();

如你所見，我們用序號來定義了兩個三角形。關鍵在於，我們雖然定義了四個頂點，但是其中兩個頂點用了兩次。這樣內存佔用減小了，從6降到了4，但是又增加了一個序號VBO，看起來好像得不償失，但是實際遊戲的模型可不僅僅是一個方形，頂點數量巨大，就值了。
後建的這個VBO類型略有不同，並非是一個普通的數組緩衝（GL_ARRAY_BUFFER），而是元素數組（GL_ELEMENT_ARRAY_BUFFER）。爲了它，還需要再申請內存ID，綁定，填充數據到緩衝（別忘了最後解綁）。

// Create a new VBO for the indices and select it (bind)
vboiId = GL15.glGenBuffers();
GL15.glBindBuffer(GL15.GL_ELEMENT_ARRAY_BUFFER, vboiId);
GL15.glBufferData(GL15.GL_ELEMENT_ARRAY_BUFFER, indicesBuffer, GL15.GL_STATIC_DRAW);
// Deselect (bind to 0) the VBO
GL15.glBindBuffer(GL15.GL_ELEMENT_ARRAY_BUFFER, 0);

vboiId是一個全局整數變量。還要注意我們保存了序號的數目（就和之前glDrawArrays教程裏保存頂點數目一樣）

Rendering with “glDrawElements” 用glDrawElements渲染

用glDrawElements渲染跟之前的glDrawArrays差不多，渲染前需要綁定序號VBO。

GL11.glClear(GL11.GL_COLOR_BUFFER_BIT);

// Bind to the VAO that has all the information about the vertices
GL30.glBindVertexArray(vaoId);
GL20.glEnableVertexAttribArray(0);

// Bind to the index VBO that has all the information about the order of the vertices
GL15.glBindBuffer(GL15.GL_ELEMENT_ARRAY_BUFFER, vboiId);

// Draw the vertices
GL11.glDrawElements(GL11.GL_TRIANGLES, indicesCount, GL11.GL_UNSIGNED_BYTE, 0);

// Put everything back to default (deselect)
GL15.glBindBuffer(GL15.GL_ELEMENT_ARRAY_BUFFER, 0);
GL20.glDisableVertexAttribArray(0);
GL30.glBindVertexArray(0);

Cleaning up our memory 清除內存

做內存管理的時候別忘了也刪除序號VBO。

// Delete the index VBO
GL15.glBindBuffer(GL15.GL_ELEMENT_ARRAY_BUFFER, 0);
GL15.glDeleteBuffers(vboiId);
The rest of our cleaning code looks exactly the same, here’s the complete cleanup code:

其他的清除代碼也一樣，以下是完整的清除代碼：

// Disable the VBO index from the VAO attributes list
GL20.glDisableVertexAttribArray(0);

// Delete the vertex VBO
GL15.glBindBuffer(GL15.GL_ARRAY_BUFFER, 0);
GL15.glDeleteBuffers(vboId);

// Delete the index VBO
GL15.glBindBuffer(GL15.GL_ELEMENT_ARRAY_BUFFER, 0);
GL15.glDeleteBuffers(vboiId);

// Delete the VAO
GL30.glBindVertexArray(0);
GL30.glDeleteVertexArrays(vaoId);

Display.destroy();

The result 結果

以下是畫出的方形（和之前一樣）：

Complete source code 完整代碼

import java.nio.ByteBuffer;
import java.nio.FloatBuffer;

import org.lwjgl.BufferUtils;
import org.lwjgl.LWJGLException;
import org.lwjgl.opengl.ContextAttribs;
import org.lwjgl.opengl.Display;
import org.lwjgl.opengl.DisplayMode;
import org.lwjgl.opengl.GL11;
import org.lwjgl.opengl.GL15;
import org.lwjgl.opengl.GL20;
import org.lwjgl.opengl.GL30;
import org.lwjgl.opengl.PixelFormat;

public class TheQuadExampleDrawElements {
    // Entry point for the application
    public static void main(String[] args) {
        new TheQuadExampleDrawElements();
    }

    // Setup variables
    private final String WINDOW_TITLE = "The Quad: glDrawElements";
    private final int WIDTH = 320;
    private final int HEIGHT = 240;
    // Quad variables
    private int vaoId = 0;
    private int vboId = 0;
    private int vboiId = 0;
    private int indicesCount = 0;

    public TheQuadExampleDrawElements() {
        // Initialize OpenGL (Display)
        this.setupOpenGL();

        this.setupQuad();

        while (!Display.isCloseRequested()) {
            // Do a single loop (logic/render)
            this.loopCycle();

            // Force a maximum FPS of about 60
            Display.sync(60);
            // Let the CPU synchronize with the GPU if GPU is tagging behind
            Display.update();
        }

        // Destroy OpenGL (Display)
        this.destroyOpenGL();
    }

    public void setupOpenGL() {
        // Setup an OpenGL context with API version 3.2
        try {
            PixelFormat pixelFormat = new PixelFormat();
            ContextAttribs contextAtrributes = new ContextAttribs(3, 2)
                .withProfileCore(true)
                .withForwardCompatible(true);

            Display.setDisplayMode(new DisplayMode(WIDTH, HEIGHT));
            Display.setTitle(WINDOW_TITLE);
            Display.create(pixelFormat, contextAtrributes);

            GL11.glViewport(0, 0, WIDTH, HEIGHT);
        } catch (LWJGLException e) {
            e.printStackTrace();
            System.exit(-1);
        }

        // Setup an XNA like background color
        GL11.glClearColor(0.4f, 0.6f, 0.9f, 0f);

        // Map the internal OpenGL coordinate system to the entire screen
        GL11.glViewport(0, 0, WIDTH, HEIGHT);
    }

    public void setupQuad() {
        // Vertices, the order is not important.
        float[] vertices = {
                -0.5f, 0.5f, 0f,    // Left top         ID: 0
                -0.5f, -0.5f, 0f,   // Left bottom      ID: 1
                0.5f, -0.5f, 0f,    // Right bottom     ID: 2
                0.5f, 0.5f, 0f      // Right left       ID: 3
        };
        // Sending data to OpenGL requires the usage of (flipped) byte buffers
        FloatBuffer verticesBuffer = BufferUtils.createFloatBuffer(vertices.length);
        verticesBuffer.put(vertices);
        verticesBuffer.flip();

        // OpenGL expects to draw vertices in counter clockwise order by default
        byte[] indices = {
                // Left bottom triangle
                0, 1, 2,
                // Right top triangle
                2, 3, 0
        };
        indicesCount = indices.length;
        ByteBuffer indicesBuffer = BufferUtils.createByteBuffer(indicesCount);
        indicesBuffer.put(indices);
        indicesBuffer.flip();

        // Create a new Vertex Array Object in memory and select it (bind)
        // A VAO can have up to 16 attributes (VBO's) assigned to it by default
        vaoId = GL30.glGenVertexArrays();
        GL30.glBindVertexArray(vaoId);

        // Create a new Vertex Buffer Object in memory and select it (bind)
        // A VBO is a collection of Vectors which in this case resemble the location of each vertex.
        vboId = GL15.glGenBuffers();
        GL15.glBindBuffer(GL15.GL_ARRAY_BUFFER, vboId);
        GL15.glBufferData(GL15.GL_ARRAY_BUFFER, verticesBuffer, GL15.GL_STATIC_DRAW);
        // Put the VBO in the attributes list at index 0
        GL20.glVertexAttribPointer(0, 3, GL11.GL_FLOAT, false, 0, 0);
        // Deselect (bind to 0) the VBO
        GL15.glBindBuffer(GL15.GL_ARRAY_BUFFER, 0);

        // Deselect (bind to 0) the VAO
        GL30.glBindVertexArray(0);

        // Create a new VBO for the indices and select it (bind)
        vboiId = GL15.glGenBuffers();
        GL15.glBindBuffer(GL15.GL_ELEMENT_ARRAY_BUFFER, vboiId);
        GL15.glBufferData(GL15.GL_ELEMENT_ARRAY_BUFFER, indicesBuffer, GL15.GL_STATIC_DRAW);
        // Deselect (bind to 0) the VBO
        GL15.glBindBuffer(GL15.GL_ELEMENT_ARRAY_BUFFER, 0);
    }

    public void loopCycle() {
        GL11.glClear(GL11.GL_COLOR_BUFFER_BIT);

        // Bind to the VAO that has all the information about the vertices
        GL30.glBindVertexArray(vaoId);
        GL20.glEnableVertexAttribArray(0);

        // Bind to the index VBO that has all the information about the order of the vertices
        GL15.glBindBuffer(GL15.GL_ELEMENT_ARRAY_BUFFER, vboiId);

        // Draw the vertices
        GL11.glDrawElements(GL11.GL_TRIANGLES, indicesCount, GL11.GL_UNSIGNED_BYTE, 0);

        // Put everything back to default (deselect)
        GL15.glBindBuffer(GL15.GL_ELEMENT_ARRAY_BUFFER, 0);
        GL20.glDisableVertexAttribArray(0);
        GL30.glBindVertexArray(0);
    }

    public void destroyOpenGL() {      
        // Disable the VBO index from the VAO attributes list
        GL20.glDisableVertexAttribArray(0);

        // Delete the vertex VBO
        GL15.glBindBuffer(GL15.GL_ARRAY_BUFFER, 0);
        GL15.glDeleteBuffers(vboId);

        // Delete the index VBO
        GL15.glBindBuffer(GL15.GL_ELEMENT_ARRAY_BUFFER, 0);
        GL15.glDeleteBuffers(vboiId);

        // Delete the VAO
        GL30.glBindVertexArray(0);
        GL30.glDeleteVertexArrays(vaoId);

        Display.destroy();
    }
}

Credit

Mathias Verboven (moci)

【LWJGL2 WIKI】【現代OpenGL篇】用DrawElements畫方形

Introduction 介紹

Do you remember the following… 知識回顧

Defining our vertices 定義頂點

Adding indices 增加序號

Rendering with “glDrawElements” 用glDrawElements渲染

Cleaning up our memory 清除內存

The result 結果

Complete source code 完整代碼

Credit

【LWJGL官方教程】遊戲邏輯

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