打开 GL ES 2.0 画一个正方形

Question

我正在尝试遵循 Android 开发人员“使用 OpenGL ES 显示图形”教程。 在第一部分中，我必须画一个三角形，并且没问题。现在我应该使用可以按照教程定义的 Square 类来绘制一个正方形。 当我尝试绘制时（使用我主要从 Triangle 类复制的 draw() 方法），屏幕仅显示一个三角形，它是定义正方形的一半（显然我已经注释掉了 triangle.draw() 线.. .).

这是我的代码，有人可以解决吗？

Square.java：

public class Square {

    private FloatBuffer vertexBuffer;
    private ShortBuffer drawListBuffer;

    private final int mProgram;

    private final String vertexShaderCode =
            "attribute vec4 vPosition;" +
                    "void main() {" +
                    "  gl_Position = vPosition;" +
                    "}";

    private final String fragmentShaderCode =
            "precision mediump float;" +
                    "uniform vec4 vColor;" +
                    "void main() {" +
                    "  gl_FragColor = vColor;" +
                    "}";

    // number of coordinates per vertex in this array
    static final int COORDS_PER_VERTEX = 3;
    static float squareCoords[] = {
            -0.5f,  0.5f, 0.0f,   // top left
            -0.5f, -0.5f, 0.0f,   // bottom left
            0.5f, -0.5f, 0.0f,   // bottom right
            0.5f,  0.5f, 0.0f }; // top right

    // Set color with red, green, blue and alpha (opacity) values
    float color[] = { 0.63671875f, 0.76953125f, 0.22265625f, 1.0f };

    private short drawOrder[] = { 0, 1, 2, 0, 2, 3 }; // order to draw vertices

    public Square() {
        // initialize vertex byte buffer for shape coordinates
        ByteBuffer bb = ByteBuffer.allocateDirect(squareCoords.length * 4);
        bb.order(ByteOrder.nativeOrder());
        vertexBuffer = bb.asFloatBuffer();
        vertexBuffer.put(squareCoords);
        vertexBuffer.position(0);

        // initialize byte buffer for the draw list
        ByteBuffer dlb = ByteBuffer.allocateDirect(drawOrder.length * 2);
        dlb.order(ByteOrder.nativeOrder());
        drawListBuffer = dlb.asShortBuffer();
        drawListBuffer.put(drawOrder);
        drawListBuffer.position(0);

        int vertexShader = MyGLRenderer.loadShader(GLES20.GL_VERTEX_SHADER,
                vertexShaderCode);
        int fragmentShader = MyGLRenderer.loadShader(GLES20.GL_FRAGMENT_SHADER,
                fragmentShaderCode);

        // create empty OpenGL ES Program
        mProgram = GLES20.glCreateProgram();

        // add the vertex shader to program
        GLES20.glAttachShader(mProgram, vertexShader);

        // add the fragment shader to program
        GLES20.glAttachShader(mProgram, fragmentShader);

        // creates OpenGL ES program executables
        GLES20.glLinkProgram(mProgram);
    }

    private int mPositionHandle;
    private int mColorHandle;

    private final int vertexCount = squareCoords.length / COORDS_PER_VERTEX;
    private final int vertexStride = COORDS_PER_VERTEX * 4; // 4 bytes per vertex

    public void draw() {
        // Add program to OpenGL ES environment
        GLES20.glUseProgram(mProgram);

        // get handle to vertex shader's vPosition member
        mPositionHandle = GLES20.glGetAttribLocation(mProgram, "vPosition");

        // Enable a handle to the triangle vertices
        GLES20.glEnableVertexAttribArray(mPositionHandle);

        // Prepare the triangle coordinate data
        GLES20.glVertexAttribPointer(mPositionHandle, COORDS_PER_VERTEX,
                GLES20.GL_FLOAT, false,
                vertexStride, vertexBuffer);

        // get handle to fragment shader's vColor member
        mColorHandle = GLES20.glGetUniformLocation(mProgram, "vColor");

        // Set color for drawing the triangle
        GLES20.glUniform4fv(mColorHandle, 1, color, 0);

        // Draw the square
        GLES20.glDrawArrays(GLES20.GL_TRIANGLES, 0, vertexCount);

        // Disable vertex array
        GLES20.glDisableVertexAttribArray(mPositionHandle);
    }
}

Answer 1

见Triangle primitives

你有4个顶点坐标，排列如下：

static float squareCoords[] = {
   -0.5f,  0.5f, 0.0f,   // top left
   -0.5f, -0.5f, 0.0f,   // bottom left
    0.5f, -0.5f, 0.0f,   // bottom right
    0.5f,  0.5f, 0.0f }; // top right

0       3
 x     x
 |     |
 |     |
 x-----x 
1       2

此模式匹配原始类型GL_TRIANGLE_FAN，但不匹配原始类型GL_TRIANGLES。

更改原始类型以解决您的问题：

GLES20.glDrawArrays(GLES20.GL_TRIANGLE_FAN, 0, vertexCount);

---

注意对于基本类型GL_TRIANGLES，您需要 6 个顶点坐标，例如有以下安排：

0  3      5
 x x-----x
 | \ \   |
 |   \ \ |
 x-----x x 
1      2  4

原始类型GL_TRIANGLES 匹配您的索引缓冲区：

private short drawOrder[] = { 0, 1, 2, 0, 2, 3 };

如果要从索引列表中绘制图元，则必须使用glDrawElements：

GLES20.glDrawElements(GLES20.GL_TRIANGLES, 6, GLES20.GL_SHORT, drawListBuffer);