在android打开GLES 2.0中绘制一条线

Question

嗨，我是android open gl的新手。我尝试在open gles 2.0中创建几个2d对象。现在我试图在android open gles 2.0中绘制一条线。我的代码看起来像

public class UserLine
{
     private final String vertexShaderCode =
            "uniform mat4 uMVPMatrix;" +
            "attribute vec4 vPosition;" +
            "void main() {" +
            "  gl_Position = uMVPMatrix * vPosition;" +
            "}";

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

        private final FloatBuffer vertexBuffer;
        private final ShortBuffer drawListBuffer;
        private final int mProgram;
        private int mPositionHandle;
        private int mColorHandle;
        private int mMVPMatrixHandle;

        static final int COORDS_PER_VERTEX = 3;
        private final int vertexStride = COORDS_PER_VERTEX * 4;

        private float[] pathCords = 
            {
            0.00f, 0.0f, 0.0f,

            0.5f, 0.3f, 0.0f
            };
        private short[] pathDrawOrder = {0,1}
        private float[] color = {1.0f, 0.0f, 0.0f, 1.0f};
    public UserLine()
    {
        ByteBuffer bb = ByteBuffer.allocateDirect(pathCords.length * 4);
        bb.order(ByteOrder.nativeOrder());
        vertexBuffer = bb.asFloatBuffer();
        vertexBuffer.put(pathCords);
        vertexBuffer.position(0);

        ByteBuffer dlb = ByteBuffer.allocateDirect(pathDrawOrder.length * 2);
        dlb.order(ByteOrder.nativeOrder());
        drawListBuffer = dlb.asShortBuffer();
        drawListBuffer.put(pathDrawOrder);
        drawListBuffer.position(0);

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

        mProgram = GLES20.glCreateProgram();             
        GLES20.glAttachShader(mProgram, vertexShader);   
        GLES20.glAttachShader(mProgram, fragmentShader); 
        GLES20.glLinkProgram(mProgram); 
    }

    public void draw(float[] mvpMatrix) {

    GLES20.glUseProgram(mProgram);
    mPositionHandle = GLES20.glGetAttribLocation(mProgram, "vPosition");
    GLES20.glEnableVertexAttribArray(mPositionHandle);
    GLES20.glVertexAttribPointer(mPositionHandle, COORDS_PER_VERTEX,GLES20.GL_FLOAT, false,
            vertexStride, vertexBuffer);

    mColorHandle = GLES20.glGetUniformLocation(mProgram, "vColor");
    GLES20.glUniform4fv(mColorHandle, 1, color, 0);
    mMVPMatrixHandle = GLES20.glGetUniformLocation(mProgram, "uMVPMatrix");
    GLES20.glUniformMatrix4fv(mMVPMatrixHandle, 1, false, mvpMatrix, 0);

    GLES20.glDrawElements(GLES20.GL_LINES, color.length,
            GLES20.GL_UNSIGNED_SHORT, drawListBuffer);

    GLES20.glDisableVertexAttribArray(mPositionHandle);
    GLES20.glDisable(mColorHandle);
    }
}

所以这是我的问题根据这段代码，它必须绘制一条线，但沿着该线它还会绘制一条额外的线，这不是预期的行为。

我做错了什么或是默认行为。需要帮忙。谢谢。

Answer 1

我认为问题在于：

GLES20.glDrawElements(GLES20.GL_LINES, color.length, GLES20.GL_UNSIGNED_SHORT, drawListBuffer);

第二个参数是计数，我不明白为什么你把它设置为颜色的长度。这只是您传递到着色器的线条的颜色，因此长度始终为4：

private float[] color = {1.0f, 0.0f, 0.0f, 1.0f};

你正在使用pathDrawOrder中指定的2个点绘制一条线，所以我认为使用它的长度代替你所需要的：

GLES20.glDrawElements(GLES20.GL_LINES, pathDrawOrder.length, GLES20.GL_UNSIGNED_SHORT, drawListBuffer);