在android中画一条线GLES 2.0

Question

Hi I am new in android open gl. I tries to create few 2d objects in open gles 2.0. Now I tried to draw a line in android open gles 2.0. my code looks like

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);
    }
}

SO here is my problem According to this code it must draw one line but along that line it also draw one extra line which is not expected behavior.

Am I doing some thing wrong or it that default behavior. Need Help. Thank you.

Answer 1

I think the problem is here:

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

The second parameter is the count and I don't understand why you have it set to the length of color. That's just the color of the line you pass into the shader so the length is always 4:

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

You are drawing a line with 2 points, as specified in pathDrawOrder, so I think using the length of that instead is what you need:

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