ios - 如何在OpenGLES中的球运动内画一条线？

Question

目前我正在使用 iphone 中的 OpenGLES 在球类游戏应用程序中工作，使用 GL_TRIANGLE_STRIP 从左到右绘制球和运动动作，然后 GL_LINES 画一条线，但线不能在球运动内部绘制，如何解决这个问题？

我尝试了示例代码：

#1

- (void)update
{
    self.timeSinceLastSpawn += self.timeSinceLastUpdate;
    self.timeSinceLastSpawn = 0;

    if (self.timeSinceLastSpawn > 2.0)
    {
        [self target]; // Call the Method target
    }

     for (SGGSprite * sprite in self.children) //SGGSprite is a class
     {
          [sprite update:self.timeSinceLastUpdate]; // call the update method in SGGSprite class.
     }
}

#2
- (void)update:(float)dt 
{
    GLKVector2 curMove = GLKVector2MultiplyScalar(self.moveVelocity, dt); 
    self.position = GLKVector2Add(self.position, curMove); 
}

#3
- (void)target
{
    SGGSprite * target = [[SGGSprite alloc] initWithFile:@"Ballimage.png" effect:self.effect];
    [self.children addObject:target]; 

    int minY = target.contentSize.height/2;
    int maxY = 320 - target.contentSize.height/2;
    int rangeY = maxY - minY;
    int actualY = (arc4random() % rangeY) + minY;

    if(actualY <= 100)
    {
        actualY = 215;
    }

    target.position = GLKVector2Make(480 + (target.contentSize.width), actualY);    
    int minVelocity = 480.0/12.0;
    int maxVelocity = 480.0/6.0;
    int rangeVelocity = maxVelocity - minVelocity;
    int actualVelocity = (arc4random() % rangeVelocity) + minVelocity;
    target.moveVelocity = GLKVector2Make(-actualVelocity, 0);
}

#4
- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect 
{
        glClearColor(1, 1, 1, 1);
        glClear(GL_COLOR_BUFFER_BIT); 
        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
        glEnable(GL_BLEND);

        for (SGGSprite * sprite in self.children)  // SGGSprite is a class, render method in SGGSprite class
        {
            [sprite render];             
        }
}

#5
- (void)render
{
    self.effect.texture2d0.name = self.textureInfo.name;
    self.effect.texture2d0.enabled = YES;
    self.effect.transform.modelviewMatrix = self.modelMatrix; // Call the method modelMatrix
    [self.effect prepareToDraw];

    long offset = (long)&_quad;
    glEnableVertexAttribArray(GLKVertexAttribPosition);
    glEnableVertexAttribArray(GLKVertexAttribTexCoord0);

    glVertexAttribPointer(GLKVertexAttribPosition, 2, GL_FLOAT, GL_FALSE, sizeof(TexturedVertex), (void *) (offset + offsetof(TexturedVertex, geometryVertex)));
    glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, sizeof(TexturedVertex), (void *) (offset + offsetof(TexturedVertex, textureVertex)));
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}

#6
- (GLKMatrix4) modelMatrix
{
    GLKMatrix4 modelMatrix = GLKMatrix4Identity;    
    modelMatrix = GLKMatrix4Translate(modelMatrix, -self.position.x, -self.position.y, 0);
    modelMatrix = GLKMatrix4Translate(modelMatrix, self.contentSize.width, self.contentSize.height+140, 0);
    return modelMatrix; 
}

// To draw a line, when the user tab on a screen.

-(void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event 
{
    UITouch *touch = [[event allTouches] anyObject];
    CGPoint location = [touch locationInView:touch.view]; 

    float xPosition = location.x;
    float yPosition = location.y;

        X =  xPosition/50;
        resultX = X - 4.8;

        Y = yPosition/61.538461538461538;
        resultY = Y - 3.2;

        self.effect = [[GLKBaseEffect alloc] init];
        [self.effect prepareToDraw];
        Test1 = YES;

        const GLfloat line[] =
        {
            0.0f, 0.5f,
            resultX, resultY,
        };

        // Create buffer object array
        GLuint bufferObjectNameArray;

        // Have OpenGL generate a buffer name and store the buffer object array 
        glGenBuffers(1, &bufferObjectNameArray);

        // Bind the buffer object array to the GL_ARRAY_BUFFER target buffer  
        glBindBuffer(GL_ARRAY_BUFFER, bufferObjectNameArray);

        // Send the line data over to the target buffer
        glBufferData(
                     GL_ARRAY_BUFFER,   // the target buffer 
                     sizeof(line),      // the number of bytes to put into the buffer
                     line,              // a pointer to the data being copied 
                     GL_STATIC_DRAW);   // the usage pattern of the data 

        // Enable vertex data to be fed down the graphics pipeline to be drawn
        glEnableVertexAttribArray(GLKVertexAttribPosition);

        // Specify how the GPU looks up the data
        glVertexAttribPointer(
                              GLKVertexAttribPosition, // the currently bound buffer holds the data 
                              2,                       // number of coordinates per vertex 
                              GL_FLOAT,                // the data type of each component 
                              GL_FALSE,                // can the data be scaled 
                              2*4,                    // how many bytes per vertex (2 floats per vertex)
                              NULL);         
        glDrawArrays(GL_LINES, 0, 2);
}