在三角形顶部绘制一条线

Question

我在Xcode中创建了一个新的iPhone OpenGL项目。我用三角形填充了我的背景，并给了它们一个纹理，如下所示：

CGImageRef spriteImage; 
CGContextRef spriteContext; 
GLubyte *spriteData; 
size_t width, height; 

// Sets up matrices and transforms for OpenGL ES 
glViewport(0, 0, backingWidth, backingHeight); 
glMatrixMode(GL_PROJECTION); 
glLoadIdentity(); 
//glRotatef(-90,0,0,1); 
glOrthof(-1.0f, 1.0f, -1.5f, 1.5f, -1.0f, 1.0f); 
glMatrixMode(GL_MODELVIEW); 

// Clears the view with black 
glClearColor(0.0f, 0.0f, 0.0f, 1.0f); 

// Sets up pointers and enables states needed for using vertex arrays and textures 
glVertexPointer(2, GL_FLOAT, 0, vertices); 
glEnableClientState(GL_VERTEX_ARRAY); 
//glColorPointer(4, GL_FLOAT, 0, triangleColors); 
//glColor4f(0.0f,1.0f,0.0f,1.0f); 
//glEnableClientState(GL_COLOR_ARRAY); 
glTexCoordPointer(2, GL_FLOAT, 0, spriteTexcoords); 
glEnableClientState(GL_TEXTURE_COORD_ARRAY); 

// Creates a Core Graphics image from an image file 
spriteImage = [UIImage imageNamed:@"Bild.png"].CGImage; 
// Get the width and height of the image 
width = CGImageGetWidth(spriteImage); 
height = CGImageGetHeight(spriteImage); 
// Texture dimensions must be a power of 2. If you write an application that allows users to supply an image, 
// you'll want to add code that checks the dimensions and takes appropriate action if they are not a power of 2. 

if(spriteImage) { 
    // Allocated memory needed for the bitmap context 
    spriteData = (GLubyte *) calloc(width * height * 4, sizeof(GLubyte)); 
    // Uses the bitmap creation function provided by the Core Graphics framework. 
    spriteContext = CGBitmapContextCreate(spriteData, width, height, 8, width * 4, CGImageGetColorSpace(spriteImage), kCGImageAlphaPremultipliedLast); 
    // After you create the context, you can draw the sprite image to the context. 
    CGContextDrawImage(spriteContext, CGRectMake(0.0, 0.0, (CGFloat)width, (CGFloat)height), spriteImage); 
    // You don't need the context at this point, so you need to release it to avoid memory leaks. 
    CGContextRelease(spriteContext); 

    // Use OpenGL ES to generate a name for the texture. 
    glGenTextures(1, &spriteTexture); 
    // Bind the texture name. 
    glBindTexture(GL_TEXTURE_2D, spriteTexture); 
    // Set the texture parameters to use a minifying filter and a linear filer (weighted average) 
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); 
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); 

    // Specify a 2D texture image, providing the a pointer to the image data in memory 
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, spriteData); 
    // Release the image data 
    free(spriteData); 

    // Enable use of the texture 
    glEnable(GL_TEXTURE_2D); 
    // Set a blending function to use 
    glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA); 
    // Enable blending 
    glEnable(GL_BLEND); 

我有两个问题，BC。我对OpenGL并不熟悉。

我想写一个方法，我给两个点作为参数，我希望这两个点之间的直线被绘制在我的三角形（背景）之上。

 - (void) drawLineFromPoint1:(CGPoint)point1 toPoint2:(CGPoint)point2 { 
GLfloat triangle[] = {   //Just example points 
     0.0f, 0.0f,  
     0.1f, 0.0f, 
     0.1f, 0.0f, 
     0.1f, 0.1f 
    }; 
    GLfloat triangleColors[] = { 
     0.5f, 0.5f, 0.5f, 1.0f 
    }; 
    //now draw the triangle 
} 

就是这样的。现在，我想有一个第二个方法，它会清除这条线（而不是背景）

我的画法是这样的：

- (void)drawView 
{ 
    // Make sure that you are drawing to the current context 
    [EAGLContext setCurrentContext:context]; 

    glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer); 
    glClear(GL_COLOR_BUFFER_BIT); 
    glDrawElements(GL_TRIANGLES, number_vertices, GL_UNSIGNED_SHORT, indices); 
    glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer); 
    [context presentRenderbuffer:GL_RENDERBUFFER_OES]; 

} 

将是巨大的，如果你可以给é一些提示/帮助，

欢呼

Answer 1

传统的方法是重绘一切每当移动或删除一条线。

Answer 2

嗯，我得到它的工作。我错过了将我的drawView中的顶点指针设置为我的三角形。这里现在的作品：

- (void)drawView 
    { 

    [EAGLContext setCurrentContext:context]; 

    glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer); 
    glClear(GL_COLOR_BUFFER_BIT); 
    glVertexPointer(2, GL_FLOAT, 0, vertices); 
    glDrawElements(GL_TRIANGLES, number_vertices, GL_UNSIGNED_SHORT, indices); 
    [self drawLines]; 
    glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer); 
    [context presentRenderbuffer:GL_RENDERBUFFER_OES]; 

} 

    - (void) drawLines{ 
    glDisable(GL_TEXTURE_2D); 

    GLfloat points[4]; 
    for (Dataset *data in buttons) { 
     CGPoint s = [data screenPosition]; 
     CGPoint p = [data slot]; 
     points[0] = (GLfloat)(768-s.y); 
     points[1] = (GLfloat)(1024-s.x); 
     points[2] = (GLfloat)(768-p.y); 
     points[3] = (GLfloat)(1024-p.x); 
     glVertexPointer(2, GL_FLOAT, 0, points); 
     glDrawArrays(GL_LINE_STRIP, 0, 2); 

    } 
    glEnable(GL_TEXTURE_2D); 


}