Draw a line on top of triangles - iphone

Question

I created a new iPhone OpenGL Project in Xcode. I filled my background with triangles and gave them a texture, see below:
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);
I have got two questions, bc. I am not so familiar with OpenGL.
I want to write a method, which I give two points as parameters and I want a Line between these two points to be drawn above my triangles (background).
- (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
}
Something like that. Now I want to have a 2nd method, which erases this line (and not the background)
My drawing method looks like this:
- (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];
}
Would be great if you can give e some hints/help,
cheers

Answer 1

The conventional approach would be to redraw everything whenever you move or erase a line.

Answer 2

Well, I got it to work. I just missed to set the Vertex-Pointer in my drawView to my triangles. This here now works:
- (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);
}