OpenGL render issue

Question

(Also asked on the Apple Developer forums)

I am new to OpenGL, so be kind :). I seem to have a rendering issue with OpenGL. My app is supposed to draw a line as the user traces his finger across the screen. So, touchesMoved calls the draw function below and passes a NSMutableArray currentStroke which contains point data.

The issue I am having is that the last triangle drawn for each line (each line ends with touchesEnded) flickers. I think it has to do with the render buffer... If I call the draw function twice from touchesEnded, the flickering goes away. I just think that is a very, very jimmy-rigged solution. Anyone have an idea of what could cause the flickering and a solution?

- (void)draw {

glLoadIdentity();

//Configuring OpenGL
glEnable(GL_TEXTURE_2D);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);



//Texture Loading
GLuint      texture[1];
glGenTextures(1, &texture[0]);
glBindTexture(GL_TEXTURE_2D, texture[0]);

//Configuring Image for OpenGL Texture: Switch off mipmap; Set linear scaling
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR); 
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);

//Set image to texture
NSString *path = [[NSBundle mainBundle] pathForResource:@"at2" ofType:@"png"];
    NSData *texData = [[NSData alloc] initWithContentsOfFile:path];
    UIImage *image = [[UIImage alloc] initWithData:texData];
    if (image == nil)
        NSLog(@"No Image");

    GLuint width = CGImageGetWidth(image.CGImage);
    GLuint height = CGImageGetHeight(image.CGImage);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    void *imageData = malloc( height * width * 4 );
    CGContextRef context2 = CGBitmapContextCreate( imageData, width, height, 8, 4 * width, colorSpace, kCGImageAlphaPremultipliedLast |kCGBitmapByteOrder32Big );
    CGColorSpaceRelease( colorSpace );
    CGContextClearRect( context2, CGRectMake( 0, 0, width, height ) );
    CGContextTranslateCTM( context2, 0, height - height );
    CGContextDrawImage( context2, CGRectMake( 0, 0, width, height ), image.CGImage );

    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageData);

    CGContextRelease(context2);

    free(imageData);
    [image release];
    [texData release];

static const GLfloat texCoords[] = {
        0.0, 1.0,
        1.0, 1.0,
        1.0, 0.0,
        0.0, 0.0
 };




glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_NORMAL_ARRAY);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);

GLfloat vertices[8];
GLfloat xadjust;
GLfloat yadjust;

//Iterate through points and create triangles

if ( (currentStroke != nil) && ([currentStroke count] > 1)) {

  for (int i=0; i<([currentStroke count]-1); i++) {
    vertices[0] = [[currentStroke objectAtIndex:i] CGPointValue].x;
    vertices[1] = self.frame.size.height - [[currentStroke objectAtIndex:i] CGPointValue].y;
    vertices[2] = [[currentStroke objectAtIndex:i+1] CGPointValue].x;
    vertices[3] = self.frame.size.height - [[currentStroke objectAtIndex:i+1] CGPointValue].y;

    //Minimum width of triangle
    xadjust = -4;


    //Minimum height of triangle, increase of y aspect of triangle is short
    if ((vertices[3]-vertices[1])<1&&(vertices[3]-vertices[1])>=0) {
      yadjust = 2;
    } else if ((vertices[3]-vertices[1])>-1&&(vertices[3]-vertices[1])<=0){ 
      yadjust = -3;
    } else  {
      yadjust = 2;
    }

    vertices[4] = [[currentStroke objectAtIndex:i] CGPointValue].x+xadjust;
    vertices[5] = self.frame.size.height - [[currentStroke objectAtIndex:i] CGPointValue].y+yadjust;
    vertices[6] = [[currentStroke objectAtIndex:i+1] CGPointValue].x+xadjust;
    vertices[7] = self.frame.size.height - [[currentStroke objectAtIndex:i+1] CGPointValue].y+yadjust;

    glTexCoordPointer(2, GL_FLOAT, 0, texCoords);

    glVertexPointer(2, GL_FLOAT, 0, vertices);

    glEnableClientState(GL_VERTEX_ARRAY);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);


  }




  glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
  [context presentRenderbuffer:GL_RENDERBUFFER_OES];


  glDisable(GL_BLEND);
  glDisableClientState(GL_VERTEX_ARRAY);
  glDisableClientState(GL_NORMAL_ARRAY);
  glDisableClientState(GL_TEXTURE_COORD_ARRAY);

}

Answer 1

Not directly related to the question, but you load the image each and every time you draw, don't you ? (or maybe not, but I'm puzzled by objective-c's syntax) If you do, it's not a good thing. Especially initWithData:texData and glTexImage2D should be done only once, outside draw().

For the flickering, it's usually a double-buffering issue, but on iPhone it's always a double-buffer.

I think you should call glBindRenderbufferOES before the actual draw calls, though.

Answer 2

oh, I thing I got it.

for (int i=0; i<([currentStroke count]-1); i++)
    vertices[2] = [[currentStroke objectAtIndex:i+1] CGPointValue].x;

so you access [currentStroke count] -1 +1 -> out of bounds for (int i=0; i<([currentStroke count]-2); i++), and make sure count > 2...