Paint app for ipad [opengl-es] line strokes not proper

I have a strange problem with openGL. I am working on paint app for both iphone and ipad. I am using opengl-es for my app. In my app I am filling colors in outline images, drawings a line onscreen based on where the user touches. I just use the "touchesMoved" function to draw a line between two points. Since I would like lines to stay on screen, in my renderLineFromPoint function, but for some reason some of the points of the line just drop out, and it appears completely random. However ipad simulator and iphone device/simulator gives desired output. Line stroke appears as shown in figure.

I am creating buffer using following code:

- (BOOL)createFramebuffer{
// Generate IDs for a framebuffer object and a color renderbuffer
glGenFramebuffersOES(1, &viewFramebuffer);
glGenRenderbuffersOES(1, &viewRenderbuffer);

glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
// This call associates the storage for the current render buffer with the EAGLDrawable (our CAEAGLLayer)
// allowing us to draw into a buffer that will later be rendered to screen wherever the layer is (which corresponds with our view).

[context renderbufferStorage:GL_RENDERBUFFER_OES fromDrawable:(id开发者_开发百科<EAGLDrawable>)self.layer];


glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, viewRenderbuffer);

glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);


NSLog(@"Backing Width:%i and Height: %i", backingWidth, backingHeight);


// For this sample, we also need a depth buffer, so we'll create and attach one via another renderbuffer.
glGenRenderbuffersOES(1, &depthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, depthRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_DEPTH_COMPONENT16_OES, backingWidth, backingHeight);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, depthRenderbuffer);

if(glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES) != GL_FRAMEBUFFER_COMPLETE_OES)
{
    NSLog(@"failed to make complete framebuffer object %x", glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES));
    return NO;
}

return YES;

}

I am using following code snippet for renderLineFromPoint

- (void) renderLineFromPoint:(CGPoint)start toPoint:(CGPoint)end{
static GLfloat*     vertexBuffer = NULL;
static NSUInteger   vertexMax = 64;

NSUInteger          vertexCount = 0,
                    count,
                    i;

[EAGLContext setCurrentContext:context];
glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);

// Convert locations from Points to Pixels
//CGFloat scale = self.contentScaleFactor;
CGFloat scale;
if ([self respondsToSelector: @selector(contentScaleFactor)])
{

    scale=self.contentScaleFactor;

}
else{


//scale = 1.000000;

    if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)] == YES && [[UIScreen mainScreen] scale] == 2.00) {
        // RETINA DISPLAY

        scale = 2.000000;
    }
    else {
        scale = 1.000000;
    }

}


NSLog(@"start point %@", NSStringFromCGPoint(start));

NSLog(@"End Point %@", NSStringFromCGPoint(end));



start.x *= scale;
start.y *= scale;
end.x *= scale;
end.y *= scale;

// Allocate vertex array buffer
if(vertexBuffer == NULL)
//  vertexBuffer = malloc(vertexMax * 2 * sizeof(GLfloat));


vertexBuffer = malloc(vertexMax * 2 * sizeof(GLfloat));


// Add points to the buffer so there are drawing points every X pixels
count = MAX(ceilf(sqrtf((end.x - start.x) * (end.x - start.x) + (end.y - start.y) * (end.y - start.y)) / kBrushPixelStep), 1);

NSLog(@"count %d",count);

for(i = 0; i < count; ++i) {
    if(vertexCount == vertexMax) {
        vertexMax = 2 * vertexMax;
        vertexBuffer = realloc(vertexBuffer, vertexMax * 2 * sizeof(GLfloat));

        NSLog(@"if loop");

            }

    vertexBuffer[2 * vertexCount + 0] = start.x + (end.x - start.x) * ((GLfloat)i / (GLfloat)count);
    vertexBuffer[2 * vertexCount + 1] = start.y + (end.y - start.y) * ((GLfloat)i / (GLfloat)count);

    vertexCount += 1;
}


NSLog(@"Scale  vertex %f",scale);

//NSLog(@"%i",vertexCount);




// Render the vertex array
glVertexPointer(2, GL_FLOAT, 0, vertexBuffer);

glDrawArrays(GL_POINTS, 0, vertexCount);


// Display the buffer
glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
[context presentRenderbuffer:GL_RENDERBUFFER_OES];

}

touchbegan function code:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
CGRect              bounds = [self bounds];
UITouch*    touch = [[event touchesForView:self] anyObject];
firstTouch = YES;
// Convert touch point from UIView referential to OpenGL one (upside-down flip)
location = [touch locationInView:self];
location.y = bounds.size.height - location.y;

}

touchmoved function code:

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{  

CGRect              bounds = [self bounds];
UITouch*            touch = [[event touchesForView:self] anyObject];

// Convert touch point from UIView referential to OpenGL one (upside-down flip)
if (firstTouch) {
    firstTouch = NO;
    previousLocation = [touch previousLocationInView:self];
    previousLocation.y = bounds.size.height - previousLocation.y;

} else {
    location = [touch locationInView:self];
    location.y = bounds.size.height - location.y;
    previousLocation = [touch previousLocationInView:self];
    previousLocation.y = bounds.size.height - previousLocation.y;
}

// Render the stroke
[self renderLineFromPoint:previousLocation toPoint:location];

}

I had a similar issue with missing points and it was related to the drawableProprties of my CAEAGLLayer.

Within CAEAGLLayer's drawableProperties, do you have the kEAGLDrawablePropertyRetainedBacking set to YES? If not then the Backing of your drawing is not being retained from frame to frame which can cause missing points and flickering.