R – Fastest / most efficient way to draw moving speech bubbles on screen – CoreAnimation, Quartz2D

core-animationiphonequartz-2d

I am adding some functionality to an iPhone app, and could use some help in picking the fastest / most efficient / best practice approach for solving this problem:

At the upper-half of my screen, I have speech bubbles (think comic book) that are UIImageViews translating across the screen (dynamic x & y position). It is a UIImageView because there is an image as the background of the speech bubble.

Each speech bubble has a matching image moving around the bottom of the screen (elsewhere in the layer tree)

I would like to draw a tail (that triangle bit from a speech bubble) so the point of the triangle is tracking the lower image, with the base of the triangle being attached to the bottom of the upper UIImageView. (technically the base doesn't have to be butted against, it can overlap as long as I can match the color of my background image to the triangle).

I have already done all the tracking & drawn a line with CGContextStrokePath methods, and now I am stuck on how to replace the line with a triangle.

I have looked at drawing a triangle in Quartz and filling it. My concern is the speech bubbles are repositioned every 1/10th of a second, and it looks like drawing just the line used for proof of concept had a pretty severe performance / visual smoothness impact.

One idea I have is to do the trigonometry myself, and stretch & rotate an image of a triangle to connect each of these speech bubbles with the lower spot. Something is telling me there is a more efficient / more elegant solution, but I am not able to see it looking through the documentation. Any help on how you have or would approach this issue is appreciated. Thanks.

Best Answer

If the speech bubbles are fixed in size, just use a static UIImage. Set the image view's layer.position property at the point of the triangle. Then you can use view animation to move the bubbles around.

If you need the speech bubbles to be different sizes, I'd create a resizeable image using resizableImageWithCapInsets. Then I'd do the same as above to position it.

If there was something special about the speech bubble that I could't achieve with either a static image or a resizable image, I'd probably create a custom CA Layer or layers to get the effect I wanted (Like a gradient layer with a shape layer as it's mask layer)

Related Solutions

Objective-c – How to set order of display in Objective-C

You can try implementing this through Quartz Core using layers (see the CALayer class documentation). Basically, you can have layers hierarchies. You associate each UIView to a different layer, then the layers are rendered together providing a single, composite layer. Besides, you can also apply transforms and animations to layers.

You need to import the QuartzCore header and do something like

#import <QuartzCore/QuartzCore.h>
UIView *mainView = [[UIView alloc] initWithFrame...
UIImageView *imageView = ...

CaLayer *mainViewLayer = mainView.layer;
[mainViewLayer addSubLayer: imageView.layer];

Then, when mainView appears on the screen, all the sublayers are merged together and rendered on screen. What happens is that each view renders its layer, while mainViewLayer is rendered merging together the two layers.

Let me know if this works for you.

You can have as many layers as you like. You can create an arbitrary hierarchi by using the CALayer methods

– addSublayer: 
– removeFromSuperlayer
– insertSublayer:atIndex:
– insertSublayer:below:
– insertSublayer:above:
– replaceSublayer:with:

R – How to create a new image by drawing on top of an existing one using Quartz

These are the steps:

Create a CGBitmapContext matching the image's colorspace and dimensions.
Draw the image into that context.
Draw whatever you want on top of the image.
Create a new image from the context.
Dispose off the context.

Here's a method that takes an image, draws something on top of it and returns a new UIImage with modified contents:

- (UIImage*)modifiedImageWithImage:(UIImage*)uiImage
{
    // build context to draw in
    CGImageRef image = uiImage.CGImage;
    CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
    CGContextRef ctx = CGBitmapContextCreate(NULL,
                                             CGImageGetWidth(image), CGImageGetHeight(image),
                                             8, CGImageGetWidth(image) * 4,
                                             colorspace, kCGImageAlphaPremultipliedLast);
    CGColorSpaceRelease(colorspace);

    // draw original image
    CGRect r = CGRectMake(0, 0, CGImageGetWidth(image), CGImageGetHeight(image));
    CGContextSetBlendMode(ctx, kCGBlendModeCopy);
    CGContextDrawImage(ctx, r, image);
    CGContextSetBlendMode(ctx, kCGBlendModeNormal);

    // draw something
    CGContextAddEllipseInRect(ctx, CGRectInset(r, 10, 10));
    CGContextSetRGBStrokeColor(ctx, 1.0f, 1.0f, 1.0f, 0.5f);
    CGContextSetLineWidth(ctx, 16.0f);
    CGContextDrawPath(ctx, kCGPathStroke);

    CGContextAddEllipseInRect(ctx, CGRectInset(r, 10, 10));
    CGContextSetRGBStrokeColor(ctx, 0.7f, 0.0f, 0.0f, 1.0f);
    CGContextSetLineWidth(ctx, 4.0f);
    CGContextDrawPath(ctx, kCGPathStroke);

    // create resulting image
    image = CGBitmapContextCreateImage(ctx);
    UIImage* newImage = [[[UIImage alloc] initWithCGImage:image] autorelease];
    CGImageRelease(image);
    CGContextRelease(ctx);

    return newImage;
}

To restore to old image, just keep a reference to it.

The cropping thing is not related to the above and you should create a new question for that.

Best Answer

Related Solutions

Objective-c – How to set order of display in Objective-C

R – How to create a new image by drawing on top of an existing one using Quartz

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