Last time, we looked at how Core Animation lets you perform simple animations, including animating along paths.

Now, using Core Animation, let’s look at a way to simulate a 3D perspective. Core Animation is not ideal for 3D graphics—use OpenGL for that—but it’s handy to be able to use 3D perspective at times for simpler things.

For this example, we’ll display a two-room house with a front door.

The house’s seven walls fall neatly on a grid. Walls 1 through 4 run east-west, while 5 through 7 run north-south. Each wall is a layer. All walls are sublayers of the floorLayer, which is in turn a sublayer of the rootLayer.

Set up

-(void)setupHostView {
    
    // Set up the root layer.  Nothing unusual here.
    rootLayer = [[CALayer layer] retain];
    rootLayer.needsDisplayOnBoundsChange = YES;
    rootLayer.backgroundColor = CGColorGetConstantColor(kCGColorBlack);
    [roomView setLayer:rootLayer];
    [roomView setWantsLayer:YES];
    
    // Set up the floor layer.  Nothing unusual here.
    floorLayer = [[CALayer layer] retain];
    floorLayer.delegate = roomView;
    floorLayer.bounds = rootLayer.bounds;
    floorLayer.frame = rootLayer.frame;
    floorLayer.autoresizingMask = kCALayerHeightSizable | kCALayerWidthSizable;
    floorLayer.fillMode = rootLayer.fillMode;
    floorLayer.position = CGPointMake(400, 200);
    [rootLayer addSublayer: floorLayer];
    
    // Finally, apply the 3D perspective.  Without this, things look quite odd.
    floorLayer.sublayerTransform = [self get3DTransform];
}

Applying the 3D perspective relies on a poorly-documented feature of Core Animation’s CATransform3D structure, a 4-by-4 matrix used to perform matrix transformations. Apple’s documentation says that changes to CATransform3D.m34 “affect the sharpness of the transform.” For our purposes, this means “make things look 3D”.

- (CATransform3D) get3DTransform {
    CATransform3D transform = CATransform3DIdentity;
    transform.m34 = 1.0 / -2000;
    return transform;
}

Creating the walls

Creating the walls is relatively straightforward. Each wall has an origin and a size.

- (CALayer *)makeWallAtOrigin:(CGPoint)origin size:(CGSize)size color:(CGColorRef)color {
    
    // Create the wall with the desired background color.
    CALayer *wall = [CALayer layer];
    wall.backgroundColor = color;
    wall.anchorPoint = CGPointZero;
    CGRect frame;
    frame.origin = origin;
    frame.size = size;
    wall.frame = frame;
    wall.bounds = frame;
    return wall;
}

Walls 3, 4, and 6 lie further back and need to be translated along the z axis.

- (void)moveWall:(CALayer*)wall toDepth:(float)depth {
    NSNumber* value = [NSNumber numberWithFloat:depth];
    [wall setValue:value forKeyPath:@"transform.translation.z"];
}

North-south walls 5, 6, and 7 need to be rotated.

- (void)rotateWall:(CALayer*)wall withDegrees:(float)degrees {
    
    // Rotation occurs relative to the layer's anchorPoint, which by
    // default is in the middle of the layer.  So unless you move the
    // anchorPoint, rotation will cause the layer to pivot on its center.
    // We want rotation to pivot on the wall's left point, so we'll set
    // anchorPoint to 0. 
    wall.anchorPoint = CGPointZero;
    float radians = DegreesToRadians(degrees);
    NSNumber* value = [NSNumber numberWithFloat:radians];
    [wall setValue:value forKeyPath:@"transform.rotation.y"];
}

Putting it all together

- (void)addWalls:(CALayer *)parentLayer {
    float w = [self cellWidth];
    float h = [self cellHeight];
    float x = parentLayer.frame.size.width/2 - (w*5)/2;
    float y = parentLayer.frame.size.height/2 - (h*3/2);
    
    // Wall 1 
    CALayer* wall1 = [self makeWallAtOrigin:CGPointMake(x + w*1, y + h)
        withSize:CGSizeMake(w*1, h) color:_lightGrayColor image:image1];
    [parentLayer addSublayer:wall1];
    
    // Wall 2 
    CALayer* wall2 = [self makeWallAtOrigin:CGPointMake(x + w*3, y + h)
        withSize:CGSizeMake(w*1, h) color:_lightGrayColor image:image2];
    [parentLayer addSublayer:wall2];
    
    // Wall 3 
    CALayer* wall3 = [self makeWallAtOrigin:CGPointMake(x + w*1, y + h)
        withSize:CGSizeMake(w*2, h) color:_darkGrayColor image:image3];
    [self moveWall:wall3 toDepth:w*-6];
    [parentLayer addSublayer:wall3];
    
    // Wall 4 
    CALayer* wall4 = [self makeWallAtOrigin:CGPointMake(x + w*1, y + h)
        withSize:CGSizeMake(w*3, h) color:_darkGrayColor image:image4];
    [self moveWall:wall4 toDepth:w*-10];
    [parentLayer addSublayer:wall4];
    
    // Wall 5 
    CALayer* wall5 = [self makeWallAtOrigin:CGPointMake(x + w*1, y + h)
        withSize:CGSizeMake(w*6, h) color:_lightGrayColor image:image5];
    [self rotateWall:wall5 withDegrees:90];
    [parentLayer addSublayer:wall5];
    
    // Wall 6 
    CALayer* wall6 = [self makeWallAtOrigin:CGPointMake(x + w*1, y + h)
        withSize:CGSizeMake(w*4, h) color:_orangeColor image:image6];
    [self moveWall:wall6 toDepth:(w*-6 + 1)];
    [self rotateWall:wall6 withDegrees:90];
    [parentLayer addSublayer:wall6];
    
    // Wall 7 
    CALayer* wall7 = [self makeWallAtOrigin:CGPointMake(x + w*4, y + h)
        withSize:CGSizeMake(w*10, h) color:_darkGrayColor image:image7];
    [self rotateWall:wall7 withDegrees:90];
    [parentLayer addSublayer:wall7];
}

Download and try it

Download (requires Leopard): application | source code