Graphics on the iPhone

There are two main graphics frameworks on the iPhone:
- Quartz
- OpenGL
Quartz treats the view as a vitual canvas and follows what's called the painter's model: drawing commands are applied in much the same way as paint on a canvas.
As we have already seen to some extent with Android, OpenGL ES is implemented as a state machine.
OpenGL maintains a virtual 3D world. You perform a sequence of actions on this world, mapping it from state to state.
Actions might involve placing objects, rotating things, setting material and lighting to use, and finally projecting doen to 2D.

Getting Context in Quartz

When you draw with Quartz, you'll usually add the drawing code to the view doing the drawing.
For example, you might create a subclass of UIView and add Quartz function calls to that class's drawRect: method.
drawRect: is the method in UIView that gets called each time a view needs to redraw itself.
In Quartz, drawing happens in a graphics context, also called just a context.
Every view, has such a context.
When you want to draw into a view, you need to retrieve the current context and use that to make various Quartz drawing calls.

To retrieve the context, you can do:

CGContextRef context = UIGraphicsGetCurrentContext();

Note: this is a Core Graphics is C function. It is not Objective-C.

Drawing into a Context

Once you've got a graphics context, you can draw into it by passing the context to a variety of Core Graphics functions.

For example,

  CGContextSetLineWidth(context, 2.0);
  CGContextSetStrokeColorWithColor(context, 
     [UIColor redColor].CGColor);
  CGContextMoveToPoint(context, 100.0f, 100.0f);
  CGContextAddLineToPoint(context, 200.0f, 200.0f);
  CGContextStrokePath(context);

This code sets up the stroke width as being 2pixels wide, sets the color as red (Core Graphics supports both a stroke and fill color), sets the starting coordinates of a stroke, then specifies where to draw to, and finally to draw it.
So in Core Graphics, we first create a shape, then tell Quartz to draw it using the information we have set-up.

The Coordinate System

Coordinates in Core Graphics are pairs of floating point numbers.
The upper left corner of the context is (0,0). As you move down y increase, as you move right x increase.
This coordinate is different from OpenGL's in which (0,0) would represent the lower-left corner if one was drawing in 2D.
To specify a point in the coordinate system, some Quartz functions require two floating point numbers as parameters.
Other functions, ask for the point to be embedded in a CGPoint struct that has two float fields: x, and y.
Another common struct is a CGRect, which is a pair of structs, a CGPoint origin, which specifies the top y, left x coordinates or a rectangular region, and a CGSize called size which specifies the width and the height of the rectangle.
It is completely okay for a point to be offscreen, to have negative x and y values, etc.

Quiz

Which of the following statements is true:

Specular lighting is a general, nondirectional glow that lights a scene.
Each texture coordinate that we used to map my face to the 3D cube involved three coordinates.
To control how much our object had rotated in the scene we used the total elasped time since the init() method of our thread was called.

Specifying Color

There is an Objective-C class that can be used to help specify a color in Core Graphics: UIColor.
Since Core Graphics is a C library, you don't use this class directly but instead use its properties to get a CGColor struct reference.

This is what we did with the line:

CGContextSetStrokeColorWithColor(context, [UIColor redColor].CGColor);

One way to represent colors in Quartz is to use the additive RGBA modes.
This "adds" the red, green and blue components together in a linear combination with parameter alpha of the current color to make a new color.
Combining the three additive colors, gives a color range referred to as the color gamut.
The method of combining colors you learned in grade school, is probably a subtractive model based on red, yellow, and blue. It is almost never used in graphics.
Quartz does support other color models such as HSV, HSL, CMYK.
When working with OpenGL ES though colors are required to be RGBA.

UIColor also has the following method for specifying a color:

[UIColor colorWithRed:1.0f green: 0.0f blue 0.0f alpha:1.0f];

Drawing Images in a Context

Quartz 2D support drawing images directly into a context.
To do this, you can use the Objective-C class UImage to work with a Core Graphics struct called CGImage.

UIImage contains methods to draw its image into the current context:

CGPoint drawPoint = CGPointMake(100.0f, 100.0f);
[image drawAtPoint: drawPoint];

Drawing Shapes: Polygons, Lines and Curves

Quartz 2D contains a number of functions to make it easy to create complex shapes.
For example, to draw an ellipse, you define the rectangle into which the ellipse fits, and then call the appropriate Core Graphics functions:
```
CGRect theRect = CGRectMAke(0, 0, 100, 100);
CGContextAddEllipseInRect(context, theRect);
CGContextDrawPath(context, kCGPathFillStroke);
```
There are similar methods for rectangles.
There are also methods for arcs, Bezier Curves and paths.

Graphics iPhone

Outline