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Setting the graphic state
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Table 19.5 Specific functions allow you to draw rectangles (continued) Function Arguments Summary Draws a stroked rectangle.
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Draws a stroked rectangle, with the stroke being the designated width.
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The CGContextClearRect function can be particularly useful for erasing a window when you re ready to draw something new to it. Now that we ve told you how to draw objects in the simplest way possible, we re ready to move on and start talking about how to draw objects in more complex ways by modifying state.
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19.4 Setting the graphic state
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The graphic state is how Quartz will be drawing. It includes a variety of information such as what colors are being used for fills or strokes, which clipping paths constrain the current drawing path, what transformations are being applied to the drawing, and a number of other less important variables. State is maintained in a stack. You can save a state at any time; it doesn t change how things are being drawn, but it does push that current state onto the top of a stack for later retrieval. Later, you can restore a state, which pops the top state off the stack, putting things back to how they were before the last save. We ve mentioned these functions before, but we ve also listed them here in table 19.6.
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Table 19.6 State-related functions that help define how you draw Function Arguments Summary Pushes state onto a stack Pops state off of a stack
CGContextSaveGState CGContextRestoreGState
context context
As we ve already noted, there are a lot of things that you can store in graphic state. We re going to cover many of them here, starting with colors.
19.4.1 Setting colors In Quartz, you select colors by setting the fill color, the stroke color, or both in the current graphical state. Once you ve done this, any fill or stroke commands following the color commands will appear in the appropriate colors. Note that color is irrelevant while you are drawing the individual elements of a path the color commands apply only to the painting of the complete path at the end. You can select colors from a variety of color spaces, which are different ways to choose colors. They include RGB (red-green-blue), RGBA (red-green-blue-alpha), CMYK (cyan-magenta-yellow-black), and CGColor (the underlying Core Graphics color model). On the iPhone, you ll usually want to either use the RGBA color space or use a command that lets you select a color using standard UIKit methods. Table 19.7 lists the four most relevant of these functions.
Table 19.7
Graphics: Quartz, Core Animation, and OpenGL
The most important of numerous coloring functions Function Arguments Summary Sets the fill to the RGBA value Sets the stroke to the RGBA value Sets the fill to the CGColor Sets the stroke to the
CGContextSetRGBFillColor
context, red, green, blue, alpha context, red, green, blue, alpha context, CGColor context, CGColor
CGContextSetRGBStrokeColor
CGContextSetFillColorWithColor CGContextSetStrokeColorWithColor
CGColor
The two RGB functions allow you to set a color using values from 0 to 1 for each of red, green, blue, and alpha transparency (opacity). We saw an example of this in listing 19.2:
CGContextSetRGBFillColor(ctx, 1, 0, 0, 1);
The last two functions in table 19.7 allow you to set the color using any CGColor, and you ll understand how useful that is when you realize that you can read a CGColor property from any UIColor you create:
CGContextSetFillColorWithColor(ctx, [[UIColor redColor] CGColor]);
Given that you re already familiar and comfortable with the UIColors, we expect that this latter function will be a popular one. Having now covered the main ways to apply colors to your graphic state, we re ready to move on to the next topic: how to change how you draw through graphical state transformations.
19.4.2 Making transformations Transformations modify how you draw to your graphical context. They do this by changing the grid upon which you re drawing by moving its origin, rotating, or resizing. Why would you want to do these transformations
They can be useful for drawing photographs (or other images), because the transformations allow you to scale or rotate the picture. They can make it a lot easier to do certain types of mathematical drawing. For example, it s probably easier to draw a symmetric mathematical construct if you ve got your origin in the center of the screen rather than up at the top left corner. They can allow you to flip your screen if you end up in a context (or using a function) with an inverse coordinate system.
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