how to print barcode in vb.net 2008 Setting the graphical state in Objective-C

Generating QR Code in Objective-C Setting the graphical state

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Setting the graphical state
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Now that we ve looked at two different ways to create complex paths, we ll take a step back and look at how to draw much simpler objects in a simpler way.
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13.3.3 Drawing rectangles
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Drawing paths takes some work, but if you want to draw a rectangle, Quartz makes it easy. All you have to do is use one of a few functions listed in table 13.5. These functions take care of the path creation, drawing, and painting for you in a single step.
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Table 13.5 Specific functions allow you to draw rectangles Function CGContextClearRect CGContextFillRect Arguments context, CGRect context, CGRect Summary Erases a rectangle. Draws a filled rectangle. The more complex variant CGContextFillRects allows you to fill a whole array of rectangles. Draws a stroked rectangle. Draws a stroked rectangle, with the stroke being the designated width.
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CGContextStrokeRect CGContextStrokeRectWithWidth
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context, CGRect context, CGRect, 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 discuss how to draw objects in more complex ways by modifying state.
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13.4 Setting the graphical state
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The graphical state is how Quartz draws. It includes a variety of information such as what colors are used for fills or strokes, which clipping paths constrain the current drawing path, what transformations are 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 13.6.
Table 13.6 State-related functions that help define how you draw Function CGContextSaveGState CGContextRestoreGState Arguments context context Summary Pushes the state onto a stack Pops the state off a stack
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Graphics: Quartz, Core Animation, and OpenGL
As we ve already noted, you can store a lot of things in graphical state. We ll cover many of them here, starting with colors.
13.4.1 Setting colors
In Quartz, you select colors by setting the fill color, the stroke color, or both in the current graphic state. After you ve done this, any fill or stroke commands following the color commands appear in the appropriate colors. Note that color is irrelevant while you re 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 and iPad, you ll usually either use the RGBA color space or use a command that lets you select a color using standard UIKit methods. Table 13.7 lists the four most relevant of these functions.
Table 13.7 The most important of numerous coloring functions Function CGContextSetRGBFillColor Arguments context, red, green, blue, alpha context, red, green, blue, alpha context, CGColor 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 CGColor
CGContextSetRGBStrokeColor
CGContextSetFillColorWithColor
CGContextSetStrokeColorWithColor
context, 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). You saw an example of this earlier:
CGContextSetRGBFillColor(ctx, 1, 0, 0, 1);
The last two functions in table 13.7 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 graphical state, we re ready to move on to the next topic: how to change how you draw through graphical state transformations.
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