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

Painting Quick Response Code in Objective-C Setting the graphical state

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Setting the graphical state
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13.4.2 Making transformations
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Transformations modify how you draw to your graphic context. They do this by changing the grid on 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 they 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 have your origin in the center of the screen rather than in the upper-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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CTM TRANSFORMATIONS
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The simplest way to apply a transformation is to use one of the functions that modify the current transformation matrix (CTM), which is a matrix that s applied to all drawing done in your current graphical state. These functions are described in table 13.8.
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Table 13.8 CTM transformation functions that allow you to change how you draw Function CGContextRotateCTM CGContextScaleCTM CGContextTranslateCTM Arguments context, radian rotation context, x-scale, y-scale context, x-change, y-change Summary Rotates the grid Scales the grid Moves the origin
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There are two gotchas that you should watch for. First, note that the ordering of transformations is somewhat pickier than the ordering of color commands. You need to start your transformation before you add the relevant lines to your path, and you need to maintain it until after you paint that path. Second, although these transformations can be applied in any sequence, order matters. Following are two transformation commands that can be applied together:
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CGContextTranslateCTM(ctx, 100, 100); CGContextRotateCTM(ctx, .25*M_PI);
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These functions move a drawing 100 to the right and 100 down and rotate it by 45 degrees. Figure 13.2 shows the untransformed picture (which you ve seen before), the results if these commands are applied with the translation before the rotation, and the results if they re applied in the opposite order. Clearly, you need to be careful and think about ordering when you re applying CTM transformations. But CTM transformations aren t the only way to change your drawing space.
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Graphics: Quartz, Core Animation, and OpenGL
Figure 13.2 As these variant transformations show, order matters. The left picture is untransformed, the middle one is translated and then rotated, and the right one is rotated and then translated. AFFINE TRANSFORMATIONS
Just as you can create a reusable path and then apply that to the context with the CGContextAddPath function, you can also create a reusable transformation matrix (using the affine transformation functions) and then apply that to the context with the CGContextConcatCTM function. This is managed by a set of six core functions, listed in table 13.9. Half of them create a new matrix, applying a transformation at the same time, and the other half apply a transformation to an existing matrix. The last function is the one that applies an affine transformation to your current graphical state.
Table 13.9 Affine transformations for creating reusable transformations Function CGAffineMakeRotation Arguments radian rotation Summary Makes an array with the rotation Makes an array with the scale Makes an array with the translation Rotates the array Scales the array
CGAffineMakeScale CGAffineMakeTranslation
x-scale, y-scale x-change, y-change
CGAffineTransformRotate CGAffineTransformScale
array, radian rotation array, x-scale, y-scale
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Setting the graphical state
Table 13.9 Affine transformations for creating reusable transformations (continued) Function CGAffineTransformTranslate Arguments array, x-change, y-change context, array Summary Translates the array
CGContextConcatCTM
Applies the transformation
The following code applies a rotation followed by a translation using a reusable affine matrix:
CGAffineTransform myAffine = CGAffineTransformMakeRotation(.25*M_PI); CGAffineTransformTranslate(myAffine, 100, 100); CGContextConcatCTM(ctx, myAffine);
In addition to creating reusable affine transformations, you can also modify the transforms at a much lower level. Any affine transformation is constructed from a 3 x 3 matrix that s then multiplied across the individual vectors of your path using matrix multiplication. If you have specific needs, you can use the CGAffineTransformMake function to create a matrix by hand. Using it looks like this:
CGAffineTransform flip = CGAffineTransformMake(1,0,0,-1,0,0);
You can find information about how the matrix works and about some other functions in the CGAffine reference. The next sort of state you may want to change is one that makes fairly large-scale changes to your drawings: the clipping path.
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