Chapter 12. Drawing Offscreen Quartz 2d Graphics For Mac

Core Graphics, often referred to as Quartz, or Quartz 2D, is the drawing system that underlies all iOS drawing — UIKit drawing is built on top of it — so it is low-level and consists of C functions. There are a lot of them! This chapter will familiarize you with the fundamentals; for complete information, you’ll want to.

• alpha value

  The graphics state parameter that Quartz uses to determine how to composite newly painted objects to the existing page. At full intensity (alpha = 1.0), newly painted objects are opaque. At zero intensity, newly painted objects are invisible (alpha = 0.0).



• axial gradient

  A fill that varies along an axis between two defined end points. All points that lie on a line perpendicular to the axis have the same color value. Also called a linear gradient.



• bitmap

  A rectangular array (or raster) of pixels, each pixel representing a point in an image. Bitmap images are also called sampled images.



• blend mode

  Specifies how Quartz combines the foreground painting with the background painting.



• clipping area

  A path used to constrain the drawing of other objects within its bounds.



• color space

  A one-, two-, three-, or four-dimensional environment whose components (or channels) represent intensity values. For example, RGB space is a three-dimensional color space whose stimuli are the red, green, and blue intensities that make up a given color; and red, green, and blue are color channels.



• concatenation

  An operation that combines two matrices by multiplying them together.



• current graphics state

  The parameters values that determine how Quartz renders results as it paints.



• current point

  The last location Quartz used when painting a path.



• current transformation matrix

  An affine transform that Quartz uses to map points from one coordinate space to another.



• device color space

  A color space that is tied to the system of color representation for a particular device. This type of color space is not suitable for interchanges of color data between different devices.



• device-independent color space

  A color representation that is portable between devices and that is used for the interchanges of color data from the native color space of one device to the native color space of another device. Colors in a device-independent color space appear the same when displayed on different devices, to the extent that the capabilities of the device allow.



• even-odd rule

  A fill rule that determines when to paint a pixel. The outcome does not depend on the direction that path segments are drawn. Compare with nonzero winding number rule.



• fill

  An operation that paints the area within a path.



• generic color space

  A device-independent color space chosen automatically by Mac OS X to produce the best color for the drawing destination.



• gradient

  A fill that varies from one color to another. See also axial gradient and radial gradient.



• graphics context

  An opaque data type (CGContextRef) that encapsulates the information Quartz uses to draw images to an output device, such as a PDF file, a bitmap, or a window on a display. The information inside a graphics context includes graphics drawing parameters and a device-specific representation of the paint on the page.



• identity transform

  An affine transform that, when applied to input coordinates, always returns the input coordinates.



• image mask

  A bitmap that specifies an area to paint, but not the color. An image mask acts like a stencil to specify where to place color on the page.



• inversion

  An operation that produces original coordinates from transformed ones.

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• layer context

  An offscreen drawing destination (CGLayerRef) designed for optimal performance. A a layer context is a much better choice for offscreen drawing than a bitmap graphics context.



• line cap

  The style that Quartz uses to draw the endpoint of a line—butt, round, or projecting square.



• line dash pattern

  The repeating series of line segments and spaces used to paint a dashed line.



• line join

  The style that Quartz uses to draw the junction between connected line segments—miter, round, or bevel.



• line width

  The total width of a line, expressed in user space units.



• linear gradient

  See axial gradient.



• nonzero winding number rule

  A fill rule that determines when to paint a pixel. The outcome depends on the direction that path segments are drawn. Compare with even-odd rule.



• page

  The virtual canvas that Quartz paints to.



• painter’s model

  A drawing model in which each successive drawing operation applies a layer of paint to a page.



• path

  One or more shapes (known as subpaths) that Quartz paints as a unit. A subpath can consist of straight lines, curves, or both. It can be open or closed.



• pattern

  A sequence of drawing operations that Quartz can repeatedly paint to a graphics context.



• pattern space

  An abstract space that maps to the default user space by the transformation matrix (the pattern matrix) you specify when you create the pattern. Pattern space is separate from user space. The untransformed pattern space maps to the base (untransformed) user space, regardless of the state of the current transformation matrix.



• premultiplied alpha

  A source color whose components are already multiplied by an alpha value. Premultiplying speeds up the rendering of an image by eliminating an extra multiplication operation per color component. See also alpha value.



• radial gradient

  A fill that varies radially along an axis between two defined ends, which typically are both circles. Points share the same color value if they lie on the circumference of a circle whose center point falls on the axis. The radius of the circular sections of the gradient are defined by the radii of the end circles; the radius of each intermediate circle varies linearly from one end to the other.



• rendering intent

  Specifies how Quartz maps colors from the source color space to those that are within the gamut of the destination color space of a graphics context.



• rotation

  An operation that moves the coordinate space the specified angle.



• scaling

  An operation that changes the scale of the coordinate space by the specified x and y factors, effectively stretching or shrinking coordinates. The magnitude of the x and y factors governs whether the new coordinates are larger or smaller than the original. A negative factor flips the corresponding axis.



• shadow

  An image painted underneath, and offset from, a graphics object such that the shadow mimics the effect of a light source cast on the graphics object.

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• stroke

  An operation that paints a line that straddles a path.



• tiling

  The process of rendering pattern cells to a portion of a page. Quartz has three tiling options—no distortion, constant spacing with minimal distortion, and constant spacing.



• translation

  An operation that moves the origin of the coordinate space by the number of units specified for the x and y axes.



• transparency layer

  A composite of two or more objects that Quartz treats as a single object when applying effects, such as shadows.



• user space

  The device-independent coordinate system that you use when drawing using Quartz 2D.

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