70 lines
2.7 KiB
Go
70 lines
2.7 KiB
Go
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// Copyright 2014 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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/*
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Pacakge geom defines a two-dimensional coordinate system.
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The coordinate system is based on an left-handed Cartesian plane.
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That is, X increases to the right and Y increases down. For (x,y),
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(0,0) → (1,0)
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↓ ↘
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(0,1) (1,1)
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The display window places the origin (0, 0) in the upper-left corner of
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the screen. Positions on the plane are measured in typographic points,
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1/72 of an inch, which is represented by the Pt type.
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Any interface that draws to the screen using types from the geom
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package scales the number of pixels to maintain a Pt as 1/72 of an inch.
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*/
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package geom
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/*
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Notes on the various underlying coordinate systems.
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Both Android and iOS (UIKit) use upper-left-origin coordinate systems
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with for events, however they have different units.
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UIKit measures distance in points. A point is a single-pixel on a
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pre-Retina display. UIKit maintains a scale factor that to turn points
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into pixels. On current retina devices, the scale factor is 2.0.
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A UIKit point does not correspond to a fixed physical distance, as the
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iPhone has a 163 DPI/PPI (326 PPI retina) display, and the iPad has a
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132 PPI (264 retina) display. Points are 32-bit floats.
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Even though point is the official UIKit term, they are commonly called
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pixels. Indeed, the units were equivalent until the retina display was
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introduced.
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N.b. as a UIKit point is unrelated to a typographic point, it is not
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related to this packages's Pt and Point types.
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More details about iOS drawing:
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https://developer.apple.com/library/ios/documentation/2ddrawing/conceptual/drawingprintingios/GraphicsDrawingOverview/GraphicsDrawingOverview.html
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Android uses pixels. Sub-pixel precision is possible, so pixels are
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represented as 32-bit floats. The ACONFIGURATION_DENSITY enum provides
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the screen DPI/PPI, which varies frequently between devices.
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It would be tempting to adopt the pixel, given the clear pixel/DPI split
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in the core android events API. However, the plot thickens:
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http://developer.android.com/training/multiscreen/screendensities.html
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Android promotes the notion of a density-independent pixel in many of
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their interfaces, often prefixed by "dp". 1dp is a real physical length,
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as "independent" means it is assumed to be 1/160th of an inch and is
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adjusted for the current screen.
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In addition, android has a scale-indepdendent pixel used for expressing
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a user's preferred text size. The user text size preference is a useful
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notion not yet expressed in the geom package.
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For the sake of clarity when working across platforms, the geom package
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tries to put distance between it and the word pixel.
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*/
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