120 lines
4.3 KiB
Go
120 lines
4.3 KiB
Go
// Copyright 2014 The Go Authors. All rights reserved.
|
|
// Use of this source code is governed by a BSD-style
|
|
// license that can be found in the LICENSE file.
|
|
|
|
/*
|
|
Package geom defines a two-dimensional coordinate system.
|
|
|
|
The coordinate system is based on an left-handed Cartesian plane.
|
|
That is, X increases to the right and Y increases down. For (x,y),
|
|
|
|
(0,0) → (1,0)
|
|
↓ ↘
|
|
(0,1) (1,1)
|
|
|
|
The display window places the origin (0, 0) in the upper-left corner of
|
|
the screen. Positions on the plane are measured in typographic points,
|
|
1/72 of an inch, which is represented by the Pt type.
|
|
|
|
Any interface that draws to the screen using types from the geom package
|
|
scales the number of pixels to maintain a Pt as 1/72 of an inch.
|
|
*/
|
|
package geom // import "golang.org/x/mobile/geom"
|
|
|
|
/*
|
|
Notes on the various underlying coordinate systems.
|
|
|
|
Both Android and iOS (UIKit) use upper-left-origin coordinate systems
|
|
with for events, however they have different units.
|
|
|
|
UIKit measures distance in points. A point is a single-pixel on a
|
|
pre-Retina display. UIKit maintains a scale factor that to turn points
|
|
into pixels. On current retina devices, the scale factor is 2.0.
|
|
|
|
A UIKit point does not correspond to a fixed physical distance, as the
|
|
iPhone has a 163 DPI/PPI (326 PPI retina) display, and the iPad has a
|
|
132 PPI (264 retina) display. Points are 32-bit floats.
|
|
|
|
Even though point is the official UIKit term, they are commonly called
|
|
pixels. Indeed, the units were equivalent until the retina display was
|
|
introduced.
|
|
|
|
N.b. as a UIKit point is unrelated to a typographic point, it is not
|
|
related to this packages's Pt and Point types.
|
|
|
|
More details about iOS drawing:
|
|
|
|
https://developer.apple.com/library/ios/documentation/2ddrawing/conceptual/drawingprintingios/GraphicsDrawingOverview/GraphicsDrawingOverview.html
|
|
|
|
Android uses pixels. Sub-pixel precision is possible, so pixels are
|
|
represented as 32-bit floats. The ACONFIGURATION_DENSITY enum provides
|
|
the screen DPI/PPI, which varies frequently between devices.
|
|
|
|
It would be tempting to adopt the pixel, given the clear pixel/DPI split
|
|
in the core android events API. However, the plot thickens:
|
|
|
|
http://developer.android.com/training/multiscreen/screendensities.html
|
|
|
|
Android promotes the notion of a density-independent pixel in many of
|
|
their interfaces, often prefixed by "dp". 1dp is a real physical length,
|
|
as "independent" means it is assumed to be 1/160th of an inch and is
|
|
adjusted for the current screen.
|
|
|
|
In addition, android has a scale-indepdendent pixel used for expressing
|
|
a user's preferred text size. The user text size preference is a useful
|
|
notion not yet expressed in the geom package.
|
|
|
|
For the sake of clarity when working across platforms, the geom package
|
|
tries to put distance between it and the word pixel.
|
|
*/
|
|
|
|
import "fmt"
|
|
|
|
// Pt is a length.
|
|
//
|
|
// The unit Pt is a typographical point, 1/72 of an inch (0.3527 mm).
|
|
//
|
|
// It can be be converted to a length in current device pixels by
|
|
// multiplying with PixelsPerPt after app initialization is complete.
|
|
type Pt float32
|
|
|
|
// Px converts the length to current device pixels.
|
|
func (p Pt) Px() float32 { return float32(p) * PixelsPerPt }
|
|
|
|
// String returns a string representation of p like "3.2pt".
|
|
func (p Pt) String() string { return fmt.Sprintf("%.2fpt", p) }
|
|
|
|
// Point is a point in a two-dimensional plane.
|
|
type Point struct {
|
|
X, Y Pt
|
|
}
|
|
|
|
// String returns a string representation of p like "(1.2,3.4)".
|
|
func (p Point) String() string { return fmt.Sprintf("(%.2f,%.2f)", p.X, p.Y) }
|
|
|
|
// A Rectangle is region of points.
|
|
// The top-left point is Min, and the bottom-right point is Max.
|
|
type Rectangle struct {
|
|
Min, Max Point
|
|
}
|
|
|
|
// String returns a string representation of r like "(3,4)-(6,5)".
|
|
func (r Rectangle) String() string { return r.Min.String() + "-" + r.Max.String() }
|
|
|
|
// PixelsPerPt is the number of pixels in a single Pt on the current device.
|
|
//
|
|
// There are a wide variety of pixel densities in existing phones and
|
|
// tablets, so apps should be written to expect various non-integer
|
|
// PixelsPerPt values. In general, work in Pt.
|
|
//
|
|
// Not valid until app initialization has completed.
|
|
var PixelsPerPt float32
|
|
|
|
// Width is the width of the device screen.
|
|
// Not valid until app initialization has completed.
|
|
var Width Pt
|
|
|
|
// Height is the height of the device screen.
|
|
// Not valid until app initialization has completed.
|
|
var Height Pt
|