Difference between revisions of "KDE Visual Design Group/HIG/Units"

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== From design to code ==
 
== From design to code ==
For any mockup, please help the developers by specifing all messurements, either in the mockup itself or in an extra guide to the mockup. It is a lot of work and it is errorprone for developers trying to messure everything from a mockup.  
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For any mockup, please help the developers by specifying all measurements, either in the mockup itself or in an extra guide to the mockup. It is a lot of work and it is error prone for developers trying to measure everything from a mockup. Even if the mockup is in a file format that would allow exact measurements, don't expect the developer to know how to do it.
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{| style="border-spacing: 20px 0px; border-collapse: separate; margin: 5px 0 20px 0;"
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| style="width: 216px;"|[[Image:Design.qml.png|center]]
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| style="width: 216px;"|[[Image:Design_Good.qml.png|center]]
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|-
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| style=" vertical-align: top;"|<span style="color: red;">BAD</span><br/>There are no measures.<br/>
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| style=" vertical-align: top;"|<span style="color: green;">GOOD</span><br/>Try to be as detailed as necessary, but you don't have to provide measurement for objects that can be easily calculated. For example the size of the dark rectangle can be easily obtained.
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|}
  
 
=== recomended spacings ===
 
=== recomended spacings ===
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|[[Image:Margin.qml.png|Use of base units]]
 
|[[Image:Margin.qml.png|Use of base units]]
 
|}
 
|}
 
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<br/><br/>
 
<syntaxhighlight lang="qml" line>
 
<syntaxhighlight lang="qml" line>
 
Row {
 
Row {
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|[[Image:Arbitrary.qml.png|Use of arbitrary px values]]
 
|[[Image:Arbitrary.qml.png|Use of arbitrary px values]]
 
|}
 
|}
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<br/><br/>
 
<syntaxhighlight lang="qml" line>
 
<syntaxhighlight lang="qml" line>
 
Row {
 
Row {

Revision as of 07:28, 14 September 2017

Purpose

This pages gives an overview about different units used in KDE and Plasma

Pixel

A (physical) pixel or dot is a physical point in a raster image, or the smallest addressable element in an all points addressable display device. Be careful not to confuse it with DPI independent pixels

DPI - pixels per inch

Pixel density is the number of (physical) pixels or dots that fit into an inch. Different screens have different DPI.

screen density = screen width (or height) in pixels / screen width (or height) in inches

Different DPIs on desktop and mobile

DPI is often used interchangeably with PPI, pixel per inch.

PPI / DPI independent pixels

A DPI independet pixel is scaled to look uniform on any screen regardless of the DPI. A lot of platforms, eg iOS, Android, the Web, replaced the old (physical) px with a DPI px. So most the time you read about pixel/px they actually talk about DPI independent pixels. Qt (and QML) support DPI independent pixels in newer versions, but because KDE and Plasma support older versions of Qt too, one can not assume that pixels used in Qt or QML apps are DPI independent.

Different DPIs on desktop and mobile

A rectangle defined with physical pixels and DPI independent pixels.



Except explicilty stated otherwise, all HIG pages, draft, mockups, ... pixels/px are always DPI independent pixels.

DPI independent pixels in KDE

As a developer, if you want to use DPI independent pixels use units.devicePixelRatio as a multiplier on physical pixels. Since units.devicePixelRatio is a float, make sure to round the results. Most of the time you want to floor it.

Fonts

Since KDE allows the user to change the font settings any dimensions defined with px, no matter if they are DPI independent or not, make problems together with text.

Using DPI independet pixel with different font setting

base units in plasma

There are special base units in plasma:

  • units.smallSpacing, for mockup and design = 4px
  • units.largeSpacing, for mockup and design = 18px
  • units.gridUnit, for mockup and design = 18px

These are not only DPI independent but scale according to the font settings too. While designing, be careful not to rely on the ratio between units.smallSpacing and units.largeSpacing because these change depending on font settings.

Using units.smallSpacing with different font setting

A rectangle defined with units.smallSpacing

From design to code

For any mockup, please help the developers by specifying all measurements, either in the mockup itself or in an extra guide to the mockup. It is a lot of work and it is error prone for developers trying to measure everything from a mockup. Even if the mockup is in a file format that would allow exact measurements, don't expect the developer to know how to do it.

Design.qml.png
Design Good.qml.png
BAD
There are no measures.
GOOD
Try to be as detailed as necessary, but you don't have to provide measurement for objects that can be easily calculated. For example the size of the dark rectangle can be easily obtained.

recomended spacings

If you design try to use the recomended values for margin and paddings, to ensure a uniform appearance. See Placement and Spacing for more details.

Use of base units



1 Row {
2 	spacing: units.largeSpacing
3 	Rectangle {
4 		...
5 	}
6 	Rectangle {
7 		...
8 	}
9 }
1 Row {
2 	spacing: 2 * units.smallSpacing
3 	Rectangle {
4 		...
5 	}
6 	Rectangle {
7 		...
8 	}
9 }

arbitrary px values

When needed, you can use arbitrary px values for your mockups. As a developer you need to use units.devicePixelRatio to make these values DPI independent.

Use of arbitrary px values



 1 Row {
 2 	spacing: units.smallSpacing
 3 	Rectangle {
 4 		height: units.largeSpacing
 5 		width: Math.floor(2 * units.devicePixelRatio)
 6 	}
 7 	Text {
 8 		...
 9 	}
10 }

ratio

Sometimes the ratio between dimensions is more important then the actually values.

Ratio.qml.png
 1 Grid {
 2 	columns: 3
 3 	...
 4 	Repeater {
 5 		model: 9
 6 		...
 7 		Rectangle {
 8 			width: grid.width / 3
 9 			height: grid.height / 3
10 			...
11 		}
12 	}
13 }

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