High DPI

From Lazarus wiki
Jump to: navigation, search

Deutsch (de) | English (en) | Español (es)

Contents

Introduction

DPI (dots per inch) is the relation between size in pixels and the actual display size. Here dot is an equivalent for pixel in printing terminology. Applications can either use pixel sizes, or take into account the actual display size. In this second case, sizes are given in points.

On Windows 95 and later, it is possible to change the DPI ratio to make elements bigger. High DPI means any custom DPI setting with more than 96 DPI (the default setting) *.

High DPI awareness means that an application takes this DPI setting into account. This article was designed for Windows 7. For Windows 8 Metro Applications read this http://blogs.msdn.com/b/b8/archive/2012/03/21/scaling-to-different-screens.aspx

Pixels and points

For example 300 DPI means that there are 300 pixels (or dots) per inch. There are 72 points per inch, so :

300 pixels ↔ 1 inch

300/72 pixels ↔ 1 point

4.16 pixels ↔ 1 point

Now with 96 DPI :

72 pixels ↔ 1 inch

1.33 pixel ↔ 1 point

Now with 144 DPI :

144 pixels ↔ 1 inch

2 pixels ↔ 1 point

Example - Fixed Font Sizes (not HighDPI)

Here is a form with an undefined font size (set to zero, which is the default value). It has been designed at 96 DPI (100%), and it looks like this :

Testdpi100.png

Now, at 120 DPI (125%), it becomes :

Testdpi125.png

As you can see, the font gets bigger and so the text is clipped. The window title gets bigger, but the client area of the window remains the same size. Note that these changes in size can occur by using an application with a different Windows theme, or with another operating system.

To avoid this, you must set the font size to a non-zero value. Note that Font.Size is expressed in points and Font.Height is expressed in pixels. In fact, only the value of Font.Height is stored, and Font.Size changes according to current DPI value. So if we set the font size, it will be fixed to a certain size in pixels.

If we try again with a fixed font size of 9 points, then at 96 DPI (100%), we get this :

Testdpi100fixedM12P9.png

Now if the same program is run at 120 DPI (125%), it becomes :

Testdpi125fixedM12P9.png

The result is the almost the same. The title bar is bigger, but the client area and the font size is the same. Note that in fact, the size in points of the font has changed.

The conclusion from this is that it is possible to avoid inconsistency in the display by fixing font sizes. But we do not take into account that the graphical elements may be smaller according to actual DPI of the screen. With DPI awareness, it is possible to make an application behave as if it knew the real size of the pixels.

Setting High DPI under Windows

Windows Vista / Windows 7

In Windows 7 go to Control Panel > Appearance and Personalization > Display (or just Control Panel > Display in recent updates).

Select Smaller 100% (default), Medium 125% or Larger 150%. If you select 100% (96 DPI) this is the default Windows DPI setting, (High DPI is not the default).

If you select 125% (120 DPI) the option "Use Windows XP style DPI scaling" is enabled. Applications you run under this setting are scaled as if running under Windows XP.

If you select 150% (144 DPI) the option "Use Windows XP style DPI scaling" is disabled (DPI Virtualization is enabled), and applications you run under this setting must be High DPI Awareness to prevent system scaling which will produce a blurred image.

You can also set your custom DPI setting via the option "Set custom text size (DPI)" and enable/disable the DPI Virtualization.

Example - DPI Aware Application (For Vista +)

CPickSniff is an application to capture screen colors. We will use it as an example to see how High DPI works in Windows.

Default DPI

This is the app running at 96 DPI (100%). It's the default mode, when scaling isn't necessary.

cpicksniff defaultdpi.png

Windows DPI Scaling

This is same app running at 144 DPI (150%) without a manifest, so Windows scales it like a bitmap. The result is a blurred image.

cpicksniff blured.png

With Manifest

Running at 144 DPI (150%). This time the app includes a manifest but the application contains no code to handle scaling. Items aren't scaled whereas fonts are scaled (Windows does this automatically), so text is clipped.

cpicksniff nohighdpi.png

High DPI

Finally with both a manifest and a coded scaling handler, the app is in High DPI.

cpicksniff highdpi.png

STEP 1 - Declare High DPI Awareness

To do this we need a manifest file that includes the declaration, with Lazarus 0.9.30 we can do this by going to Options > Project Options > then selecting the options "Use Manifest to Enable Themes (Windows)" and "Dpi Aware application (for Vista +)".

STEP 2 - Scale Forms and Controls

To do this we can call ScaleDPI procedure in the OnCreate event of each form in your project.

First copy the code below and save it to a text file "uscaledpi.pas":

unit uscaledpi;
 
{$mode objfpc}{$H+}
 
interface
 
uses
  Forms, Graphics, Controls;
 
procedure HighDPI(FromDPI: integer);
procedure ScaleDPI(Control: TControl; FromDPI: integer);
 
implementation
 
procedure HighDPI(FromDPI: integer);
var
  i: integer;
begin
  if Screen.PixelsPerInch = FromDPI then
    exit;
 
  for i := 0 to Screen.FormCount - 1 do
    ScaleDPI(Screen.Forms[i], FromDPI);
end;
 
procedure ScaleDPI(Control: TControl; FromDPI: integer);
var
  i: integer;
  WinControl: TWinControl;
begin
  if Screen.PixelsPerInch = FromDPI then
    exit;
 
  with Control do
  begin
    Left := ScaleX(Left, FromDPI);
    Top := ScaleY(Top, FromDPI);
    Width := ScaleX(Width, FromDPI);
    Height := ScaleY(Height, FromDPI);
  end;
 
  if Control is TWinControl then
  begin
    WinControl := TWinControl(Control);
    if WinControl.ControlCount = 0 then
      exit;
    for i := 0 to WinControl.ControlCount - 1 do
      ScaleDPI(WinControl.Controls[i], FromDPI);
  end;
end;
 
end.

Copy the "uscaledpi.pas" file to the main folder of your project:

 MyProject\uscaledpi.pas
 

In the "uses" section of your project you need to add "uScaleDPI":

unit form1;
 
{$mode objfpc}{$H+}
 
interface
 
uses
  Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs,
  uScaleDPI; // This includes ScaleDPI procedure

The OnCreate event of each form calls the procedure in this way:

procedure TForm1.FormCreate(Sender: TObject);
begin
  ScaleDPI(Self,96); // 96 is the DPI you designed the Form1  
end;

Scale All Forms

You can resize all forms at once without having to touch each form's OnCreate event. In order to do this open your project source (typically the Project1.lpr file) and add uScaleDPI in the uses clause.

Then call the procedure HighDPI below the code that initializes the forms:

begin
  RequireDerivedFormResource := True;
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.CreateForm(TForm2, Form2);
  Application.CreateForm(TForm3, Form3);
  HighDPI(96);  // 96 is the DPI you designed the Form1, Form2 & Form3
  Application.Run;
end.

The result looks like this:

program Project1;
 
{$mode objfpc}{$H+}
 
uses
  {$IFDEF UNIX}{$IFDEF UseCThreads}
  cthreads,
  {$ENDIF}{$ENDIF}
  Interfaces, Forms,
  Unit1, Unit2, Unit3,
  uScaleDPI;
 
{$R *.res}
 
begin
  RequireDerivedFormResource := True;
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.CreateForm(TForm2, Form2);
  Application.CreateForm(TForm3, Form3);
  HighDPI(96);
  Application.Run;
end.

Advanced

Some controls have more properties or different property names like TToolBar buttons (ButtonHeight / ButtonWidth instead Width / Height). Also if you use fixed font sizes the behaviour can change in different OSs.

You can edit the ScaleDPI procedure to include code to scale all controls in the way you want.

This is the uscaledpi used in LazPaint. This is very useful for scaling ToolBars and ToolBox.

This is not the final High DPI unit, for example you can use under Windows different LCL widgets, like Qt and this can change the final result.

Link: uscaledpi.pas in LazPaint

Example - Using AutoSize

You can enable the 'AutoSize' option for each control you have (including Forms). Then test the effect under the different DPI modes, with different 'skinning' themes (if available in your target OS) and different font sizes. This can be very useful technique, and has been employed to solve several Lazarus IDE HighDPI issues.

For example using the default AutoSize and ChildSizing most controls can be automatically sized and positioned. But the spacing must be scaled:

  with WinControl.ChildSizing do
  begin
    HorizontalSpacing := ScaleX(HorizontalSpacing, FromDPI);
    LeftRightSpacing := ScaleX(LeftRightSpacing, FromDPI);
    TopBottomSpacing := ScaleY(TopBottomSpacing, FromDPI);
    VerticalSpacing := ScaleY(VerticalSpacing, FromDPI);
  end;

For more information see:

The uscaledpi.pas with this additional code:

 unit uscaledpi;
 
{$mode objfpc}{$H+}
 
interface
 
uses
  Forms, Graphics, Controls;
 
procedure HighDPI(FromDPI: integer);
procedure ScaleDPI(Control: TControl; FromDPI: integer);
 
implementation
 
procedure HighDPI(FromDPI: integer);
var
  i: integer;
begin
  if Screen.PixelsPerInch = FromDPI then
    exit;
 
  for i := 0 to Screen.FormCount - 1 do
    ScaleDPI(Screen.Forms[i], FromDPI);
end;
 
procedure ScaleDPI(Control: TControl; FromDPI: integer);
var
  i: integer;
  WinControl: TWinControl;
begin
  if Screen.PixelsPerInch = FromDPI then
    exit;
 
  with Control do
  begin
    Left := ScaleX(Left, FromDPI);
    Top := ScaleY(Top, FromDPI);
    Width := ScaleX(Width, FromDPI);
    Height := ScaleY(Height, FromDPI);
  end;
 
  if Control is TWinControl then
  begin
    WinControl := TWinControl(Control);
    if WinControl.ControlCount = 0 then
      exit;
 
    with WinControl.ChildSizing do
    begin
      HorizontalSpacing := ScaleX(HorizontalSpacing, FromDPI);
      LeftRightSpacing := ScaleX(LeftRightSpacing, FromDPI);
      TopBottomSpacing := ScaleY(TopBottomSpacing, FromDPI);
      VerticalSpacing := ScaleY(VerticalSpacing, FromDPI);
    end;
 
    for i := 0 to WinControl.ControlCount - 1 do
      ScaleDPI(WinControl.Controls[i], FromDPI);
  end;
end;
 
end.

External Links

Personal tools