How to draw a scaled version of images from the system ImageList?

Question

I create and initialize the TImageList...

ImageList:=TImageList.Create(self);
ImageList.ShareImages:=true;

I get the system ImageList handle for small icons (16x16)...

ImageList.Handle:=SHGetFileInfo('',0,FileInfo,SizeOf(FileInfo),SHGFI_SYSICONINDEX or SHGFI_SMALLICON);

And now I want to draw the icons on my control canvas scaled to 50x50 pixels. How I do this ?

Answer 1

Upscaling small icons is never good idea. 16X16 icon is too small to use it in any other way other as-is.

For start you can get larger shell images using

ImageList.Handle:=SHGetFileInfo('',0,FileInfo,SizeOf(FileInfo),SHGFI_SYSICONINDEX or SHGFI_ICON);

Those images will be generally 32x32 pix, but to be on safe side (if Windows are running in high DPI mode) you can get correct size from system

uses
  Winapi.Windows;

var
  IconWidth, IconHeight: integer;

  IconWidth := GetSystemMetrics(SM_CXICON);
  IconHeight := GetSystemMetrics(SM_CYICON);

You can also get even larger shell images with SHGetImageList http://msdn.microsoft.com/en-us/library/windows/desktop/bb762185%28v=vs.85%29.aspx

To retrieve icon from you ImageList (if ImageList contains icons, and in your case it does) you can use:

ImageList.GetIcon(Index: Integer; Image: TIcon);

For scaling icons, while preserving transparency, to custom dimension you can use following code:

procedure StretchDrawIcon(Canvas: TCanvas; Dest: TRect; Icon: TIcon);
begin
  DrawIconEx(Canvas.Handle, Dest.Left, Dest.Top, Icon.Handle, Dest.Right - Dest.Left, Dest.Bottom - Dest.Top, 0, 0, DI_NORMAL);
end;