Concerning SDL2 as a GTK2 form control in Pascal/Lazarus

I hop between Windows 10 and Linux Mint constantly. The basic Lazarus components work fine without much changes need, if any at all. However, I wanted to use SDL2 as a form component because CPU rendering was too slow for my needs. I had for the first time to actually use compile time conditionals for these GTK2 specific problems to get SDL2 running without any issues on Linux Mint.

SDL2 for Pascal

Before continuing, I have to mention that I’m using the SDL2-for-Pascal units found on GitHub. There you can find more details about how to install and use the Pascal units. For more information visit FreePascal-meets-SDL.net.

Facing some problems

I’m using the bare minimum of SDL2 just to draw a single texture to screen. SDL2 works auto-magically on Windows, but I needed to do some actual work on Linux to make it work properly.

• acquiring the pointer needed for SDL_CreateWindowFrom() was more complex and needed GTK2 specific libraries;
• with trial and error I discovered that TOpenGLControl from LazOpenGLContext package works best as an SDL2 window (while on Windows a simple TPanel works just fine);
• OnPaint doesn’t seem to work on TOpenGLControl when calling Invalidate(). I only managed to get it working by assigning its Parent an OnPaint event while also being forced to call Invalidate() on said Parent (but both OnPaint and Invalidate() work on TOpenGLControl in Windows, go figure);
• having to manually call SDL_SetWindowSize() for proper control resizing (happens auto-magically in Windows).

Some relevant code

Starting with the uses block, these are the most relevant units for making SDL2 work as a form control.

uses
  {$IFDEF LCLGTK2}
  gtk2, gdk2x,
  {$ENDIF}
  OpenGLContext, SDL2;

Getting the pointer for the control handle is a bit more complicated in Mint than it is on Windows when it comes to SDL2 window creation (I found this snippet online but can’t recall where).

{$IFDEF LCLGTK2}
// Linux with GTK2 widget-set
sdlWindow := SDL_CreateWindowFrom(Pointer(GDK_WINDOW_XWINDOW(PGtkWidget(PtrUInt(OpenGLControl.Handle))^.window)));
{$ELSE}
// My targets are Windows and Linux (GTK2) so a simple ELSE works
sdlWindow := SDL_CreateWindowFrom(Pointer(OpenGLControl.Handle));
{$ENDIF}

Naturally, SDL_SetWindowSize() needs to be called preferably in the OnPaint event (keeping in mind that OnPaint nor Invalidate() seem to not be working with TOpenGLControl on Linux so assign/use them on the TOpenGLControl’s parent instead).

{$IFDEF LCLGTK2}
SDL_GetWindowSize(sdlWindow, @currentWidth, @currentHeight);
if (currentWidth <> actualWidth) or (currentHeight <> actualHeight) then
  SDL_SetWindowSize(sdlWindow, actualWidth, actualHeight);
{$ENDIF}