How to program DirectFB in C# on Raspberry PI (and other embedded devices / SoCs)

Raspberry PI is a very popular and extremely affordable ARM based computer. Among other things, it runs several flavors of Linux, including Debian (called Raspbian in R-Pi world.) Lots of interest in R-Pi world revolves around its graphics and how to unleash its power the easy way (which is not necessarily the right one.) For example, this post  raises an interesting question - how to access kernel's framebuffer via DirectFB in C# (mono.) I've been studying DirectFB for a few days now, and even though C is my language of choice, I have some experience in C#, so I decided to accept the challenge. Here is what I did:

1. Install mono runtime and c sharp compiler
apt-get install mono-runtime mono-mcs

2. Install directfb package for development
apt-get install libdirectfb-dev

3. Build and run directfb example drawing a single line. This is needed to test that DirectFB works properly on Raspberry PI. I copied the sample code into the line.c file, then compiled as follows.
Note that in order to run this successfully, your account needs to have read/write rights to the /dev/fb0 device (userspace interface to the kernel's framebuffer.) Alternatively, you can drop to the root shell.
gcc line.c -o line `pkg-config directfb --cflags --libs` sudo ./line If everything's fine, you should see a horizontal line on  a black background.    

4. Now it's time to try the same in C#. The obvious option is P/Invoke. If I were to develop a C# library for DirectFB, I would (have to) replicate all DirectFB structures in C# code. This is however an overkill for a proof of concept, so here I am cutting corners. It's not the best approach, but definitely the fast one. So I create another shared library in C that wraps the needed set of DirectFB API calls and exposes them to the C# world in a much simpler way. These calls accept no parameters and return void, eliminating the need to replicate numerous DirectFB structures in C#. The corresponding files are dfbwrap.h and dfbwrap.c:

dfbwrap.h
#ifndef __DFBWRAP_H__ #define __DFBWRAP_H__ #include <directfb.h> void Init(int argc, char **argv); void DrawLine(int left, int top, int right, int bottom, unsigned int rgba); void Flip(); void Dispose(); #endif

dfbwrap.c (compare this to the DirectFB line sample - they are almost identical)
#include "dfbwrap.h" static IDirectFB *dfb = NULL; static IDirectFBSurface *primary = NULL; static int screen_width = 0; static int screen_height = 0; #define DFBCHECK(x...) \ { \ DFBResult err = x; \ \ if (err != DFB_OK) \ { \ fprintf( stderr, "%s <%d>:\n\t", __FILE__, __LINE__ ); \ DirectFBErrorFatal( #x, err ); \ } \ } void Init(int argc, char **argv) { DFBSurfaceDescription dsc; DFBCHECK (DirectFBInit (&argc, &argv)); DFBCHECK (DirectFBCreate (&dfb)); DFBCHECK (dfb->SetCooperativeLevel (dfb, DFSCL_FULLSCREEN)); dsc.flags = DSDESC_CAPS; dsc.caps = DSCAPS_PRIMARY | DSCAPS_FLIPPING; DFBCHECK (dfb->CreateSurface( dfb, &dsc, &primary )); DFBCHECK (primary->GetSize (primary, &screen_width, &screen_height)); DFBCHECK (primary->FillRectangle (primary, 0, 0, screen_width, screen_height)); } void DrawLine(int left, int top, int right, int bottom, unsigned int rgba) { DFBCHECK (primary->SetColor (primary, (rgba >> 24) & 0xFF, (rgba >> 16) & 0xFF, (rgba >> 8) & 0xFF, rgba & 0xFF)); DFBCHECK (primary->DrawLine (primary, left, top, right, bottom)); } void Flip() { DFBCHECK (primary->Flip (primary, NULL, 0)); } void Dispose() { primary->Release( primary ); dfb->Release( dfb ); }

The line below compiles and links it into libdfbwrap.so which we will use in our C# app:
gcc dfbwrap.c `pkg-config directfb --cflags --libs` -fpic -shared -o libdfbwrap.so

5. Create and build C# app. I put my C# code into the client.cs:
using System; using System.Runtime.InteropServices; using System.Threading; namespace client { class MainClass { public static void Main (string[] args) { Init (args.Length, args); DrawLine(100,100,200,100,0xFF0000FF); DrawLine(200,100,200,200,0x00FF00FF); DrawLine(200,200,100,200,0x0000FFFF); DrawLine(100,200,100,100,0xFFFFFFFF); Flip (); Thread.Sleep(3000); Dispose(); } [DllImport("libdfbwrap")] static extern void Init(int argc, string[] args); [DllImport("libdfbwrap")] static extern void DrawLine(int left, int top, int right, int bottom, uint rgba); [DllImport("libdfbwrap")] static extern void Flip(); [DllImport("libdfbwrap")] static extern void Dispose(); } }

This is a build command:
mcs client.cs

6. Finally, run the result (don't forget about proper access for your account to /dev/fb0 or drop to root shell)
mono client.exe

If everything's fine, you'll see a square with red, green, blue, and white sides for 3 seconds.

DirectFB API has functions to draw a lot more primitives and also to render text which you can access the same way.