How to display graphical text without X using C?

Question

Using the Raspberry Pi 2 with the official 7" touchscreen, I want to display some high quality text without using an Xorg server. I'm looking for APIs that provide simple functions such as:

Render_text ("hello", x,y, blue, size, "nicefontX");

Such a library could be build on top of the linux kernel framebuffer or openGLES/VG. I have found at least one such thing that appears to be pi specific.

Again, I am interested in C -- I have already found many things involving python.

Answer 1

Did you consider 2D Game engines/libraries ? Because OpenGL/OpenVG are very low-level and don't do font handling , the added "belt and suspenders" of those libs will make your life much easier :

http://www.raylib.com/

https://github.com/raysan5/raylib

Answer 2

I got 2D vector graphics library working, using it from C Language, currently with a 128x128 OLED display on a headless Raspberry Pi 3B, using the following recipe. Note: X-Windows (X11) is not installed (e.g. Raspian lite edition). As for performance considerations, OpenVG's claim to fame is that it utilizes available HW acceleration layer(s) rather than just load the CPU.

1. Install: Raspian lite edition

2. Install: fbcp-ili9341: "Blazingly fast display driver for SPI-based LCD displays for Raspberry Pi A,B,2,3, and Zero"

   I built that from source, and configured the the GPIO pins SPI pins via the (documented) -D commands on the compiler line. For example:

   # cmake -DSSD1351=ON -DGPIO_TFT_DATA_CONTROL=25 -DGPIO_TFT_RESET_PIN=24 -DSPI_BUS_CLOCK_DIVISOR=30 -DARMV8A=ON \  ..
   

   and then proved it work sanity tested it by writing stuff to /dev/fb0.

3. Didn't need to install OpenGL libraries on my Raspian systems, because they seemed to be pre-installed at /opt/vc/lib, opt/vc/include.

   However, I created the following symlinks:

   # cd /opt/vc/lib
   # ln -s libbrcmGLESv2.so libGLESv2.so
   # ln -s libbrcmEGL.so libEGL.so
   

4. OpenVG "The Standard for Vector Graphics Acceleration"

   https://github.com/ajstarks/openvg
   

5. Compiled the following C program and ran it. Worked perfectly.

   $ cc -o demo demo.c -I/opt/vc/include -Lopt/vc/lib -lshapes
   
   #include <stdio.h>
   #include <stdlib.h>
   #include <stddef.h>
   #include <stdint.h>
   #include <unistd.h>
   #include "VG/openvg.h"
   #include "VG/vgu.h"
   #include "fontinfo.h"
   #include "shapes.h"
   
   int main() {
       int width, height;
       char s[3];
   
       init(&width, &height);                  // Graphics initialization
       Start(width, height);                   // Start the picture
       Background(0, 50, 0);                   // Black background
       Fill(44, 77, 232, 1);                   // Big blue marble
       Circle(width / 2, 0, width);            // The "world"
       Fill(255, 255, 255, 1);                 // White text
       TextMid(width / 2, height / 2, 
           "hello, world", 
            SerifTypeface, width / 10);        // Greetings 
       End();                                  // End the picture
       fgets(s, 2, stdin);                     // look at the pic, end with [RETURN]
       finish();                               // Graphics cleanup
       exit(0);
   }