rpi-rgb-led-matrix/examples-api-use

Running some demos

Let's start by running some demos, then we can dive into code. The demo-main.cc has some testing demos. Via command line flags, you can choose the display type you have (16x32 or 32x32), and how many you have chained and paralleled. For detailed description of these flags see the main README section about it.

$ make
$ sudo ./demo
usage: ./demo <options> -D <demo-nr> [optional parameter]
Options:
        -D <demo-nr>              : Always needs to be set
        -L                        : 'Large' display, composed out of 4 times 32x32
        -t <seconds>              : Run for these number of seconds, then exit.
        -R <rotation>             : Sets the rotation of matrix. Allowed: 0, 90, 180, 270. Default: 0.
        --led-rows=<rows>         : Panel rows. 8, 16, 32 or 64. (Default: 32).
        --led-chain=<chained>     : Number of daisy-chained panels. (Default: 1).
        --led-parallel=<parallel> : For A/B+ models or RPi2,3b: parallel chains. range=1..3 (Default: 1).
        --led-pwm-bits=<1..11>    : PWM bits (Default: 11).
        --led-brightness=<percent>: Brightness in percent (Default: 100).
        --led-scan-mode=<0..1>    : 0 = progressive; 1 = interlaced (Default: 0).
        --led-show-refresh        : Show refresh rate.
        --led-inverse             : Switch if your matrix has inverse colors on.
        --led-swap-green-blue     : Switch if your matrix has green/blue swapped on.
        --led-pwm-lsb-nanoseconds : PWM Nanoseconds for LSB (Default: 130)
        --led-no-hardware-pulse   : Don't use hardware pin-pulse generation.
        --led-slowdown-gpio=<0..2>: Slowdown GPIO. Needed for faster Pis and/or slower panels (Default: 1).
        --led-daemon              : Make the process run in the background as daemon.
        --led-no-drop-privs       : Don't drop privileges from 'root' after initializing the hardware.
Demos, choosen with -D
        0  - some rotating square
        1  - forward scrolling an image (-m <scroll-ms>)
        2  - backward scrolling an image (-m <scroll-ms>)
        3  - test image: a square
        4  - Pulsing color
        5  - Grayscale Block
        6  - Abelian sandpile model (-m <time-step-ms>)
        7  - Conway's game of life (-m <time-step-ms>)
        8  - Langton's ant (-m <time-step-ms>)
        9  - Volume bars (-m <time-step-ms>)
        10 - Evolution of color (-m <time-step-ms>)
        11 - Brightness pulse generator
Example:
        ./demo -t 10 -D 1 runtext.ppm
Scrolls the runtext for 10 seconds

To run the actual demos, you need to run this as root so that the GPIO pins can be accessed; as soon as that established, the program will drop the privileges.

Here is how demo '1' looks. It requires a ppm (type raw) with a height of 32 pixel - it is infinitely scrolled over the screen; for convenience, there is a little runtext.ppm example included:

 $ sudo ./demo -D 1 runtext.ppm

Here is a video of how it looks

Using the API

While there is the demo program and the utilities, this code can be used independently as a library to be used in your own programs. The includes are in include/, the library to link is built in lib/. This is a C++ also with C bindings. There is also a Python binding.

Getting started

The relevant part to start with is to look at led-matrix.h.

You can would typically use the CreateMatrixFromFlags() factory to create an RGBMatrix and then go from there.

#include "led-matrix.h"

using rgb_matrix::RGBMatrix;

int main(int argc, char **argv) {
  // Set some defaults
  RGBMatrix::Options my_defaults;
  my_defaults.chain_length = 3;
  my_defaults.show_refresh_rate = true;
  rgb_matrix::RuntimeOptions runtime_defaults;
  runtime_defaults.drop_privileges = 1;
  RGBMatrix *matrix = rgb_matrix::CreateMatrixFromFlags(&argc, &argv,
                                                        &my_defaults,
                                                        &runtime_defaults);
  if (matrix == NULL) {
    PrintMatrixFlags(stderr, my_defaults, runtime_defaults);
    return 1;
  }

  // Do your own command line handling with the remaining options.

  //  .. now use matrix

  delete matrix;   // Make sure to delete it in the end.
}

The RGBMatrix is essentially a canvas, it provides some basic functionality such as SetPixel(), Fill() or Clear(). If you want to do more, you might be interested in functions provided in the graphics.h header.

If you have animations, you might be interested in double-buffering. There is a way to create new canvases with CreateFrameCanvas(), and then use SwapOnVSync() to change the content atomically. See API documentation for details.

Start with the minimal-example.cc to start.

If you are interested in drawing text and the font drawing functions in graphics.h, have a look at the text example:

sudo ./text-example -f ../fonts/8x13.bdf
hello

Integrating in your own application

Until this library shows up in your favorite Linux distribution, you can just include the library via github; it is pretty easy to be up-to-date.

I suggest to add this code as a sub-module in your git repository. That way you can use that particular version and easily update it if there are changes:

 git submodule add https://github.com/hzeller/rpi-rgb-led-matrix.git matrix

(Read more about how to use submodules in git)

This will check out the repository in a subdirectory matrix/. The library to build would be in directory matrix/lib, so let's hook that into your toplevel Makefile. I suggest to set up some variables like this; you only need to change the location RGB_LIB_DISTRIBUTION is pointing to; in the sub-module example, this was the matrix directory:

 RGB_LIB_DISTRIBUTION=matrix
 RGB_INCDIR=$(RGB_LIB_DISTRIBUTION)/include
 RGB_LIBDIR=$(RGB_LIB_DISTRIBUTION)/lib
 RGB_LIBRARY_NAME=rgbmatrix
 RGB_LIBRARY=$(RGB_LIBDIR)/lib$(RGB_LIBRARY_NAME).a
 LDFLAGS+=-L$(RGB_LIBDIR) -l$(RGB_LIBRARY_NAME) -lrt -lm -lpthread

Also, you want to add a target to build the libary in your sub-module

 # (FYI: Make sure, there is a TAB-character in front of the $(MAKE))
 $(RGB_LIBRARY):
	 $(MAKE) -C $(RGB_LIBDIR)

Now, your final binary needs to depend on your objects and also the $(RGB_LIBRARY)

 my-binary : $(OBJECTS) $(RGB_LIBRARY)
     $(CXX) $(CXXFLAGS) $(OBJECTS) -o $@ $(LDFLAGS)

As an example, see the PixelPusher implementation which is using this library in a git sub-module.

If you are writing your own Makefile, make sure to pass the -O3 option to the compiler to make sure to generate fast code.

Note, all the types provided are in the rgb_matrix namespace. That way, they won't clash with other types you might use in your code; in particular pretty common names such as GPIO or Canvas might run into clashing trouble.

Anyway, for convenience you just might add using-declarations in your code:

 // Types exported by the RGB-Matrix library.
 using rgb_matrix::Canvas;
 using rgb_matrix::GPIO;
 using rgb_matrix::RGBMatrix;
 using rgb_matrix::ThreadedCanvasManipulator;

Or, if you are lazy, just import the whole namespace:

 using namespace rgb_matrix;

Read the minimal-example.cc to get started, then have a look into demo-main.cc.

Remapping coordinates

You might choose a different physical layout than the wiring provides.

Say you have 4 displays with 32x32 and only a single output like with a Raspberry Pi 1 or the Adafruit HAT -- if we chain them, we get a display 32 pixel high, (4*32)=128 pixel long. If we arrange the boards in a square, we get a logical display of 64x64 pixels:

In action:

How can we make this 'folded' 128x32 screen behave like a 64x64 screen ?

In the API, there is an interface to implement, a CanvasTransformer that allows to program re-arrangements of pixels in any way. You can plug such a CanvasTransformer into the RGBMatrix to use the new layout (void RGBMatrix::SetTransformer(CanvasTransformer *transformer)).

Sometimes you even need this for the panel itself: In newer panels (often with 1:4 multiplexing) the pixels are often not mapped in a straight-forward way, but in a snake arrangement for instance. The CanvasTransformer allows you to work around that (sorry, I have not seen these panels myself so that I couldn't test that; but if you come accross one, you might want to send a pull-request with a new CanvasTransformer).

Back to the 64x64 arrangement:

There is a sample implementation class LargeSquare64x64Transformer that maps the 128x32 pixel logical arrangement into the 64x64 arrangement doing the coordinate mapping. In the demo program and the led-image-viewer, you can activate this with the -L option.