Update instructions

README.md

@@ -11,11 +11,12 @@ project, a **point quadtree** is implemented.
 The idea is that each node represents a rectangel in 2D space. The rectangle can
 store a limited number of points. If its capacity is reached, a node gets
 divided into 4 NW, NE, SE, SW children (leaves), where its points get
-re-distributed. Some nice theoretical resources are found in [1], [2].
+re-distributed. Some nice theoretical resources are found in [1], [2], [3].
 
 To get some intuition, the schematic below shows what happens to a node of
 capacity of 2 when 3 points are sequentially inserted.
 ```
+spatial representation:
 capacity=2                                       NW                 NE
 +--------------------+  +--------------------+  +--------------------+
 |                    |  |                    |  |          |         |
@@ -29,46 +30,67 @@ capacity=2                                       NW                 NE
 |                    |  |                    |  |          |         | 
 +--------------------+  +--------------------+  +--------------------+
                                                 SW                  SE
+tree representation:
+     +--+                      +--+                      +--+
+     |* |                      |**|                      |  |
+     +--+                      +--+                      +--+
+                                                          | 
+                                                          |
+                                               +------+---+---+------+
+                                               |      |       |      |
+                                              +--+   +--+   +--+   +--+
+                                              |* |   |  |   |* |   |* |
+                                           NW +--+ NE+--+ SE+--+ SW+--+
 ```
 
 # 2. Usage
 
 ## 2.1 Project structure
 ```
 .
-├── examples
-│   ├── 01_particle_sim.c    <- demo
+├── examples                 <- directory where demos can be added
+│   └── 01_particle_sim.c    <- demo
 ├── include
-│   ├── quad.h               <- library
-│   └── viz.h                <- GNUplot visualizer
+│   ├── quad.h               <- library 
+│   ├── viz_gplot.h          <- gnuplot plotter
+│   ├── viz.h                <- plotter interface
+│   └── viz_ppm.h            <- ppm frame plotter
 ├── src
 │   ├── quad.c               <- library
-│   └── viz.c                <- GNUplot visualizer
+│   ├── viz_gplot.c          <- gnuplot plotter
+│   └── viz_ppm.c            <- ppm frame plotter
 └── test
-    ├── nanotest.h           <- testing framework
-    └── test.c               <- unit tests
+    ├── nanotest.h           <- unit test framework
+    └── tests.c              <- unit tests
 ```
-The visualizer plots the quadtree in real time via a pipe fed to GNUplot. The
-library is independent from the visualizer.
+The visualizer plots the quadtree in real time either via a pipe fed to GNUplot
+or directly as .ppm frames. The quadtree library is independent from the
+visualizer.
 
 ## 2.2. Dependencies and required tools
 
 * `make` to compile the project.
-* `gnuplot` to run the particle simulation demo.
 * `gcc` is the default compiler, set in the Makefile.
+*  either `gnuplot` or media player like `mpv` to run the particle simulation
+demo.
 
 ## 2.3. Compilation
 
-To compile the library with any demo(s):
-```
-make
-```
+You can compile the example(s) with either GNUplot or .ppm frame emitter as the
+visualizer.
+
+| gnuplot  | ppm frames       |
+| -------- | ---------------- |
+| `make`   | `make -DUSE_PPM` |
+
 Then all demos under `examples` will be compiled against the library. All
 executables will be generated in the root. A particle simulation to demonstrate
 how the tree works has been written. To run it:
-```
-./01_particle_sim | mpv -
-```
+
+| gnuplot               | ppm frames                                            |
+| --------------------- | ----------------------------------------------------- |
+| `./01_particle_sim`   | `./01_particle_sim | mpv --no-correct-pts --fps=30 -` |
+
 To compile and run the unit tests:
 ```
 make test