> ## Documentation Index
> Fetch the complete documentation index at: https://docs.pocketbyte.co/llms.txt
> Use this file to discover all available pages before exploring further.
# Your First App
This guide walks you through writing your first piece of software for Pocketbyte and how you can use the simulator to test your app without needing a DevKit.
## Prerequisites
Before you begin, please ensure that the ESP-IDF is installed on your computer and that you have set up your project folder with the Pocketbyte Library added as a component.
Visit [Development Setup](development-setup) to learn how to do this.
## Introduction
On Pocketbyte, apps are single, contained programs. They behave exactly how you would expect, for example, an Arduino program to function.
## Initialization
When writing your program, you must explicitly initialize the subsystem that you will be using.
For example, if you are using audio, you must call `pb_audio_init()` at the start of your program's entry point.
See the library's header files for more info.
## Graphics
The Pocketbyte Library features two different graphics backends: [LVGL](https://lvgl.io/) and [PAX](https://github.com/robotman2412/pax-graphics).
You must decide which backend to use before writing a line of code. They are *not interchangeable,* and if you decide to switch later on, you will have to rewrite a substantial portion of your code.
Alternatively, you can also access the raw display handle to blit pixels without using a dedicated graphics library. LVGL and PAX are just layers above this.
### LVGL
* Retained mode
* Cool, polished-looking UI
* Lots of pre-built widgets
* Great for making traditional apps and utility programs
* Expansive dev ecosystem
* If you already know it, it will be easy to write Pocketbyte software
* Extremely slow - *do not* use it for games!
### PAX
* Immediate mode
* Blazing fast - the best backend for games
* No built-in UI library
* Can be more difficult than LVGL
* Great for drawing primitives
## Examples
"Talk is cheap. Show me the code." - Linus Torvalds.
### LVGL
```c theme={null}
#include "pocketbyte.h"
#include "gfx_lvgl.h"
void app_main(void)
{
pb_gfx_lvgl_init();
lv_obj_t *label = lv_label_create(lv_scr_act());
lv_label_set_text(label, "Hello, Pocketbyte! :D");
lv_obj_center(label);
while (pb_timing_loop(60)) {
pb_gfx_lvgl_tick();
}
}
```
### PAX
```c theme={null}
#include "pocketbyte.h"
#include "gfx_pax.h"
void app_main(void)
{
pb_gfx_pax_init();
while (pb_timing_loop(60)) {
pax_buf_t *gfx = pb_gfx_pax_get_buf();
pax_background(gfx, pax_col_rgb(255, 0, 0));
pax_draw_text(gfx, pax_col_rgb(255, 255, 255), pax_font_sky_mono, 18, 0, 0, "Hello, Pocketbyte! :D");
pb_gfx_pax_flush();
}
}
```
***
As you can see, when developing Pocketbyte applications, the grunt of the work revolves around the graphics.
Because Pocketbyte uses third-party libraries, such as LVGL and PAX, to provide this functionality, 90% of what you'll need to know are actually in the LVGL and PAX documentation.
## What Else?
The examples above are simply "Hello World!" applications.
The Pocketbyte Library has many other features not yet discussed, such as input, audio, module detection, Wi-Fi, NVS, and even IMU interfacing for motion controls.
Most of what you need can be found in the header files of the library itself. I tried my best to make it as intuitive as possible, so you can easily write your software even without a full API reference.
You can find the source code on GitHub: [https://github.com/PocketByteTechnology/library](https://github.com/PocketByteTechnology/library)
## Testing Your Code on the Pocketbyte Simulator
[The Pocketbyte Simulator has been unofficially published on GitHub recently.](https://github.com/PocketByteTechnology/simulator)
On the README, you will find the basic instructions necessary to get it up and running.
You can actually test your software without needing a DevKit!
Enjoy!
