U8g2 is a graphics library developed at [olikraus/u8g2](https://github.com/olikraus/u8g2) with support for many different displays. It is the successor of [U8glib](https://github.com/olikraus/u8glib) which is not developed any further.
The NodeMCU firmware supports the following displays in I²C and SPI mode:
Hook up SDA and SCL to any free GPIOs. Eg. [graphics_test.lua](../../../lua_examples/u8glib/u8g_graphics_test.lua) expects SDA=GPIO16 and SCL=GPIO17. They are used to set up nodemcu's I²C driver before accessing the display:
```lua
id = i2c.HW0
sda = 16
scl = 17
i2c.setup(id, sda, scl, i2c.FAST)
```
### SPI connection
Hook up pins to any free GPIOs:
- SCLK
- MOSI
- CS
- D/C
- RES (optional for some displays)
Again, refer to the initialization sequence eg. in [graphics_test.lua](../../../lua_examples/u8glib/u8g_graphics_test.lua):
```lua
sclk = 19
mosi = 23
bus = spi.master(spi.HSPI, {sclk=sclk, mosi=mosi})
```
### Library Usage
The Lua bindings for this library closely follow u8g2's object oriented C++ API. Based on the u8g2 class, you create an object for your display type.
This object provides all of u8g2's methods to control the display. Refer to [graphics_test.lua](../../../lua_examples/u8glib/u8g_graphics_test.lua) to get an impression how this is achieved with Lua code. Visit the [u8g2 homepage](https://github.com/olikraus/u8g2) for technical details.
### Displays
I²C and HW SPI based displays with support in u8g2 can be enabled. To get access to the respective constructors, enable the desired entries for I²C and SPI displays in u8g2's sub-menu (run `make menuconfig`).
### Fonts
u8g2 comes with a wide range of fonts for small displays. They can be supplied as strings or compiled into the firmware image to decrease the RAM footprint. Simply add the desired fonts to the font selection sub-entry via `make menuconfig`.
They'll become available as `u8g2.<font_name>` in Lua.
### Bitmaps
XBM bitmaps are supplied as strings to `drawXBM()`. This off-loads all data handling from the u8g2 module to generic methods for binary files. See [graphics_test.lua](../../../lua_examples/u8glib/u8g_graphics_test.lua).
In contrast to the source code based inclusion of XBMs in upstream u8g2 library, it's required to provide precompiled binary files. This can be performed online with [Online-Utility's Image Converter](http://www.online-utility.org/image_converter.jsp): Convert from XBM to MONO format and upload the binary result.
Each display type can be initialized to provide the framebuffer contents in run-length encoded format to a Lua callback. This mode is enabled when a callback function is specified for the setup function. Hardware display and framebuffer callback can be operated in parallel. If the callback function is the only parameter then no signals for a hardware display are generated, leaving a virtual display.
The callback function can be used to process the framebuffer line by line. It's called with either `nil` as parameter to indicate the start of a new frame or with a string containing a line of the framebuffer with run-length encoding. First byte in the string specifies how many pairs of (x, len) follow, while each pair defines the start (leftmost x-coordinate) and length of a sequence of lit pixels. All other pixels in the line are dark.
