GxEPD2_4G

Arduino Display Library for SPI E-Paper Displays

4 grey levels are available on some e-papers that support differential refresh
4 grey level support can be used instead of differential refresh
"old data" and "new data" controller buffers provides 2 bits per pixel
the 4 states per pixel can produce 4 grey levels using a special waveform
the 4 grey level waveform table is taken from demo from Good Display
the 4 grey levels may differ from display to display, and may be temperature dependent
ghosting can't be avoided; using clearScreen() helps

the display panels are for 3.3V supply and 3.3V data lines
never connect data lines directly to 5V Arduino data pins, use e.g. 4k7/10k resistor divider
series resistor only is not enough for reliable operation (back-feed effect through protection diodes)
4k7/10k resistor divider may not work with flat cable extensions or Waveshare 4.2 board, use level converter then
do not forget to connect GND
the actual Waveshare display boards now have level converters and series regulator, safe for 5V
use 4k7 pull-down on SS for ESP8266 for boards with level converters

This library uses paged drawing to limit RAM use and cope with missing single pixel update support
buffer size can be selected in the application by template parameter page_height, see GxEPD2_Example
Paged drawing is implemented as picture loop, like in U8G2 (Oliver Kraus)
see https://github.com/olikraus/u8glib/wiki/tpictureloop
Paged drawing is also available using drawPaged() and drawCallback(), like in GxEPD
// GxEPD style paged drawing; drawCallback() is called as many times as needed
void drawPaged(void (*drawCallback)(const void*), const void* pv)
paged drawing is done using Adafruit_GFX methods inside picture loop or drawCallback

full screen buffer is selected by setting template parameter page_height to display height
drawing to full screen buffer is done using Adafruit_GFX methods without picture loop or drawCallback
and then calling method display()

bitmap drawing support to the controller memory and screen is available:
either through the template class instance methods that forward calls to the base display class
or directly using an instance of a base display class and calling its methods directly

GxEPD2 uses Adafruit_GFX for Graphics and Text support, which is well documented there
GxEPD2 uses meaningful method names, and has some comments in the header files
consult the header files GxEPD2_BW.h, GxEPD2_4G.h and GxEPD2_GFX.h
for the concept of paged drawing and picture loop see:
https://github.com/olikraus/u8glib/wiki/tpictureloop

renamed top level files and classes to allow co-existence of installed GxEPD2_4G and GxEPD2
fixed drawGreyPixmap() issues
additional fixes

added method drawGreyPixmap() to buffered graphics
void drawGreyPixmap(const uint8_t pixmap[], int16_t depth, int16_t x, int16_t y, int16_t w, int16_t h);