This likely needs to be answered by someone familiar with the GxEPD2 ePaper Library since I can only attach a fraction of the library.
I'm new ePaper, the GxEPD2 library is working great for me 
But I have much to learn.
I have been looking at the different procedures to help me understand how to best use them and have found some code that confuses me. I'm fair and c programming but not so much with c++ to the answer may be in how I'm interpreting the code.
My confusion is with the _rst variable on an "if" test.
The constructor assigns the EPD panel Reset to (_rst) pin. Which is connected to GPIO2 of the ESP8266.
This being the case, I don't understand what the test (below) of if (_rst >= 0) means. I would think it would always be >= 0 being a GPIO input.
Am I missing something?
from GxEPD2 by Jean-Marc Zingg
both the below snippets from file: GxEPD2_270.cpp
// constructor
GxEPD2_EPD::GxEPD2_EPD(int8_t cs, int8_t dc, int8_t rst, int8_t busy, int8_t busy_level, uint32_t busy_timeout,
uint16_t w, uint16_t h, GxEPD2::Panel p, bool c, bool pu, bool fpu) :
WIDTH(w), HEIGHT(h), panel(p), hasColor(c), hasPartialUpdate(pu), hasFastPartialUpdate(fpu),
_cs(cs), _dc(dc), _rst(rst), _busy(busy), _busy_level(busy_level), _busy_timeout(busy_timeout),
_diag_enabled(false),
_spi_settings(4000000, MSBFIRST, SPI_MODE0)
// _reset (also same for hibernate)
void GxEPD2_EPD::_reset()
{
if (_rst >= 0)
{
if (_pulldown_rst_mode)
{
digitalWrite(_rst, LOW);
pinMode(_rst, OUTPUT);
delay(_reset_duration);
pinMode(_rst, INPUT_PULLUP);
delay(200);
}
else
{
digitalWrite(_rst, HIGH);
pinMode(_rst, OUTPUT);
delay(20);
digitalWrite(_rst, LOW);
delay(_reset_duration);
digitalWrite(_rst, HIGH);
delay(200);
}
_hibernating = false;
}
}