Bit mask bug in Epd::SetPartialWindow causes visual glitch at certain coordinates

[LingDong-]

Dear Waveshare team,

I believe there is a bug in Epd::SetPartialWindow (in Arduino/epd4in2b_V2, and probably a few other demos) that causes a visual glitch.

More specifically, these two lines:

https://github.com/waveshareteam/e-Paper/blob/master/Arduino/epd4in2b_V2/epd4in2b_V2.cpp#L107

SendData(((x & 0xf8) + w  - 1) >> 8);
SendData(((x & 0xf8) + w  - 1) | 0x07);

Should be changed to:

SendData(((x & 0xff8) + w  - 1) >> 8);
SendData(((x & 0xff8) + w  - 1) | 0x07);

The glitch occurs when x is smaller than 255 and x+w is larger than 255, x & 0xf8 will clear all the bits above 8, which will make the computed HRED smaller than HRST, violating the datasheet. The result on the screen looks like jagged strips of torn pieces. To reproduce the glitched effect, one can change the number 200 to 280 in this line:

epd.SetPartialWindowRed(paint.GetImage(), 280, 200, paint.GetWidth(), paint.GetHeight());

The bug can be easily fixed with a larger mask that does not discard the higher bits, e.g. 0xff8 or 0xfff8 etc. I've confirmed that this works on the 4.2inch module.

Thank you for publishing these demos, and for taking a look at this issue! Please feel free to let me know if I missed anything.