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The DrawLine routine in 1_54in_epaper/src/epdpaint.cpp does not draw horizontal lines. Perhaps the same for vertical lines but I did not test for that. It does not implement the Bresenham algorithm fully.

Here is my revised code. I have also factored out the "2 * err" repeated code.

void Paint::DrawLine(int x0, int y0, int x1, int y1, int colored) {
/* Bresenham algorithm */
int dx = x1 - x0 >= 0 ? x1 - x0 : x0 - x1;
int sx = x0 < x1 ? 1 : -1;
int dy = y1 - y0 <= 0 ? y1 - y0 : y0 - y1;
int sy = y0 < y1 ? 1 : -1;
int err = dx + dy;

int e2;

while(true)

{
if ((x0 == x1) && (y0 == y1)) break;

    DrawPixel(x0, y0 , colored);
    e2 = 2 * err;
    
    if (e2 >= dy) {
       if (x0 == x1) break;     
        err += dy;
        x0 += sx;
    }
    if (e2 <= dx) {
       if (y0 == y1) break;
        err += dx; 
        y0 += sy;
    }
}

}