Using the LCD - topikachu/ev3dev GitHub Wiki

NOTE: You need to be a member of the video group to use the LCD.

Basics

The ev3 has a 178 x 128 pixels monochrome LCD. The ev3dev video driver provides standard Linux framebuffer interface and it's possible to write pixel data into /dev/fb0 directly.

The pixel buffer format

Each byte in the buffer represents 8 pixels. Leftmost pixel is in the least significant bit. Value 0 is white and value 1 is black because the driver fb_fix_screeninfo#visual is set to FB_VISUAL_MONO01.
Each row need to be 4 bytes aligned. So it's (178+31)/324 = 24 bytes per row, same value as fb_fix_screeninfo#line_length. The total buffer length is 24128 = 3072 bytes.
See the ev3 video driver code for more detail.

Suppose below is the pixel buffer

    (76543210)(FEDCBA98)
row0 10000100  00000000 00000000 ... (total 24 bytes)
row1 10000100  00000000 00000000 ... (total 24 bytes)
row2 10000100  00000000 00000000 ... (total 24 bytes)
... (total 128 rows)

It draws two vertical lines at column 2(0 based index) and column 7 (also 0 based index).

POC

Use python to draw something interesting.
Below code only test on ev3dev-jessie-2014-07-12. ev3dev may change video driver in the future. We recommend developers getting your own console, activate it and set it to graphics mode before write to framebuffer. Plese see https://github.com/ev3dev/brickdm/blob/master/src/BrickDisplayManager.vala as an example

#!/usr/bin/env python

SCREEN_WIDTH = 178
SCREEN_HEIGHT = 128
HW_MEM_WIDTH = int((SCREEN_WIDTH + 31) / 32) * 4
SCREEN_MEM_WIDTH = int((SCREEN_WIDTH + 7) / 8)
LCD_BUFFER_LENGTH = SCREEN_MEM_WIDTH * SCREEN_HEIGHT
LCD_HW_BUFFER_LENGTH = HW_MEM_WIDTH * SCREEN_HEIGHT
import os
import array


def main():
    buf = [0] * LCD_HW_BUFFER_LENGTH

    # draw a vertical line in column 100 (0 based index)
    for row in range(0, SCREEN_HEIGHT):
        buf[row * HW_MEM_WIDTH + int(100 / 8)] = 1 << (100 % 8)

    # draw a horizontal line in row 64 (0 based index)
    for col in range(0, SCREEN_MEM_WIDTH):
        buf[64 * HW_MEM_WIDTH + col] = 0xff


    import math
    # draw a circle, center at (40,40), radius is 20
    for x in range(0, 20):
        y = math.sqrt(20 * 20 - x * x)
        buf[(40 + int(y)) * HW_MEM_WIDTH + int((40 + x) / 8)] = 1 << ((40 + x) % 8)
        buf[(40 - int(y)) * HW_MEM_WIDTH + int((40 + x) / 8)] = 1 << ((40 + x) % 8)
        buf[(40 + int(y)) * HW_MEM_WIDTH + int((40 - x) / 8)] = 1 << ((40 - x) % 8)
        buf[(40 - int(y)) * HW_MEM_WIDTH + int((40 - x) / 8)] = 1 << ((40 - x) % 8)

    f = os.open('/dev/fb0', os.O_RDWR)
    s = array.array('B', buf).tostring()
    os.write(f, s)
    os.close(f)

if __name__ == '__main__':
    main()