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Using the EV3 LCD

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  3. Using the EV3 LCD

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 using write or mmap.

The Frame Buffer Format

You can get information about the framebuffer by running fbset -i.

robot@ev3dev:~$ fbset -i

mode "178x128"
    geometry 178 128 178 128 1
    timings 0 0 0 0 0 0 0
    rgba 1/0,1/0,1/0,0/0
endmode

Frame buffer device information:
    Name        : ST7586
    Address     : 0xc39be000
    Size        : 3072
    Type        : PACKED PIXELS
    Visual      : MONO01
    XPanStep    : 0
    YPanStep    : 0
    YWrapStep   : 0
    LineLength  : 24
    Accelerator : No

The 1 at the end of geometry means that there is 1 bit per pixel. So, each byte in the buffer represents 8 pixels. The leftmost pixel is in the least significant bit.

Visual : MONO01 tells us that the value 0 is white and the value 1 is black.

LineLength : 24 means that each horizontal line is 24 bytes. If you do the math, 178 pixels / 8 bits per byte * 1 bit per pixel = 22.25 bytes. This means not all of the bits in a line are actually displayed.

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).

Example

Use python to draw something interesting.

#!/usr/bin/env python

# Hard coding these values is not a good idea because the values could
# change. But, since this is an example, we want to keep it short.
SCREEN_WIDTH = 178 # pixels
SCREEN_HEIGHT = 128 # pixels
LINE_LENGTH = 24 # bytes
SIZE = 3072 # bytes

import os
import array


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

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

    # draw a horizontal line in row 64 (0 based index)
    for col in range(0, LINE_LENGTH):
        buf[64 * LINE_LENGTH + 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)) * LINE_LENGTH + int((40 + x) / 8)] = 1 << ((40 + x) % 8)
        buf[(40 - int(y)) * LINE_LENGTH + int((40 + x) / 8)] = 1 << ((40 + x) % 8)
        buf[(40 + int(y)) * LINE_LENGTH + int((40 - x) / 8)] = 1 << ((40 - x) % 8)
        buf[(40 - int(y)) * LINE_LENGTH + 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()