How to write/draw to LCD screen?

[ndward]

Me again, the webmaster of ev3python.com where I try to make it easier for beginners to make a start with ev3dev and ev3 Python.

I haven't been able to write text or graphics to the EV3 LCD screen using EV3 Python except by running the ridiculously complex code on http://www.ev3dev.org/docs/tutorials/using-ev3-lcd/ Thirty lines of code just to draw two lines and a circle?! There has to be a better way. And how do I display test, or an image file. Please help!

[ddemidov]

The Screen class has draw property, which is just an instance of pillow's (python imaging library) ImageDraw.Draw class. See pillow's documentation here: https://pillow.readthedocs.io/en/3.3.x/

All operations are applied to internally help image, which is dumped to the actual LCD when update() method is called. Here is an example that draws an alternating smiley-frawny face on EV3 screen:

from time import sleep
import ev3dev.auto as ev3

screen = ev3.Screen()

smile = True

while True:
    screen.clear()

    # Screen.draw returns a PIL.ImageDraw handle
    screen.draw.ellipse(( 20, 20,  60, 60))
    screen.draw.ellipse((118, 20, 158, 60))

    if smile:
        screen.draw.arc((20, 80, 158, 100), 0, 180)
    else:
        screen.draw.arc((20, 80, 158, 100), 180, 360)

    smile = not smile

    # Update lcd display
    screen.update()

    sleep(1)

[ndward]

Your code gives me the following error:
File "smiley.py", line 4, in
screen = ev3.Screen()
File "/usr/local/lib/python3.4/dist-packages/python_ev3dev-0.6.0-py3.4.egg/ev3dev/core.py", line 2310, in init
File "/usr/lib/python3/dist-packages/PIL/Image.py", line 2030, in new
return Image()._new(core.fill(mode, size, color))
TypeError: integer argument expected, got float

Once I've managed to write or draw to the screen, how do I get Brickman back?

[ddemidov]

I am still using python2 on ev3 (its the default I think). It looks like division on line 2310 should be changed to integer division (//) for this to work in python3. I am afraid I won't be able to make a pull request in a couple of weeks (on vacation, away from computer). For now, you could try to run the script with python2.

[ndward]

No worries, for I'm just leaving also for a two week vacation.

[ddemidov]

Once I've managed to write or draw to the screen, how do I get Brickman back?

When you run your scripts from brickman menu, you should get your own screen. It will switch back to brickman when your script finishes. If you run your script from ssh session, you would get weird results: brickman and your script will both write to the same screen.

[WasabiFan]

@ddemidov I can test that out and open a PR if it fixes the problem.

Also, about the screen thing: I think there's a way to do it via SSH as well but can't remember the command. @dlech will have do advise on that.

[ndward]

I've never yet run a script from Brickman. I have to modify the file permissions before I can do that, don't I? Will Brickman know that it should use Python3?

[WasabiFan]

@dlech will have do advise on that.

@dlech has spoken! To run an application that writes to the display via SSH:

1. Run sudo chvt 6. This just switches the display to show you an empty terminal instead of Brickman, so that Brickman doesn't mess with you.
2. Run sudo conspy. This will make your SSH session show the same terminal that is on the EV3's display. Log in to the prompt it shows you.
3. Run your program. You should see it show up on the EV3 display, as you'd expect.
   • You can work from this terminal like normal, run your program multiple times, etc.
4. Press ESC 3 times to exit the display terminal
5. Run sudo chvt 1 to get Brickman back.

It seems a bit complicated, but it's reasonably easy. If you don't mind a little blinking cursor showing up on top of your program, I believe you can skip steps 2 and 4. This also means that the console isn't small, because instead of using the one on the tiny EV3 display, you are using a normal one and just displaying your code's shapes on the display.

---

As for running it with Brickman, I believe you just need to do two things (once per Python program).

1. Run chmod +x my-file.py, using the real name of your file
2. Add the line #!/usr/bin/env python3 to the top of your file. Make sure it is the very fist line!

[dlech]

You have to press ESC 3 times to exit conspy

[WasabiFan]

Fixed.

[WasabiFan]

I am still using python2 on ev3 (its the default I think). It looks like division on line 2310 should be changed to integer division (//) for this to work in python3.

This change has already been made, however @ndward is using version 0.6.0, which doesn't support Python 3 in full. See #195 (comment).

[ndward]

It's common to run, via SSH, Python scripts that can only be terminated with Ctrl+C. What is the equivalent trick to stop the same scripts if they are launched from within Brickman?

[ddemidov]

You can long press back button to end any program that was started with brickman. Another useful shortcut is back+enter to reset the brick.

[ndward]

I followed the instructions above to make a script executable from Brickman:
Run chmod +x my-file.py, using the real name of your file
Add the line #!/usr/bin/env python3 to the top of your file. Make sure it is the very fist line!

And it worked for this script

#!/usr/bin/env python3
# so that script can be run from Brickman
from ev3dev.auto import *  # needed for Sound
Sound.beep()

but not for this one (I used chmod as suggested)

#!/usr/bin/env python3
# so that script can be run from Brickman
from ev3dev.auto import *
from time import sleep
m = LargeMotor('outB')

m.run_to_rel_pos(position_sp=360, duty_cycle_sp=60, stop_command="hold")
sleep(5)

The same script works fine via SSH.

How come the line
#!/usr/bin/env python3
is needed? Aren't comments ignored?
And is it OK to add anything at the end of that line?

[ddemidov]

When the first line starts with a #! followed by a command, its called a shebang. Bash knows to execute the command specified and to pass the rest of the script as its argument.

but not for this one (I used chmod as suggested)

Make sure the file has unix line-endings. When you forget to convert from Windows format, bash thinks that an extra \r in the end of the first line is part of the command.