Occasional image corruption with pseudo (and draw?)

[groucho86]

I'm sometimes getting "glitchy" imagery when my background uses "checkerboard" and I draw shapes. It happens about 40% of the time.

Code sample to (sometimes) reproduce the issue:

from wand.image import Image
from wand.color import Color
from wand.display import display
from wand.drawing import Drawing

for f in range(8):
    with Drawing() as fx_wand:
        
        with Image(width=640, height=360, background='white', pseudo='pattern:checkerboard') as img:
            img.metadata['colorspace:auto-grayscale'] = 'false' 
            img.metadata['PixelAspectRatio'] = str(1.0)
            img.colorspace='srgb'
            img.units = 'pixelsperinch'
            img.format='png'      
            img.resolution = 220       
            fx_wand.push_pattern('green_circle', 0, 0, 10, 10)
            fx_wand.fill_color = 'green'
            fx_wand.stroke_color = 'black'
            fx_wand.circle(origin=(5, 5), perimeter=(5, 0))
            fx_wand.pop_pattern()
            fx_wand.set_fill_pattern_url('#green_circle')
            fx_wand.rectangle(top=5, left=5, width=40, height=40)
            fx_wand.draw(img)
            display(img)

Out of the 8 rendered images, 3 exhibited glitches. Is this a bug or is there a proper method to avoid it?

[emcconville]

That looks like a buffer that's not cleared -- not a Wand issue. Are you using X11 or Wayland (or XWayland)?

First thought would be to replace display(img) with img.save(filename='output_{0}.png'.format(f)), and see if the artifacts are not present when you see them on the screen. If so, this is a buffer issue in the desktop's window manager.
If the artifacts are present in the image saved to disk, then that'll be either a ImageMagick, or Python issue.

[groucho86]

Thanks Eric. I’m testing the code in pydev (Eclispe) on an M1 MacBook Pro.

Saving the images result in the same inconsistent behavior.

[emcconville]

Sorry, can't offer much help with that environment. Here's a few more experiments that might help determine the cause.

Goals

• Eliminate Wand's Drawing context, and use direct MVG input.
• Repeat MVG input in a controlled CLI environment.

Part 1: Create MVG

Copy the following into a vector.mvg file in the same pydev environment.

push pattern green_circle 0 0 10 10
 fill '#000080800000'
 stroke '#000000000000'
 circle 5 5 5 0
pop pattern
fill url(#green_circle)
rectangle 5 5 45 45

Part 2: Execute in Python/Wand

Replace the original code above with the following.

from wand.image import Image
from wand.display import display

for _ in range(8):
    with Image(width=640, height=360, background="white", pseudo="pattern:checkerboard") as img:
        img.format = 'png'
        with Image(width=640, height=360, background="transparent", filename="MVG:vector.mvg") as draw:
            img.composite(draw)
            display(img)

Part 3: Repeat in CLI

Open a terminal, and cd to the same directory used by pydev. Run the following bash commands.

for i in $(seq 8)
do
  magick -size 640x360 -background white pattern:checkerboard \
    -background transparent MVG:vector.mvg -composite png:- | display png:-
done

Although, I think osx uses zsh now-a-days. You might need to replace for i in $(seq 8) with for (( i=0; i < 8; i++))

Hopefully this will help identify the cause.