Highlight (un)recognized words about chaya HOT 4 CLOSED

This should be possible:

• render canvas to image and send image to OCR
• use the response to modify the canvas (e.g. dim everything, and brighten recognized words)
• render this modified canvas to image, and use that for the page backgrounds

from chaya.

Would be good to include bounding boxes for each line, in the line's attrs. This will also help if we want tight bounding boxes for lines/regions later.

from chaya.

Done:

in 25d8516.

from chaya.

Made the boxes slightly wider in 177f878.

from chaya.