How to encode a time series as an image to feed it into CNN? I want to try CNN in the task of stock chart pattern recognition. I suspect that feeding a line chart won't work because the image will have a lot of empty pixels.

What time series encoding options are available to get a lossless, dense image?
You can encode time series to images using image encoding methods like Recurrence Plots (RP), Gramian Angular Field (GAF), Markov Transition Field (MTF).

See the following article:

Estebsari, A.; Rajabi, R. Single Residential Load Forecasting Using Deep Learning and Image Encoding Techniques. Electronics 2020, 9, 68.
https://doi.org/10.3390/electronics9010068