Recently I downsized a JPEG of line art in Photoshop for a poster, and it looked just awful.
When I reduce the size to 300ppi, and 4" wide,
If it's really line art (no gray values) then 300ppi isn't enough. Line art needs to be closer to the output resolution (800-1200 ppi), and shouldn't be saved as JPEG.
That was just an extra comment. This is a JPEG photo I'm asking about.
but when I print out the page on my printer, the diagonals are still kind of jaggy.
Make sure you have Print>Graphics>Send Data set to All.
I did change it, but it didn't help. Thank you, though.
I've been using Photoshop and InDesign for years and years and years. This is the first time I've run into this problem - since CC.