This improved SSTV decoder corrects slant using the Hough transform. I’ve put a moving average lowpass filter before the FM demodulator part to reduce noise, as you can see, this helps a lot:

MMSSTV’s image is on the right. The lowpass filters reduce noise but they keep the details. Here’s a video of my app and MMSSTV decoding a distorted signal:

Resources

Slant correction using the Hough transform
SSTV signal generator

Open source SSTV apps:

qsstv
slowrx

Source code