Bug 366059

Audio-only (wav) exports / renders are extremely slow when GPU acceleration / movit / glsl are enabled, as soon as you turn them off the render speed of audio-only jumps up considerably.

A good way to prove this is to take a short video, generate a script for an audio only export, run the script, take the glsl.=1out of the script and re-run it. Note the change in running speed. The problem is also evident if you simply render a wave from inside kdenlive, and then do the same thing after disabliing gpu rendering.

Feels like melt is rendering the whole frame and then discarding the video rather than ignoring the video because its audio-only.

This also happened when I use flac audio-only (video_off=1 f=flac acodec=flac vn=1) - despite the explicit video_off and vn settings, the slowness was still apparent, once again as soon as I removed glsl.=1 from the generated script.

The bug exists both in 16.04 and the latest git version.

Reproducible: Always

Steps to Reproduce:
1. Drag a movie clip into a fresh project with movit enabled
2. Generate an audio-only (wav) script
3. Run the script

Actual Results:  
Very slow rendering, almost real-time.

Expected Results:  
Speedy audio-only render, much much faster than real-time.

Removing glsl.=1 from the generated script makes it run a gazillion times faster.