[Feature] Multiple passes for glsl shaders

[Selur]

I can confirm that if I use the glsl filters from https://github.com/bloc97/Anime4K/releases and use for example:

# Imports
import vapoursynth as vs
# getting Vapoursynth core
core = vs.core
# Loading Plugins
core.std.LoadPlugin(path="I:/Hybrid/64bit/vsfilters/Support/libvs_placebo.dll")
core.std.LoadPlugin(path="I:/Hybrid/64bit/vsfilters/SourceFilter/FFMS2/ffms2.dll")
# source: 'G:\TestClips&Co\files\test.avi'
# current color space: YUV420P8, bit depth: 8, resolution: 640x352, fps: 25, color matrix: 470bg, yuv luminance scale: limited, scanorder: progressive
# Loading source using FFMS2
clip = core.ffms2.Source(source="G:/TestClips&Co/files/test.avi",cachefile="E:/Temp/avi_6c441f37d9750b62d59f16ecdbd59393_853323747.ffindex",format=vs.YUV420P8,alpha=False)
# making sure input color matrix is set as 470bg
clip = core.resize.Bicubic(clip, matrix_in_s="470bg",range_s="limited")
# making sure frame rate is set to 25
clip = core.std.AssumeFPS(clip=clip, fpsnum=25, fpsden=1)
# Setting color range to TV (limited) range.
clip = core.std.SetFrameProp(clip=clip, prop="_ColorRange", intval=1)
# GLSL filter: Anime4K_Darken_VeryFast.glsl
# adjusting color space from YUV420P8 to YUV444P16 for VsGLSLFilter
clip = core.resize.Bicubic(clip=clip, format=vs.YUV444P16, range_s="limited")
clip = core.placebo.Shader(clip=clip, shader="C:/Users/Selur/Desktop/Anime4k/Anime4K_Darken_VeryFast.glsl", width=640, height=352)
# adjusting output color from: YUV444P16 to YUV420P8 for x264Model
clip = core.resize.Bicubic(clip=clip, format=vs.YUV420P8, range_s="limited")
# set output frame rate to 25.000fps
clip = core.std.AssumeFPS(clip=clip, fpsnum=25, fpsden=1)
# Output
clip.set_output()

that the preview works fine, vspipe info works fine, but encoding using:

vspipe "E:\Temp\encodingTempSynthSkript_2021-12-26@07_38_12_9010.vpy" - -c y4m | x264 --preset veryfast --crf 18.00 --profile high --level 4.1 --ref 3 --direct auto --b-adapt 0 --sync-lookahead 48 --qcomp 0.50 --rc-lookahead 40 --qpmax 51 --partitions i4x4,p8x8,b8x8 --no-fast-pskip --subme 5 --aq-mode 0 --vbv-maxrate 62500 --vbv-bufsize 78125 --sar 1:1 --non-deterministic --range tv --colormatrix bt470bg --demuxer y4m  --input-range tv --fps 25/1 --output-depth 8 --output "E:\Temp\2021-12-26@07_38_12_9010_03.264" -

fails with:

y4m [error]: bad frame header magic

Wild guess is that those shaders only work if there is an acctual display.

Since I don't think this is a bug in Hybrid, I opened an issue entry over at https://github.com/Lypheo/vs-placebo/issues/17 to see whether the author of vs-placeco can say anything about it.

Cu Selur