Understanding/controlling MLT melt slideshow?

Question

Consider the following bash script (on Ubuntu 18.04, melt 6.6.0), which uses melt to make a slideshow and play it locally in a window (SDL consumer), mostly copied from https://mltframework.org/blog/making_nice_slideshows/ ( edit: I'm aware that's its possible to specify files individually as in https://superuser.com/questions/833232/create-video-with-5-images-with-fadein-out-effect-in-ffmpeg/834035#834035 - but that approach seems to scale images during transition, and takes quite a while to "render" before playing in SDL window, while this one has nearly instant playback):

echo "
description=DV PAL
frame_rate_num=25
frame_rate_den=1
width=720
height=576
progressive=0
sample_aspect_num=59
sample_aspect_den=54
display_aspect_num=4
display_aspect_den=3
colorspace=601
" > my-melt.profile

mkdir tmppics
convert -background lightblue -fill blue -size 3840x2160  -pointsize 200  -gravity center label:"Test A" tmppics/pic_01.jpg
convert -background lightblue -fill blue -size 3840x2160  -pointsize 200  -gravity center label:"Test B" tmppics/pic_02.jpg

melt -verbose -profile ./my-melt.profile \
./tmppics/.all.jpg ttl=6 \
-attach crop center=1 \
-filter luma cycle=6 duration=4 \
-consumer sdl

• When I run the above command, the video shows the two images loop, but the frame counter keeps on going, increasing indefinitely. How do I make it stop after the exact amount of frames that the loop is long?
• As far as I can see, the size of the output video is controlled by a profile; that is, even if I don't specify -profile, a default one is assumed; is that correct?
• The original images look like this:

... and the video looks like this:

... which means the aspect ratio is wrong; additionally I can see jagged edges, meaning the scaled image in the video is not antialiased.

How do I make the image fit in video size with correct aspect ratio, with antialiasing/smoothing? (I guess it has to do with -attach crop center=1, but I couldn't find documentation on that).

• When viewing stuff in SDL and stepping through frames, are frames numbered 0-based, - or are they 1-based, and at frame 0 simply the same frame as 1 is shown?

• If I use ttl=6 and -filter luma cycle=6 duration=4, I get this:

... that is, visible transition starts at frame 7 (frame 6 is full image A), lasts for frames 7 and 8, and ends at frame 9 (which is full image B); then again at frames 13 and 14 (frame 15 is full image A)

However, if I use ttl=6 and -filter luma cycle=6 duration=2, then I get this:

... that is, there is no transition, image instantly changes at frame 7, then again at frame 13, etc.

So, I'd call the first case a transition duration of 2 frames, and the second case a duration of 0 frames - yet the options are duration=4 and duration=2, respectively. Can anyone explain why? Where are those 2 frames of difference gone?

• Can I - and if so, how - do the same kind of slideshow, except with fade to black? I'd like to define a "time to live" (ttl) of 6 frames per image, and a transition of 4 frames, such that:

  • first, 4 frames are shown of image A;
  • then one frame image A faded, followed by one frame black (amounting to 6 frames TTL for image A, the last 2 transition);
  • then two frames image B faded (amounting to 4 frames transition with previous 2), followed by two more frames image B full (so 4 frames here of image B);
  • then one frame image B faded, followed by one frame black (amounting to 6 frames TTL for image B);

... etc.

• Is it possible to persuade melt to use globbing to select images for slideshow, instead of using .all.jpg? As far as I can see on MLT (Media Lovin' Toolkit) Photo Slide Video no - but maybe there is another approach...

Answer 1

Ok, so, I spent some time looking into the commands for melt and turns out there is actually a pretty effective way of altering a bunch of images (if the number of arguments is too long or there are too many characters for your terminal to handle).

What you want to do is to use -serialise <name of file>.melt which will store your commands (you can also create this file manually). Then to execute that file, use melt <name of file>.melt along with any other options you have for your video file.

Example Format:

melt <images and what to do to them> -serialise <name of file>.melt

---

Example

---

Create the melt file (with Melt CLI)

melt image1.png out=50 image2.png out=75 -mix 25 -mixer luma image3.png out=75 -mix 25 -mixer luma image3.png out=75 -mix 25 -mixer luma image4.png out=75 -mix 25 -mixer luma <...> -serialise test.melt

---

.melt file format

test.melt

image1.png out=50 image2.png out=75 -mix 25 -mixer luma image3.png out=75 -mix 25 -mixer luma image3.png out=75 -mix 25 -mixer luma image4.png out=75 -mix 25 -mixer luma <...>

---

Run

melt test.melt -profile atsc_1080p_60 -consumer avformat:output.mp4 vcodec=libx264 an=1

---

Additional Notes

There should be an extra return character at the end of the melt file. If there isn't, Exceeded maximum line length (2048) while reading a melt file. will be outputted

Notice that -serialise <name of file>.melt will not be in the .melt file

Melt will actually take some time to load the melt file before the encoding process begins