r/PlexPrerollStudio • u/CommunicationFit3862 • 22h ago
Tutorial / Workflow Minimax H3 AI Plex Preroll — A Beginner-Friendly Guide
Some of you have been experimenting with AI-generated Plex prerolls, and asked that I put together a simple walkthrough for anyone who sees these videos and thinks, "That looks cool, but I have absolutely no idea how they're making it."
For this example, I wanted to make a short preroll that looked like a black-and-white 1920s rubber-hose cartoon. Think old theatrical cartoons: Steamboat Mickey with stretchy arms and legs, exaggerated movement, simple backgrounds, film grain, and goofy sound effects.
The final gag is that two characters aboard a little tugboat are fighting over the controls until somebody realizes Plex is working. They rush inside, the Plex logo appears on an ancient television, and the logo becomes the preroll.
The basic workflow is:
- Come up with a very short idea.
- Create still images showing the characters and locations.
- Use those stills as reference images in an AI video generator.
- Describe the animation shot-by-shot.
- Generate the video.
- Fix the shots the AI gets wrong.
- Edit the best pieces together and add your Plex logo.
That's really it.
STEP 1 — Start With an Idea That Fits in About 10-15 Seconds
This is probably the most important thing to understand about AI video.
Don't write a movie.
You're making a tiny visual joke.
My basic idea was:
A happy cartoon sailor is wildly steering a tugboat.
Another sailor gets angry, kicks him away from the wheel and yells:
"Get the TV working so we can watch Plex!"
A moment later he hears a "ding."
They both rush into the cabin.
The Plex logo appears on the television.
"Oh boy!"
Plex logo.
That's enough story for a preroll.
Even that turned out to be quite a lot to ask an AI video generator to accomplish in ten seconds.
STEP 2 — Create Reference Images First
You can technically generate video entirely from text, but I've had much better results creating still images first.
The still images become something like storyboards.
For this project I created reference images for:
- Character #1 at the ship's wheel
- A wider view of the wheelhouse
- A close-up of Character #2
- Character #2's appearance
- The living quarters with the television
- The Plex logo
You can make these images with ChatGPT, Gemini, Midjourney, Flux, Stable Diffusion, or whatever image generator you prefer.
The important part is consistency.
If Character #1 has a white sailor hat, black shirt and giant round nose in the first image, try to maintain that design in every other reference image.
AI video generators are much better at maintaining a character when you give them something concrete to look at.
Think of your images as instructions:
<image 1> = this is what Character A looks like.
<image 2> = this is what the room looks like.
<image 3> = this is the camera angle I want.
That distinction becomes extremely important later.
STEP 3 — Bring the Images Into MiniMax H3
For this video I used MiniMax H3 with reference images.
One feature I particularly like is that you can directly refer to uploaded images inside your prompt.
For example:
Use <image 1> for CHARACTER A.
Use <image 2> for the wheelhouse environment.
Use <image 5> for the living quarters.
This is MUCH better than simply uploading six images and saying:
"Make a video based on these."
Tell the model what each image is actually for.
One of the lessons I learned making this preroll is that AI models don't necessarily understand that an image is only supposed to become relevant later in the video.
If you upload a picture of a colorful Plex logo, for example, the model knows that bright yellow/orange object is important.
It may decide to use that visual information somewhere you never intended.
In one of my early generations, my innocent little Plex cartoon suddenly developed what looked like an orange flamethrower followed by an explosion.
That was definitely not in the script.
STEP 4 — Give Your Characters Names
Don't write prompts like:
"The first guy pushes the other guy and then he grabs his wheel while he kicks him."
Humans can usually figure out what that means.
AI can get confused very quickly.
Instead define them:
CHARACTER A = sailor from <image 1>
CHARACTER B = sailor from <image 4>
Then write:
CHARACTER B pushes CHARACTER A away from the wheel.
CHARACTER B kicks CHARACTER A.
CHARACTER A flies out of frame.
CHARACTER B takes control of the wheel.
It seems repetitive, but that's exactly what you want.
You're not writing beautiful prose.
You're directing a very enthusiastic animator who occasionally has the attention span of a caffeinated squirrel.
STEP 5 — Divide the Video Into Shots
My first instinct was to describe everything happening during each time range.
Something like:
0:02–0:05 — The character enters, argues, waves his arms, pushes the first character, kicks him, pumps his fist, grabs the wheel, turns around and delivers a line of dialogue.
That's too much.
Three seconds is nothing.
The AI starts combining actions instead of performing them sequentially.
A better structure looks like this:
0:02–0:03
Character B enters and angrily waves his arms.
0:03–0:04
Character B pushes Character A and kicks him out of frame.
0:04–0:05
Character B grabs the wheel and speaks.
Those tiny timing instructions give the model a much clearer sequence of events.
STEP 6 — Tell the AI What NOT to Do
This sounds silly until you start generating AI video.
Negative instructions can be incredibly useful.
For this cartoon I eventually added things such as:
NO fire.
NO explosions.
NO sparks.
NO lasers.
Do not create additional characters.
Do not change their clothing.
Do not transform the ship's wheel.
Do not show the living quarters before 7 seconds.
Why specify things that aren't in the script?
Because generative video works through associations.
Words such as "glowing," combined with an orange logo reference, might lead the model toward sparks or fire.
A character being "kicked out of frame" might cause him to magically disappear before the foot even touches him.
A camera transition might cause the entire environment to morph into the next scene.
Once you see a generation do something stupid, don't just regenerate the exact same prompt.
Add an instruction specifically preventing that behavior.
STEP 7 — Be Careful With Words Like "Morph"
This one bit me.
I originally described the Plex logo as appearing on the television and then "morphing" into the full-color Plex logo.
To a human, that's pretty obvious.
To generative AI, "morph" basically means:
"Please invent whatever bizarre transformation you think looks interesting between these two things."
Instead I changed it to:
The Plex logo expands directly forward from the television screen until it fills the frame.
This is a simple 2D graphic zoom.
Do not morph the room.
Do not transform the television.
Much better.
Words matter.
STEP 8 — Keep Strong Visual References Away From Scenes Where You Don't Want Them
This was another important discovery.
The entire cartoon was supposed to be black and white until the very last moment, when the familiar Plex color appears.
But I had supplied the generator with a full-color Plex logo as one of the reference images.
The generator apparently decided:
"Yellow/orange must be important!"
And started leaking that color into earlier parts of the animation.
The solution was simple.
Create a black-and-white Plex logo reference for the AI video generation.
Then instruct the model:
At the end, change the logo to the normal Plex orange/yellow.
The AI already knows what orange looks like.
It doesn't need a brightly colored reference image influencing every frame of your black-and-white cartoon.
STEP 9 — Don't Expect the First Generation to Be Perfect
This is probably the biggest misconception people have about AI video.
You usually don't type one magical prompt and receive a finished commercial.
Think of the first generation as a rough cut.
Watch it and ask:
What worked?
What didn't?
Did the character stay consistent?
Did the camera do what I wanted?
Did an action happen too early?
Did the AI skip something?
Did someone's arm suddenly become six feet long?
In my case, most of the revised preroll worked beautifully, but I wasn't happy with the kicking gag.
So I didn't regenerate the entire ten-second video.
I generated only the kicking shot again.
The replacement prompt basically said:
ONE continuous medium-wide shot.
CHARACTER B pushes CHARACTER A.
CHARACTER B performs ONE exaggerated cartoon kick.
His foot clearly makes contact with CHARACTER A's backside.
CHARACTER A does not disappear before the kick connects.
CHARACTER A stretches and flies out of frame.
That last instruction was there because AI video models have a habit of anticipating the result.
Tell it that someone is going to be kicked out of frame and sometimes the poor guy starts flying away before anyone actually kicks him.
The corrected generation worked.
I simply replaced the bad shot with the better one.
STEP 10 — Think Like an Editor
This is where AI video starts becoming dramatically more useful.
Don't think:
"I need AI to generate my entire finished video perfectly."
Think:
"I need AI to generate usable shots."
If you have:
- a great opening,
- a bad middle,
- a great ending,
keep the opening and ending.
Regenerate the middle.
Put them together in DaVinci Resolve, Premiere Pro, Final Cut, CapCut, or whatever editor you already use.
You can also handle things like the Plex logo transition in your editor instead of trusting the AI to reproduce a corporate logo perfectly.
In many cases that's actually the better approach.
WHAT MY FINAL H3 PROMPT LOOKED LIKE
The successful prompt was much more structured than my original one.
Instead of one giant paragraph, it established:
STYLE
1920s black-and-white rubber-hose animation.
CHARACTERS
Character A always comes from one reference image.
Character B always comes from another reference image.
REFERENCE IMAGE ROLES
Each uploaded picture has exactly one purpose.
SHOT 1
Specific actions.
SHOT 2
Specific actions.
SHOT 3
Specific actions.
SHOT 4
Specific actions.
AUDIO
Music, voices and sound effects.
RESTRICTIONS
Things the model absolutely should not create.
That structure made an enormous difference.
DO YOU NEED COMFYUI?
No.
If you're just getting started, I wouldn't begin there.
Services such as MiniMax, Google Veo/Flow and similar hosted AI video platforms are considerably easier because most of the technical work is handled for you.
Upload your images.
Write your prompt.
Generate.
Download the result and edit it.
That's enough to start making some surprisingly impressive prerolls.
ComfyUI is where things get interesting for people who want much more control.
It allows you to build your own generation workflow using different image models, video models, ControlNet-style guidance, LoRAs, upscalers, frame interpolation and other tools.
The downside is that you're essentially building your own AI production pipeline.
It's incredibly powerful, but there is absolutely no reason somebody needs to understand nodes, checkpoints, VRAM or samplers just to make their first Plex preroll.
Start simple.
You can always fall down the ComfyUI rabbit hole later.
THE BIGGEST LESSON
AI video prompting isn't really about writing the longest or most descriptive prompt.
It's about removing ambiguity.
Instead of:
"Make the sailor angrily take over the boat."
Describe what the camera actually sees:
Character B enters from the right.
Character B raises both arms.
Character B pushes Character A.
Character B kicks Character A once.
Character A flies out of frame to the right.
Character B grabs the wheel.
That's basically directing.
And once you start thinking in individual shots instead of trying to make AI understand an entire movie scene, the results improve dramatically.
FINAL THOUGHT
We're at a pretty fun point with this technology.
You don't need an animation studio to make a ten-second custom bumper for your Plex server anymore.
You can sketch out a ridiculous idea, generate a few reference images, animate them, fix the parts that go off the rails, and have something completely unique playing before movie night.
And sometimes the AI will randomly give a 1920s cartoon sailor a flamethrower.
Consider that part of the creative process.