AI video generation models, explained

What the model is actually doing

Modern video models are diffusion models. They start from visual noise and refine it, step by step, into frames that match your prompt — the same way image generators work, except each step has to produce many frames that agree with each other over time. That temporal agreement is the hard part, and it's where most of the research effort goes.

Why clips are short

Every extra second multiplies the frames the model must keep consistent. Small errors — a slightly shifted shadow, a warped finger — compound frame over frame until the clip drifts. That is why five-to-eight-second clips look sharp and minute-long generations fall apart, and why professionals assemble longer videos from several short renders.

Text-to-video vs image-to-video

Text-to-video asks the model to invent everything: subject, scene, light, motion. With so many degrees of freedom, results vary widely between runs. Image-to-video removes most of that freedom: your photo becomes the first frame, so the subject, lighting and composition are already decided. The model only invents motion — a much smaller job, which is why the results are so much more consistent.

• Text-to-video: maximum creativity, variable consistency
• Image-to-video: the photo anchors reality, the model adds motion
• expidvid supports both — upload a photo or start from a written prompt

Why prompts read like film direction

These models learned from real footage and its descriptions. Camera vocabulary — "handheld medium shot", "slow push in", "shallow depth of field" — matches how that training footage was described, so it activates far more specific behavior than adjectives like "epic" or "beautiful". This is also why a scanned description of your photo makes a strong prompt: it describes the scene the way the model expects.

What changes when a new model version ships

New versions mainly improve temporal consistency (less drift), physics (water, cloth, hair behave more plausibly) and prompt adherence (the clip matches what you asked for). What rarely changes: short clips still beat long ones, one action per shot still works best, and starting from a real photo still beats starting from nothing. The fundamentals in our prompt-writing guide survive every model update.

The takeaway

Treat the model like a very fast, very literal camera crew: give it a real scene or a precise shot description, ask for one thing, keep it short. The tools will keep improving, but that working style transfers to every new version.