Text prompts are good at describing what should be in an image and hopeless at controlling exactly where it goes. That gap is what ControlNet fills. It conditions a diffusion model on a structural input, a pose skeleton, a depth map or an edge outline, so the picture follows a layout you hand it rather than whatever the prompt happens to roll. The result is precise, repeatable composition that prompting alone cannot reach.

Quick Answer

ControlNet is an add-on for diffusion models that locks the output to a structural guide such as pose, depth or edges, giving you direct control over layout and composition. Each ControlNet model loads on top of your checkpoint and adds roughly 1 to 2GB of VRAM, so an 8GB card runs one comfortably and 12GB or more lets you stack several.

How ControlNet Works

ControlNet attaches trainable modules to the diffusion model's noise predictor while keeping the original model weights frozen. You feed it a control map, an image-shaped guide derived from a reference, and that map steers the generation alongside your text prompt. The base model still decides style, colour and detail; ControlNet decides structure.

In practice you start with a reference image, run it through a preprocessor that extracts the control map, then generate with both the map and your prompt active. The output keeps the reference's layout while taking on whatever the prompt describes.

The Main Control Types

Three control types cover most work. OpenPose extracts a stick-figure skeleton of a person, capturing the position of head, limbs and hands, so you can pin a character into an exact pose and regenerate it in any style. Depth turns a reference into a brightness map where lighter areas read as closer and darker as further, preserving the three-dimensional arrangement of a scene. Canny detects edges and hands the model a clean line drawing, holding the structural outline while letting everything else change.

You are not limited to one. A common stack for character scenes uses depth at a lower weight for the overall spatial layout, OpenPose at a higher weight to lock the pose, and a touch of Canny for edge detail. Each control you add costs more VRAM, which is the practical limit on how many you run at once. To see how current RTX cards stack up by memory across the VRAM tiers that ControlNet demands, the AI PC range at Evetech covers the full spread from 8GB to 32GB in built configurations.

What It Costs in VRAM

Each ControlNet model sits on top of the loaded checkpoint and adds around 1 to 2GB of VRAM. For SD 1.5, the lightest ecosystem, 6 to 8GB of VRAM runs a single ControlNet and a long list of available models. SDXL is heavier and wants 12 to 16GB, especially if you use a combined model that handles many control types at once. Running several ControlNets together realistically needs 12GB or more.

This is why ControlNet work is so VRAM-led. The card's memory capacity, more than its raw speed, decides how many controls you can stack and how large you can generate before you run out of room.

Using ControlNet in ComfyUI

ComfyUI is the practical home for ControlNet because its node graph exposes every stage of the pipeline and manages memory efficiently. Install the ControlNet auxiliary preprocessor node pack and you get Canny, depth, OpenPose, lineart and the rest as drag-and-drop nodes. From there you wire a preprocessor to extract the control map, feed it into a ControlNet apply node, and route that into your sampler alongside the prompt. Adjusting each control's weight is how you balance how strictly the output obeys the guide. To see how current GPUs compare on memory for this kind of pipeline, the graphics card best-sellers list is a quick reference.

Frequently Asked Questions

What does ControlNet actually control?

It controls structure and layout. By conditioning the model on a pose, depth or edge map, it forces the generated image to follow that arrangement, something a text prompt cannot do reliably on its own.

How much VRAM does ControlNet need?

Each ControlNet model adds roughly 1 to 2GB on top of your checkpoint. SD 1.5 runs on 6 to 8GB, SDXL prefers 12 to 16GB, and stacking multiple controls realistically wants 12GB or more.

What is the difference between OpenPose, depth and Canny?

OpenPose captures a human pose as a skeleton, depth captures the near-to-far arrangement of a scene as a brightness map, and Canny captures edges as a line drawing. Each preserves a different aspect of the reference.

Can I use more than one ControlNet at once?

Yes. Stacking controls, such as depth for layout plus OpenPose for pose, gives finer command of a scene. The limit is VRAM, since each active control consumes more GPU memory.

Why is ComfyUI recommended for ControlNet?

Its node-based workflow exposes each pipeline stage explicitly and manages GPU memory efficiently. With the auxiliary preprocessor pack installed, every control type is available as a node you can wire in directly.

ControlNet rewards GPU memory above almost anything else. Size your card to the controls you want to stack with the AI PC range at Evetech.