Fast Preview(FastPreview)
This node technically processes by converting the input image tensor into a PIL image object and then encoding it in memory.
Fast Preview
The Fast Preview node technically works by converting the input image tensor into a PIL image object and then performing encoding processing in memory.
The Fast Preview node is an experimental tool designed for quickly previewing generated images within a ComfyUI workflow. Its core functionality is to bypass the conventional disk-saving steps and directly display the image on the user interface, thereby providing faster visual feedback. Using a binary websocket, it avoids the overhead of Base64/JSON encoding, making previews even snappier.
Node Functionality
Technically, this node works by converting the input image tensor into a PIL image object and then performing encoding processing in memory. It supports two mainstream image formats: JPEG and PNG. Instead of converting the encoded image data into a Base64 string, it sends the raw binary data via a binary websocket, implementing a "file-less" instant preview mechanism that avoids base64/JSON overhead.
Node Parameter Description - Fast Preview
Connection Inputs (Inputs)
| Parameter Name | Data Type | Required | Default Value | Value Range/Options | Description |
|---|---|---|---|---|---|
image | IMAGE | Yes | - | - | Receives the image input to be previewed, of type IMAGE. |
Control Parameters (Parameters)
| Parameter Name | Data Type | Required | Default Value | Value Range/Options | Description |
|---|---|---|---|---|---|
format | COMBO | Yes | JPEG | "JPEG", "PNG" | JPEG or PNG. |
max_size | INT | Yes | 768 | 128 – 4096 (Step: 64) | Sets the maximum dimension (width or height) of the preview image; the image will be resized proportionally if it exceeds this value. |
Usage Scenarios
In image generation or processing workflows, you can connect this node after any node that outputs an image (of type IMAGE). For example, connecting this node at the end of a Stable Diffusion text-to-image pipeline allows you to immediately view the generation result without waiting for the image to be saved to the hard disk, facilitating rapid iteration and adjustment of generation parameters.
Notes
As this is an experimental node, its stability and compatibility may not match that of standard preview nodes. It is primarily intended for scenarios requiring quick viewing of intermediate or final results and is not suitable as a replacement for persistent image storage.
Fast Preview Node Source Code Link
The Fast Preview node is from the ComfyUI-KJNodes node package.
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