Overview
The basic Upscale Image By node resizes an image by a chosen factor. A 512 by 512 image scaled by two becomes 1024 by 1024. The result is larger, but the original blur and lack of detail remain. The method changes dimensions rather than generating new image structure, so it is not recommended when the purpose is to improve visible detail.
Upscale Image Using Model performs a different process. It requires an Upscale Model Loader connected to the node. Two useful model options are:
RealESRGAN x4, which is suitable for realistic photographs.
4x-UltraSharp, another strong general option.
A 4x model increases each dimension by four. A 512 by 512 input therefore becomes an image over 2000 pixels on each side. A 2x result requires a 2x upscaler model rather than assuming every model supports an arbitrary scale.
To compare methods, preview the decoded 512 by 512 image before the upscaler and the enlarged output after it. The model-based result shows sharper patterns in the subject's helmet and armor, whereas the basic resize preserves the original blur.
ComfyUI can pause a downstream branch with node muting. Selecting a node and pressing Ctrl+M mutes it, and everything after that point is paused. The workflow can generate the initial image, allow the user to inspect it, and proceed to upscaling only after the node is unmuted.
This two-stage method requires a fixed seed. If the seed remains randomized, queuing the graph after unmuting generates a new initial image and upscales that different result. With a fixed seed, the unchanged upstream image is reproduced, and ComfyUI can continue from the previously muted branch.
A more efficient option is the Image Chooser custom node. The batch size can be increased to four, and all four images are decoded into the chooser. The user selects one or more candidates and triggers Progress Selected Image. Only the selected output is sent into the upscaler.