# Visual Footprint

> Use this feature to compute the visual footprint of 3D assets, providing a 2D representation of their spatial extent for efficient rendering and analysis in the Pixyz SDK.

API function: [algo.getVisualComparisonFootprint](/asset-transformer-sdk/2026.4/api/python/algo_functions.md#getvisualcomparisonfootprint)

This function is used in order to compare the visual difference between two sets of occurrences. The result is in millimeters, in the world space.

![Visual Comparison](/api/media?file=/asset-transformer-sdk/media/images/algo/visual_footprint/comparison_before_after.png)

For instance, here we have an original model and a decimated version of it. We may want to create a HLOD based on the visual error between these two models. To do so, we use the [algo.getVisualComparisonFootprint](/asset-transformer-sdk/2026.4/api/python/algo_functions.md#getvisualcomparisonfootprint) function.

![Viewpoints](/api/media?file=/asset-transformer-sdk/media/images/algo/visual_footprint/viewpoints.png)

It takes two sets of occurrences that will be visually compared. It will create `nb_viewpoints` viewpoints around the scene from where a visual difference footprint will be computed based on an image size controlled by the parameter `resolution`.

![Comparison steps](/api/media?file=/asset-transformer-sdk/media/images/algo/visual_footprint/comparison_steps.png)

On the image above, we see one of these viewpoints, where `A` is the original model and `B` is a decimated version of it. The "visual difference" is then computed and shown in `C`. Finally, we compute the maximum error between the two images based on this "visual difference", as shown in `D`. The maximum error of all viewpoints is returned.

We can use the result to create a HLOD with the right visual error.

![Zooming in HLODs](/api/media?file=/asset-transformer-sdk/media/images/algo/visual_footprint/zoom.gif)

See `createHLODChainUsingVisualFootprint.py` for an example usage:

```python title="Python"
import pxz


def create_hlod_chain(files: list[str], resolution: int = 512, nb_viewpoints: int = 256) -> int:
    """Creates a chain of hlods given an list of files to import.

    Parameters
    ----------
    files : list[str]
        The files to import. The first in the list is the "original" file. The others are low-quality version of it.
        The order of the LODs doesn't matter, they will be ordered in the HLOD chain based on their resemblance
        with the original model.
    resolution: int
        Size of the viewer to compute the footprint.
    nb_viewpoints: int
        Numbers of points generated around the bounding sphere of the scene to compute the footprints.

    Returns
    -------
    int
        The occurrence representing the root of the created hlods chain.
    """

    print(f"Creating a HLOD chain of {len(files)} files:")
    print("   - Importing files... ", end="")
    occurrences = []
    for filename in files:
        occurrences.append(pxz.io.importScene(filename))
    print("Done")

    print("   - Computing footprints... ", end="")
    footprints = [0]
    for occ in occurrences[1:]:
        # Visual comparison between the first occurrence, which is the real model, and the current lod of the loop
        footprints.append(pxz.algo.getVisualComparisonFootprint([occurrences[0]], [occ], resolution, nb_viewpoints))
    print("Done")

    # Sorting occurrences by footprints
    footprints, occurrences = zip(*sorted(zip(footprints, occurrences)))

    # Creating HLOD chain
    hlods = []
    for i in range(len(files)):
        hlod = pxz.scene.createOccurrence(f"HLOD {i:03}")
        pxz.scene.setParent(occurrences[i], hlod)
        cmp_hlod = pxz.scene.addComponent(hlod, pxz.scene.ComponentType.HLODComponent)
        pxz.core.setProperty(cmp_hlod, "Error", str(footprints[i]))
        hlods.append(hlod)

    for i in range(len(files) - 1):
        pxz.scene.setParent(hlods[i], hlods[i + 1])

    print("Chain creation is done!")
    return hlods[-1]


files = [
    "path_to_original_model.pxz",
    "path_to_heavily_decimated_version_of_it.obj",
    "path_to_slightly_decimated_version_of_it.glb",
    "path_to_more_decimated_version_of_it.fbx",
    # ...
    "path_to_impostor_version_of_it",
]

hlod_chain = create_hlod_chain(files)
pxz.scene.setParent(hlod_chain, pxz.scene.getRoot())

```