Generating lightmap UVs

You can generate your own lightmap UVs for a Mesh, or Unity can automatically generate them.

Generating your own lightmap UVs

You can author your own lightmap UVs in the content creation software of your choice. You must place lightmap UVs in the second UV channel of your mesh.

A good UV set for lightmaps should adhere to the following rules:

It should be within the [0,1] x [0,1] UV space.
It should have a wide enough margin between individual charts. For more information, see UV overlap feedback.
It should not have any overlapping faces.
There should be a low difference between the angles in the UV and the angles in the original geometry.
There should be a low difference between the relative scale of triangles in the UV and the relative scale of the triangles in the original geometry), unless you want some areas to have a higher lightmap resolution.

Automatically generating lightmap UVs for a Model

You can tell Unity to automatically generate lightmap UVs for a Model, using the Model Import Settings.

Select the Model in your Project view. Unity opens the Model Import Settings in the Inspector.
In the Model Import Settings, navigate to the Model tab, and then the Geometry section.
Tick the Generate Lightmap UVs checkbox. The Lightmap UVs settings section appears below the Generate Lightmap UVs checkbox.
Optional: Configure the settings in the Lightmap UVs settings section. See Settings for more information.
Click the Apply button. Unity generates lightmap UVs into the Mesh.uv2 channel.

Lightmap UVs settings

These are the settings that appear in the Model tab of the Model Import Settings, when you enable Generate Lightmap UVs.

Propiedad:		Función:
Hard Angle		The angle between neighboring triangles (in degrees) after which Unity considers it a hard edge and creates a seam. You can set this to a value between 0 and 180. This is set to 88 degrees by default. If you set this to 180 degrees, Unity considers all edges smooth, which is realistic for organic models. The default value (88 degrees) is realistic for mechanical models.
Angle Error		The maximum possible deviation of UV angles from the angles in the source geometry (as a percentage from 0–100). This is set to 8% by default. This controls how different the triangles in UV space can be to the triangles in the original geometry. Usually this should be fairly low, to avoid artifacts when applying the lightmap.
Area Error		The maximum possible deviation of UV areas from the areas in the source geometry (as a percentage from 0–100). This is set to 15% by default. This controls how well Unity preserves the relative triangle areas. Increasing the value allows you to create fewer charts. However, increasing the value can change the resolution of the triangles, so make sure the resulting distortion does not deteriorate the lightmap quality.
Margin Method		Whether you specify the Pack Margin manually, or whether Unity automatically calculates it.
	Manual	You specify the Pack Margin manually.
	Calculate	Based on expected lightmap resolution and object scale, Unity calculates a Pack Margin just large enough to avoid UV overlaps.
Pack Margin		The margin between neighboring charts (in pixels), assuming the Mesh takes up the entire 1024x1024 lightmap. You can set this to a value between 1 and 64. A larger value increases the margin, but also increases the amount of space the chart needs. This is set to 4 pixels by default. For more information, see Pack Margin. This property is only visible when Margin Method is set to Manual.
Min Lightmap Resolution		The minimum lightmap resolution (in texels per unit) of the MeshRenderers that use this Mesh, across all Scenes. The lightmap resolution of a MeshRenderer is a combination of the MeshRenderer’s `Scale in Lightmap` property, and the `Lightmap Resolution` property of the Lighting Settings Asset of the Scene it appears in. For more information, see Min Lightmap Resolution and Min Object Scale. Unity uses this information to calculate pack margin. This property is only visible when Margin Method is set to Calculate.
Min Object Scale		The minimum transform scale that of the GameObjects that use this Mesh, across all Scenes. For more information, see Min Lightmap Resolution and Min Object Scale. Unity uses this information to calculate pack margin. This property is only visible when Margin Method is set to Calculate.

Pack Margin

To allow filtering, the lightmap contains lighting information in texels near the chart border, so always include some margin between charts to avoid light bleeding when applying the lightmap.

The lightmap resolution defines the texel resolution of your lightmaps. Lightmappers dilate some chart texels in the lightmap to avoid black edges, so the UV charts of your Mesh need to be at least two full texels apart from each other to avoid light bleeding. Use the Pack Margin setting to ensure you have enough margin between the UV charts of your geometry.

In lightmap UV space, the padding between charts need to be at least two full texels in order to avoid UV overlapping and accidental light bleeding. In this image, the black space represents the space between charts.

Min Lightmap Resolution and Min Object Scale

Placing UV charts too close together may cause cross-chart texel bleeding in the final lightmap. Placing charts too far away from each other wastes memory. The ideal packing margin of an object depends on how many lightmap texels are allotted to it.

The number of texels that Unity uses for a MeshRenderer depends on the MeshRenderer’s lightmap resolution and transform scale. To calculate a good margin, Unity needs to know the expected minimum values for these properties.

A MeshRenderer’s lightmap resolution is a combination of the Lightmap Resolution property of the Lighting Settings Asset for the Scene that the MeshRenderer appears in, and the Scale in Lightmap property of the MeshRenderer. Note that this means that the same MeshRenderer can have a different lightmap resolution in different Scenes.

You can use the Inspector to view the lightmap resolution of a MeshRenderer in a given Scene:

In the Scene view or Hierarchy, select the GameObject that has the MeshRenderer component.
In the Inspector, navigate to the MeshRenderer component’s Lightmapping section.
Open the Baked Lightmap fold-out. Unity displays the MeshRenderer’s lightmap resolution in the baked lightmap with the label Lightmap Resolution, and its transform scale in the baked lightmap with the label Lightmap Object Scale.

Angle distortion

The following screenshots demonstrate equal resolution, but with different UVs. The first image has a high Angle Error, and the result contains unintended artifacts. The second image has the default Angle Error (8%). In Meshes with more triangles, angle distortion can significantly distort the shape.

Area distortion

In the image below, two spotlights with the same parameters light the sides of a cylinder. The right-hand side of the cylinder has a higher Area Error value, which distorts the triangles and leads to a lower resolution, creating artifacts in the light.