The Corona Decal object is available starting from Corona 8. It provides a way to add complex details to scenes without using any complicated material setup or UV mapping work.
It is an object that projects a 2D texture onto any geometry, but you can also control other surface properties with it, like diffuse color, bump, IOR, roughness, and displacement by applying a material to it.
The stickers and the metal plate at the back of this bus were created using Corona Decal objects.
Where to find the Corona Decal?
You can find the Corona Decal under the Corona menu:
How to use Corona Decals
To start using decals in your scene project, simply create a Corona Decal object from the Corona menu.
The second thing you will need is an image to project; to do this, create a Corona Material (it can be any Corona Material type) and apply it to the Corona Decal object.
Since decals are like stickers, it usually makes sense to use a texture map in the opacity channel to give it a cut-out shape; otherwise, decals will look like solid squares, which might look odd.
Once everything is ready, place the Corona Decal object at the right place in your scene, then make sure it is intersecting with the desired geometry; otherwise, the decal won't be correctly projected onto the geometry.
The Corona Decal objects can project textures onto any geometry object.
Corona Decal objects need to intersect with other objects to project textures onto them.
The opacity channel can be used to define the shape of the decal, cut holes in it, or create transparent gradients showing the underlying surface.
Note: You can change the arrangement of the Corona Decal objects when using more than one in the same area by simply moving them towards or away from the geometry.
The Corona Decals can overlap.
To change the arranging order, simply move the Corona Decal object near the geometry object or away from it.
The source for the decal's mask
The Corona Decal can use either the opacity channel from a Corona Material (Physical or Legacy) or a texture map to mask the material that is applied to the decal object.
If you choose to create the decal's mask directly from a texture map, please consider that you can use either a file that contains an alpha channel (PNG, TIF, PSD, etc.) or a simple B&W/Grayscale JPEG file to mask out the material that is using the Corona Decal. This will work just like any material's opacity channel.
Example using the "From map" option:
|The resulting image after using a texture map to mask out the decal's material.||The material is not using the Opacity channel. The Corona Decal object is using its own mask.|
The Corona Decal size can be manually adjusted using the Scale tool, or by adjusting the width and height in the decal's properties.
It can be difficult to manually set the correct proportions for the decal size. We can use the same proportions as the texture map used for the decal by using the "Fit to Bitmap..." button. By clicking this button, a pop-up window will show and you'll need to select the same texture map that you're using for the decal.
Using bump or displacement with Corona Decals
The Corona Decal object allows you to combine its bump or displacement with the underlying surface or to replace them completely.
If your Corona Decal's material uses a shader or texture map to represent bump, then the Corona Decal will automatically mix the underlying surface's bump with the one from the material of the Corona Decal:
The base surface has its own bump. No decal yet.
If the decal's material has no bump at all, the underlying surface bump will show.
To mix the underlying surface and decal's bump, make sure the "Replace base bump" checkbox is disabled.
If you add a bump texture map to the decal's material, it will mix with the underlying surface material.
Alternatively, you can use the decal's bump texture map to completely replace the visible bump over the area where the Corona Decal object is located:
The underlying surface's bump can be ignored by enabling the "Replace base bump" checkbox.
To completely override the underlying surface bump then enable the "Replace base bump" checkbox.
The same behavior as mentioned above will occur when using displacement properties on a Corona Decal - you can combine it or completely override it:
The base surface has its own displacement and bump. No decal yet.
A simple decal using a material that has no bump or displacement at all, the result:
the underlying surface bump and displacement will show.
To mix either the bump or a displacement from the underlying surface and the Corona Decal's material, keep both checkboxes disabled:
If the "Replace base displacement" checkbox is disabled, the displacement from both objects will mix up.
When the decal's material has displacement, it can be mixed with the underlying surface's material.
You can decide if you'll be combining the displacement texture map from the decal object and the bump map from the underlying surface as follows:
The underlying surface's displacement can be ignored by enabling the "Replace base displacement" checkbox.
In this case, the decal's displacement is overriding the underlying surface displacement, but the bump texture is combined from both objects.
You can also enable both checkboxes to replace both the bump and the displacement textures of underlying objects:
Resulting image after enabling the "Replace base displacement" and "Replace bump" checkboxes.
In this case, a solid color was used for displacement and bump.
Note: You can also mix different bump or displacement textures by using a Corona Layered Material.
Projection properties of the Corona Decal
Generally speaking, the Corona Decal can be used over any surface; however, you may find a scenario where you need to use the decals over organic (curved) surfaces. For cases like this, you might want to use the "Projection" properties of the decal.
The Corona Decal will be projected onto surfaces that intersect with its bounding box. If we move the Decal object away from the surface, the objects won't be intersecting, resulting in no projection at all.
|The Corona Decal is visible because it is intersecting with the object.||As seen in the viewport, the Corona Decal is intersecting the object.|
|The Corona Decal is not visible because it is not intersecting with the object.||As seen in the viewport, the Corona Decal is not intersecting with the object.|
You can increase or reduce the Corona Decal's bounding box by adjusting the "Depth limit" value so the decal can intersect objects even if it's not near the surface.
The "Max angle limit" value defines the maximum angle at which the projection occurs. If the angle between the Corona Decal and the surface is lower than the specified then the surface will receive the projection.
As you can see in the following images, some clipping will occur on the decal if you modify the "Max Angle limit" value.
|The Max Angle limit is set to 180° (default value)||The Max Angle limit is set to 60°|
|The Max Angle limit is set to 45°||The Max Angle limit is set to 30°|
On the other hand, the "Max Angle Blur" value can be used to avoid sharp clippings when the "Max Angle limit" is being used.
This value will blur the edge of the clipping creating a soft transition between the areas where the Decal is projected and where it isn't.
|The "Max Angle limit" is set to 35° and the "Max Angle Blur" is set to 0%||The "Max Angle limit" is set to 35° and the "Max Angle Blur" is set to 5%|
|The "Max Angle limit" is set to 35° and the "Max Angle Blur" is set to 10%||The "Max Angle limit" is set to 35° and the "Max Angle Blur" is set to 15%|
It is possible to use the Corona Decal with native Mograph's cloner object to scatter decals.
Here's a simple example of this using a metal material with a texture of the head of a screw for the displacement.
Resulting image after scattering Corona Decal objects.
|Simple geometry is being used for this example.||A native Mograph's cloner is used to scatter Corona Decals.|
Material setup for the head of the screw.
You can also scatter decals using the new Chaos Scatter. In this example, the materials for the different Corona Decal objects are simple metallic materials using normal maps:
This is the final result after scattering Corona Decal objects using the new Chaos Scatter.
|This is the material setup for one of the materials used for the Corona Decals.||
This is how the scene was assembled.
The Chaos Scatter was used to distribute all the Corona Decals.
Here are some additional examples of how Corona Decal can be used:
A. For this example, the Corona Decal objects are used to show the golden engraving over the cover of the book (the materials are using displacement texture maps):
B. For this other example, a different metallic material was used to show the engraving over the cover of the book:
As you can see, the decals are placed around the book's geometry.
C. For the following example, a material with a damaged wall texture map is used and the "replace base displacement" and "replace base bump" checkboxes are enabled:
No decal is present in the scene.
The Corona Decal has been placed, intersecting the wall's geometry.
Cinema 4D's viewport shows how the Corona Decal object looks placed over the wall.
D. The following example is using the Self-illumination channel in the decal material:
The resulting image after enabling the Self-illumination channel in the decal material.
The object's placement in the C4D viewport.
The material setup for the decal.