Using Corona Decals in your renderings

Editor's note - thanks to kindness of Francesco Legrenzi we can share with you an excerpt from Corona: The Complete Guide book. The book analyzesall the parameters of Corona Renderer, with clear and detailed examples using more than 100 files for 3ds Max (2014-2021). Corona: The Complete Guide is a unique resource, extremely technical and professional. You will learn to use Corona Renderer like a true expert, discovering details and tricks known to few.

The Corona Decal object (CDecal) allows you to project a texture on an object without the target mesh needing valid UV mapping coordinates. The Corona Decal projected texture is loaded into an adjustable “cuboid” (gizmo), which controls the decal size and projection limits.

Open the CoronaDecal.max and start the Corona Interactive Rendering (Fig. 01).

Fig. 01 - CoronaDecal.max. Corona Decal is textured using a CoronaPhysicalMtl with two textures in the "Base color" and "Opacity" channel.
Corona Decal is displayed as a simple box, with the top face textured by the decal that is project on the receiving geometries (targets). Click on the Corona Decal icon in the Corona 8 toolbar, or go to Create TAB → Geometry → Corona → CDecal (orange arrows) to create a Corona Decal object. Pressing the CTRL key while dragging the mouse will start the creation from the geometric center of the Corona Decal object.

The pivot of the gizmo is 30cm from the top face of Corona Decal (when the total height is 100cm) because Pivot offset is set to 0.3 by default (on a scale of 0 to 1). This setting makes selecting the Corona Decal object in the viewport easier by avoiding overlap with the target objects (in this case, Floor). Think about the Corona Decal gizmo in a three-dimensional way, where the decal is projected on all intersecting geometries. According to this principle, both Floor and Teapot (purple arrows) were textured with the yellow graffiti.

Now disable the CDecal_Start layer and enable CDecal_Samples: click Render. For a complete understanding of Corona Decal we need to analyse both the render (Fig. 02)

and the orthogonal views (Fig. 03). Screenshot of the viewport when the CDecal_Samples layer is enabled. Note: the word VICTOR appears in front of the yellow graffiti, but this is a 3ds Max display issue.

We start with the cyan Corona Decal (vertically oriented), with part of its volume above the wall. Since half of the Corona Decal intersects the wall, the graffiti letter M is projected about 50cm on the top of the target object (cyan arrow). Let’s continue with the yellow and magenta Corona Decal objects. These two objects are also vertical, with the yellow graffiti covering the word VICTOR. In the render, the letter R is not visible: the geometric position of the gizmo defines who overlaps what.

The next example shows the sum of the two previous systems. The blue decal is placed under the red gizmo, with the latter penetrating the barrels (red arrow in Fig. 03): the red decal colors the green cylinder for a few centimetres from the base (green arrows in Fig. 02).

Looking at the scene closely, the yellow Corona Decal is deeper than the wall. We generate a new render, using the Corona_Camera_Back camera: the graffiti is projected on the back side of the wall.

Corona Decal Parameters

For the next parameters, disable the CDecal_Samples layer and enable CDecal. A vertical Corona Decal appears, with a CoronaPhysicalMtl applied. We loaded two maps: one for the Base color channel and one for the Opacity and Displacement channels.  On – enables or disables the texture projection of the Corona Decal object during rendering.

0 object exluded… – press this button to display the 3ds Max Exclude/Include window. We can now define which objects Corona Decal uses as a target objects by selecting the geometries in the left window and clicking the right double arrow “>>” button . We can also add an object in the list by selecting it in the viewport and clicking on the “+” button, without opening the Exclude/Include window. When we right‑click on the “+” icon, three actions can be performed: delete all objects in the list, select the objects in the list, or add the selected objects to the list.

From material – selects this option when an opacity map is available in shader applied to Corona Decal (a black and white map in CoronaPhysicalMtl in this example). Corona Decal is transparent where the opacity color is black.

Map – applies a different opacity map than the material assigned to the Corona Decal object. Mask generated with the "Map" option.

Replace base displacement – when the target object has the displacement effect enabled (the wall in this case), turning on Replace base displacement will replace this effect with the displacement of the shader assigned to Corona Decal. Vice versa, the Corona Decal displacement will be added to the displacement of the target object.

3D bricks of the target Wall object are generated with Displacement through shader. The material of CDecal_Graffiti also has a map in the Displacement channel. When Replace base displacement is disabled, the displacement effect of the Corona Decal material is added to the target shader. As a result, both displacements are visible. On the contrary, only the displacement of the Corona Decal will be taken into account.

Replace base bump – when the target object has the bump effect enabled (the wall in this case), turning on Replace base bump will replace this effect with the bump of the shader assigned to Corona Decal. Vice versa, the Corona Decal displacement will be added to the bump of the target object.

We end the chapter on Corona Decal with a comparison of two renders. In the first image I used two Corona Decal objects and the white Wall material for the wall. In the second image I disabled the CDecals layer and applied the WALL_Composite_Version material to the wall, enabling the UVW Map_Graffiti and UVW Map_Dirt modifiers.

The renders are very similar, but the complex setup of the WALL_Composite_Version material, the need for two extra UWV modifiers, the changes of Map Channels at shader and geometric levels, and the lack of flexibility of this map‑based system, make Corona Decal a fast, reliable, and extremely effective tool.

For an in-depth study on Corona Renderer, get a copy of Corona: THE COMPLETE GUIDE by Francesco Legrenzi.