It doesn't work in orthographic mode!
More info help.autodesk.com/view/3DSMAX/2024/ENU/?guid=GUID-F5518487-A253-44F0-A8D9-9CB537A8F392
Select an object to paint on.
The object need not have a material assigned, but it must have mapping coordinates. If not, Viewport Canvas will not work with it.
Open the Materials menu and on the Create/Edit Maps panel, click Viewport Canvas.
The Viewport Canvas dialog opens. If necessary, position it so doesn’t obscure your object.
Tip: Customize the dialog by adjusting the position and settings as you like and then make the changes permanent with Save Current Settings As Default. You can also dock the dialog to the left or right side of the program window: Drag it into position or right-click the dialog title bar and choose Dock Left or Right.
Click (Paint) or any other tool.
If the object does not have a material assigned, the Assign Material dialog opens, giving you the option to assign a Standard material, browse for a material using the Material/Map Browser, or cancel. For details on these options, see Assign Material Dialog. To get started quickly, choose Assign Standard Material.
After you assign a material, or if the object already has a material, a list of available map types opens. The list also shows any assigned maps.
Choose a map type from the list. For example, to paint the object’s coloring with a Standard material, choose the Diffuse Color map.
If the map type does not have a bitmap image file assigned, choosing the map type opens the Create Texture dialog, where you can use the browse button () to specify a new or existing bitmap file. Otherwise Viewport Canvas uses the current bitmap.
Note: If you use Create Texture to specify an existing bitmap file, the file is overwritten. Also, if the assigned map type is not a bitmap, you’re prompted to choose a component of the map, typically a color, for which to create a bitmap. Viewport Canvas requires a bitmap file to work with.
After setting up the material and map, if necessary, the Layers dialog opens and the mouse cursor becomes the icon for the active tool when over the active viewport, such as a circle for the Paint and Erase tools. When using the Paint tool and the cursor is over the object to be painted, the paint image is visible within the brush icon.
The background layer is now active and ready for painting on.
Choose a color or brush image and/or mask and paint on the object surface, if you wish.
However, you might want to paint in a new layer instead; see the following tip:
Tip: As the background (default) layer cannot be erased, it is highly recommended that you always paint into a layer other than the background.
On the Layers dialog, click (Add New Layer).
A new layer is added to the painting and becomes the active layer.
Drag over the object surface to paint.
While painting, you can switch tools, colors, and brush images; add, delete, and switch layers, change the layer order, and specify filters and how layers interact; and navigate the viewport to paint different parts of the object.
To exit painting mode, right-click in the active viewport or click the Paint button again.
If you’ve added one or more layers to a single-layer bitmap, the Save Texture Layers dialog opens, prompting you to specify what to do with your painting. These options are detailed here.
To preserve the layers in the current map, choose “Save as PSD and replace texture in material,” then use the file dialog that opens to specify the PSD file to save. This file becomes the new map in the object’s material.
You can return to editing that map by activating any painting tool, or, to edit a different map in the material, choose it from the Options rollout.