Simple Boolean Subtraction (2)

4. Transparency mapping

We are going to make a transparency map for the front face of the cube that will reveal the inside of the object to be subtracted. Don’t feel confused, you’ll understand soon what I’m talking about!

-         Make a new color, name it black, and another named white, like I did.
Notice that white is purely emissive, while black has no active component.

-         Apply black on the handmade object.

-         Turn to Point Edit, click the Face Select button and click the cube to select its front face. Apply white on it, and then click away to deselect and go back to Edit mode.

-         In front view, with the Arc Rotate button on, zoom in the image (drag with the middle mouse button, or Alt +Right button for two buttoned mice) to almost fill the workspace.

-         Click File->Render Preview…, enter 600 and 450 for Width and Height, check Image and Antialiased and hit OK.

-         Save the rendered image as .bmp file, like front.bmp. These last three steps are intended to make a good quality transparency map.

-         From now on, you aren’t allowed to change the relative position of the cube and the object, so be careful not to do it by accident! Save frequently!

-    Open the rendered image in a paint editor (no matter which paint editor you use) and select carefully the exact margins of the white square (the front face of the cube), as you see in the image below.



-    Copy and save the selection as a .bmp file, name it frontmap.bmp. This is your transparency map for the front face of the cube.

-         Make a new material, name it Frontface. In the Material Editor leave the default settings and click Textures, then in the Surface Texture Editor on the row with Trans.: click the “…”. In the Texture Selector click Load Texture, find frontmap.bmp, click Open, back in the Texture Selector select it, then click three times OK to close the Material Editor windows.


-         In front view go to Point Edit, Select Face mode, click the cube to select its front face and apply the newly created Frontface material. Hit the UV Texture tool, that will map the material containing the transparency information on the front face of the cube. Now click the Select arrow, click in the workspace to deselect, and then return to normal Edit mode.

-         To make sure you didn’t miss something till this point, click the object to select it and do Edit->Hide. Change to Ortho view and do a test render (in Front view you get nothing). You should obtain a cube with a hole on the front face, where the object was inserted, like in the image on the left.  If you are OK, click Edit->Show All to bring back the object.


5. Now we are ready to do the Boolean Subtraction.

-         Make a new material (call it Inside). Leave the default settings, excepting Specular: and Trans: where you enter 0 and check Two Sided. Click OK. This material does the trick to show the inside of an object, as it makes the exterior completely transparent, without even reflection.

-         Select your black object and apply the new Inside material. You are done!

In the workspace the Boolean won’t look as expected (because transparency works fine only in rendering), but make a render!  The rendered image is excellent as long as you don’t go towards 90 degrees away from the front view (image on the right).

Exhibit D

Exhibit D was done in the same way (download the project file -14KB), the only difference is that I had to map for transparency three faces of the cube (front, side, top). That means I had to repeat step 4 three times for the three views.
Again, the camera angle has to be in a certain limit to hide from viewing unwanted parts of the subtracted object, which are outside the cube. How to hide that unwanted parts to have no limitation for the viewing angle? An idea would be to cut them, if possible. 
That was possible for Exhibit E (download the project file - 22KB), where I had an extruded text as second Boolean operand. The first operand was a cube (a “wall”) with the thickness equaling the text’s extrusion length. 


I did the work in exactly the same way as for Exhibit A, but at step 4 I have applied the transparency mapped material to both the front and the opposite face of the cube simultaneously. Finally, I’ve deleted both ends of the extruded text (actually, only the capped faces) obtaining a “text-tunneled” Boolean.

Exhibit E

But what to do if we can’t cut conveniently the outside part of the subtracted object? Read about that in the next part!          

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