How to create an Alpha masked render of only your model.

[David Mac]

My thanks to William Manning for sparking off this idea. Hopefully some of you may find it useful.

I have evolved a technique which allows you to export an Alpha channel of just your model using IDX. It is a bit clunky, especially if your model is very complex, but it produces very accurate masking and should certainly do until something better comes along.

Like all instructions it looks very long and tedious but to do the actions is really quite quick and simple.

1) Render your model as required and save the render.

2) SAVE YOUR MODEL'S SKP FILE!!!! We are about to radically change it.

3) Select the entire model and fill it with 100% black colour.

(If your model has complex ingredients and components that may require ungrouping to change colours save the current camera position as a scene so you can move the model around to get at the tricky bits. When everything is black restore the camera position)

4) In IDX Lighting dialogue select Natural and move the slider fully to Dim.

5) In IDX Environment dialogue set user defined colours to 100% white for all four colours.

6) Render this with exactly the same quality settings you used to render your model. (This makes sure anti aliased edges are the same) Save the render.

This will give you an image which is a solid black mask of your model on a white background. You need to invert it and create a mask from it in Photoshop. I have made the following instructions backwardly compatible for any version of CS.

In Photoshop open both images (I shall call them Render and Mask).

In the Mask image:

1) Image > Adjustments > Invert

2) Select > All and Edit > Copy

In the Render image:

3) Double click Background in the Layers Palette to convert it to a layer.

4) Layer > Layer Mask > Reveal All

5) Alt Click the white layer mask icon in the Layers Palette to edit it directly. (The image will turn white)

6) Edit > Paste (Your mask image will be pasted into the layer mask and will show on the screen)

7) Click in the Image icon in the Layers Palette to deselect the mask and re-select the image.

Bingo!! Your model render now is surrounded by transparency.

If you try this you will see it really isn't as complicated as it looks ……..   

......... and it works ..... 

Good luck and have fun.

David Mac

[Tony]

Maybe move this topic to 'Tips and Tricks'!
It's a great idea!

You mentioned in step 3 that you need to explode your model.
A faster way is to select all materials in the materials dialog of SU and delete them and then make a solid black material and apply it to all objects. This way you don't need to explode everything and this saves a lot of time in large models.

Verry nice piece of explanation though!

Tony

[David Mac]

Thanks for that suggestion Tony. That really makes the whole thing very quick and easy. I am frankly a bit of an amateur in Sketchup. I am really a Photoshopper who uses Sketchup to produce 'elements' to combine with photography to create photographic illusions. Hence the need for Alpha channels.

[wmanning]

This is a really creative solution.  Thanks for sharing it.

And as Tony suggested, I will move it to the Tips and Tricks section.

[honoluludesktop]

David, Thanks for the tip:-)