Alpha Channel

An Alpha Channel represents information added to a raster image to indicate where it is transparent. Beginning with version 5, SketchUp supports transparent PNG alpha channels.

Many paint programs like Photoshop and Photoshop Elements also make it easy to add alpha channels. However, Windows Paint, and several other paint program do not have this capability. The RpTools Alpha Channel wizard (a SketchUp plug-in) makes it easy to add an alpha channel to an existing raster image (saved on disk), by indicating a color in the existing image which is supposed to be transparent.


Image without and with an alpha channel

The first image on the right is an image of a speaker with arcs to represent the sound.

The second image show two versions of this image imported into SketchUp. The first version is imported normally, and the white in the image appears as white in SketchUp. The second version was imported from an alpha channel image where the white was declared to be transparent. Notice that in the second image, the area that was white is now transparent when loaded into SketchUp.

Landscape with alpha channel images instead of using geometry-heavy models

Images with an alpha channel can be used as Face Me components instead of weighting down the model by adding a bunch of geometry-laden models for landscaping elements. Tutorial.

Display Quality

SketchUp has three settings for the display quality of Alpha Transparent images.
If your alpha image has discoloration at the edges, set the quality to Medium or Nicer.

RpTools Alpha Images

RpTools has two features to help with images.

Save model as alpha image

This feature will save an image of the SketchUp model with the background of the image saved as an a alpha channel.
(SketchUp has this feature for the Mac version, but not for the Windows version.)

Add Alpha Channel to existing image

This is a wizard to convert a color, or range of colors in an existing image into an alpha channel and save it as a new .PNG file.

Photoshop Tutorials

There are several ways to remove unwanted background color in a paint program. More tutorials than the two linked below can be found by searching the internet for something like “Photoshop alpha transparency.”

  • Alpha Channel Primer Creating a simple alpha channel mask
  • Masking out difficult images Good, detailed steps on adding an alpha channel mask to remove unwanted background color.
  • Design with Alpha Channel Very nice video
  • Removing the background Tutorial
  • Transparency Photoshop action – Free (This will not run in Photoshop Elements. Most downloads like this from the Photoshop Exchange are free. Other catagories like paterns, gradients, brushes, shapes and styles will work in Photoshop Elements.)
  • Filter Plug-ins are an automated method to adding transparency. The filters work in programs that support Photoshop plug-ins (Photoshop Elements, Paintshop Pro, Corel Draw, Illustrator.)
  • Transparency filter for black and white transparency from Trimoon’s Blog – Free
  • Transparency Editor with several adjustment levels from Power Retouche – Fee

Tip: In Photoshop and Photoshop Elements do the original work at whatever resolution or desirable detail level for optimum results. To reduce the impact of importing overly large images into SketchUp, try out the File > Save for Web option to save the material for importing into SketchUp. Save for Web has a lot of options to optimize a new material file without altering the original image file. It is an easy way to minimize SketchUp material file size when super, high quality detail is not needed. And do not forget to crop the image to the smallest size needed.

See Also