Transparency Considerations for STL, VRML & OBJ Files

In previous tutorials in this series, we explained the capabilities of STL, VRML, and OBJ files for 3D printing. In this tutorial, we will explain how to optimize transparency for these three files types.

  1. Step 1: How GrabCAD Print prints opaque vs transparent parts


    For STL models printed WITHOUT transparency, GrabCAD Print by default prints an opaque part with whatever material assignments you select.

    For VRML/OBJ models designed WITHOUT transparency, GrabCAD Print applies the following material assignments during printing:

    1.      A mixture of all resins for the core of the model, to try and ensure uniform material consumption and print head usage.

    2.      4 mm of VeroPureWhite as an initial coating, to provide a good, solid backdrop for the model’s texture data. The same way you would print on a white piece of paper when printing in 2D.

    3.      Finally, GrabCAD Print prints the texture assignment data to the top 1 mm of each body in the model. 


    When printing STL models WITH transparency, GrabCAD Print enables users to assign transparency as per their requirements by making material assignments directly in the settings menu. You can read more about it in our tutorial about transparency.

    When printing VRML/OBJ models designed WITH transparency, GrabCAD Print prints the model’s core with clear material, and then assigns color and transparency data from the file to the outer 1 mm only of the model. 

    Note: This is true even for models with minimal amounts of transparency assigned to the texture. Even if the data from the texture file attached to the VRML or OBJ contains just one pixel designated as clear, GrabCAD Print will print the entire core of the model with clear material.

    Because of the above explanation, transparency effects may vary based on geometry, GrabCAD setting selection, part orientation and/or model type. Depending on your selection, the transparency might be less uniform and/or noticeable.


    Below we have two printed swatches of 10 squares, each 5-mm thick and with increasing levels of transparency applied in increments of 10%.

    The top swatch is an assembly of STL file, with the varying levels of transparency applied in GrabCAD Print using the transparency slider.

    The bottom swatch is a VRML file, with the increasing levels of transparency applied in Adobe® Photoshop®, and then printed in GrabCAD Print:

    As you can see, 50% opacity applied to the STL assembly looks significantly more opaque than the same assignment made to a VRML/OBJ file. This is because the STL files apply the color/transparency assignment throughout the whole body of the part, whereas the VRML/OBJ file only assigns color/transparency to the outer 1 mm of the part.

    Note: In some geometries, this can also cause certain irregularities in the transparency’s appearance, such as a frame effect in the VRML/OBJ files. This will be more noticeable on edges and corners

    To avoid this frame effect for single color models or shells, override the texture assignment by applying a color from the color picker window, and adjust the transparency as required. (cheak out this video for a step by step of this feature)

    Some tips for applying transparency to STL, VRML, and OBJ

    For STLs, you can adjust the model’s appearance by using the core+coating method to assign transparency, as discussed in our tutorial about advanced transparency. This can be adjusted with the color picker and transparency slider until the desired affect is achieved, depending on geometry, color and material assignments, etc. 

    The image above shows the differnt ways that color and transparency can be selected from within GrabCAD. You can read the more about the different settings, what they do, and how to apply them here.

    If you are using a VRML/OBJ file that contains an image texture with transparency, you can use Photoshop’s opacity slider to adjust the image texture’s transparency assignment. Then, save the model as a VRML/OBJ file and export to GrabCAD Print.

    Be aware that Photoshop assigns opacity rather than transparency. So, for example, when using Photoshop to achieve 10% transparency, you would assign 90% opacity.  

    And that concludes working with transparency for these files types. To read more about transparency, check out the tutorials on transparency and advanced transparency (also mentioned above).

    We hope this tutorial encourages you to assign transparency for whatever file type you are using. Reach out if you have any questions or comments, and be sure to read about considerations for STL printing for PolyJet in tutorial 1 of this series.