What makes for a good full colour 3D printing file?
I have often wrote about what ‘full colour 3Dprinting’ really means – http://www.eurekamagazine.co.uk/design-engineering-blogs/guest-blog-what-is-full-colour-3d-printing-and-why-does-it-matter/149123/?platform=hootsuite
And hope at this stage there is good understanding of the concept of ‘True colour’ and the value of colour – as seen in our pics below!
Next lesson ‘what makes for a good full colour 3D printing file?
When we launched our first full colour printer, the Mcor IRIS in 2012 we realised that the industry was in its infancy with regard to the quality of full colour files. It was challenging to even get our hands on printable colour 3D files!
This of course is improving all the time but let’s have a look at what we look for in a full colour 3D file.
The polygon/facet count relates to 3D quality in the same way pixels relate to 2D quality. Figure 2 shows an example of a typical bust created for printing using a scanner. In this image both the colour data and geometric data is enabled.
Figure 2 : Example of typical part for 3D printing
In Figure 3 the colour data was disabled and the geometric data can be seen represented as the polygon mesh. Each of the black triangles is a polygon. Looking closer at the mesh, Figure 4 shows that the size of these polygons can vary as the geometry gets more complex a smaller polygon is used. Refining the mesh at local areas where it is needed rather than having the entire mesh made of very small polygons allows a higher quality on small details while keeping the polycount low in order to reduce file size.
Figure 4 : Mesh refined at detailed areas
The higher the polycount the more computing power is required to print the model. In order to keep the resolution to a minimum the mesh should contain no edges with a length below 100 microns (0.1mm). Below this size no difference can be seen in the model due to the size of each layer of paper. As a general rule of thumb a polycount should be less than 1million for a very large (≥750cc) or complex parts or below 250,000 for smaller parts (≤750cc) or parts with straighter walls.
The geometry of the part defines the polycount in that a simple shape such as a box only needs a low number of polygons (16) to define its shape as opposed to a curved surfaces such as facial features which require a finer mesh
The polycount can be kept low in high resolution scans by using image maps as opposed to per vertex colour. This is because a polygon can only contain one solid colour using per vertex colour but many different colours if a texture maps is applied.
The full colour file types compatible with our software are VRML, OBJ, and DAE. Some file formats can include textures directly in the file (e.g. VRML, and DAE), while others store the texture in a separate image-file and map-file (this is the case for obj).
In the latter case (obj-file), it’s important to remember that an obj file alone only has the information about the model’s shape and volume. It’s a mesh without any texture information.
Texture maps, or UV maps, allow a high level of colour detail while having a lower polycount. Per-Vertex colour requires a higher number of polygons to define shades as each face can only be assigned one colour. An example of a part with simple per vertex colour is shown in Table 1 Polygon count vs Colour Quality.
|Table 1 Polygon count vs Colour Quality|
Figure 5 is a simple part with 16 polygons to define the shape. With a solid colour the part can still be defined with a small number of polygons.
Figure 6 shows what the same part would look like if we applied an image while retaining the low polygon count.
Figure 7 shows what the same part would look like if a UV Map was used.
Figure 8 shows the number of polygons that would be required to retain the same level of colour detail from an image applied per vertex, such as the part found in Figure 9.
Figure 10, again shows what the part would look like if a UV Map were to be used, this time on the cube with a higher number of polygons. It can be seen from this table that the most efficient solution to achieve a high colour resolution while maintaining the lowest possible polygon count is to use a UV Map i.e. Figure 7.
Using UV maps or texture maps allow the user to manipulate the colour easily to achieve a desired colouring if for example a scan is too dark or the user requires different coloured clothing etc.
So as you can see achieving high quality colour in 3D is a tricky business so hope this blog helped you to know what to look for next time you need to print in full colour!