#CreativeFriday – Using Photoshop’s Cross Section tool to split a single mesh

Sometimes you will open a 3D model that is a single mesh into Photoshop, this might be due to the way that it was designed or the way that i was exported from a 3D package (STL files are single meshes).  For some work you might want to work on individual components of the model and break it down (e.g. painting, texturing etc. Multiple meshes will give you more control of the texture UV maps and the ability to paint or add colour at specific sections of the mesh). Inside Photoshop CC on the 3D layer there is a cross section tool, available on any scene’s Properties panel. This tool will allow you to split a mesh on the X,Y or Z axis on a straight plane. This tool will essentially create a new mesh with a new UV map for each.

The following example will walk you through how to split a mesh, or in other words, how to take a single mesh and divide it into many meshes.

Take this 3D model of a pair of sun glasses frames. It may be required that individual components like the arms and the front of the frames need enhancing in different ways (e.g. painting, texturing etc or even printing as separate parts, for assembly later).

Each 3D object has a 3D menu that will give you access to different elements of the model in Photoshop.

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Each model will have a UV map, UV maps can be opened by clicking on the individual element(s), under the diffuse layer on the  models Layer properties panel. In this case there is only 1 UV map. If the model does not have a UV map, then you can create on by selecting the menu option 3D / Generate UV’s.

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something similar to the the following 2D image will be displayed. This example shows the un-wrapped geometry . In this example, would be difficult to manage and enhance individual components. Ideally, a single UV map for each critical part might be a better way to work, it will also provide a higher resolution (especially when textures are involved, as you will see later on the body scan).

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To do this, take the single mesh and use Photoshop to be split it into three. Let us first take off the front of the frames on this model. The fastest way to is to position the model for the cross section, in this case, use the secondary view (marked yellow), and use the top view (it will be easier in this case to line everything using the top section, but other views are available if required as this depend on the model and how it needs to be split). Models can also be moved around on the main canvas by using the 3 icons (bottom left of the main canvas, just above the axis controler. This view can be moved into the main canvas, by clicking on the transfer icon (top right) within the secondary view (marked yellow). If this view is not available in your Photoshop instance, then it maybe turned off, to turn it on, navigate to the View menu item/ Show / 3D Secondary view and select it.

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once the new view has been loaded, navigate to the 3D menu panel / Scene (marked Yellow), then head over to the scene properties.

To see the cross section (marked red), check the ‘cross section’ option. The cross section will be shown on the main canvas using a plane by default. The visibility of the plane can be controlled using the opacity and visibility check boxes within this panel. The cross section can be moved on the X/Y and Z axis, as well as the ability to tilt on the X or Z axis (marked in purple below). Once the cross section is in the right place, the  ‘apply the cross section’ button (marked in red below) can be pressed to split the mesh.

 

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The new mesh is created (the front of the frames in this case), and the arms are hidden from view. The new mesh has been created in the 3D panel, and is automatically turned off from the display.

Note. Once the first cross section has been applied, I would recommend that the model parts are not moved, unless you would like to print or represent them as separate elements.

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In this example, I would like to have the arms as independent meshes as well, To do this, the front of the frames need to be moved to once of the sides of the new cross section, other wise it may get caught up in the cross section and be split as well. The Objects can be moved by clicking on the them and showing the cage, then using the arrow to move the object around the scene. The cross section can be turned on and orientated to which ever axis suits the cut, then placed into position using the options in the properties panel (marked yellow) of the 3D menu item / scene. The icon marked in green is used to switch the cross section view to the opposite side.

 

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Splitting the mesh(s) will create multiple diffuse maps under the 3D object in the Layers panel, under the diffuse section. This process should create a new UV for each element. However, if you need to change the resolution of the UV map, or re-create them, then it may be advisable to re-generate the UV’s (this option is available under the 3D menu / Generate UV’s).

The configuration dialog box that will be shown allows you to generate a different resolution textures, all the way to 4096 (the highest resolution). This size of resolution is the best for fixing textures, especially from 3D scans.
When checked :-

  • Merge Materials – Will merge materials on each mesh to be a single material
  • Preserve Appearance – If a material already exists on the mesh, it won’t be removed during the UV generation.

You will need to play with the options to get the best result from your files.

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Clicking on the appropriate element on the diffuse section of the 3D layer will show the new;y generated UV’s.

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As an example of what you may want to do at this stage, a simple example a gradient can be applied to the arm mesh only, on a separate layer in the Layers panel.

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Returning to the 3D model, the texture has been applied.

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An easier way to see the applied gradient is to select the Scene in the 3D menu, then choose the ‘Unlit Texture’ style of the surface properties (marked green), then if no geometry is visible, but you would like to see how it fits the mesh, then the ‘Lines’ can be turned on (marked orange).

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Splitting the mesh is also good when trying to work on a high resolution texture from a 3D body scan.

Take this torso scan, when the UV textures are opened (accessed by double clicking on the diffuse layer under the 3D layer in the Layers panel),

 

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you will see all the whole UV and the bitmap.

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If it’s needed to work on the high resolution textures, then it’s much better to work in each object as a separate mesh. This objective can be achieved by apply a cross section that isolates the both meshes.

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Doing this will get a much higher resolution UV / texture to work on.

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Now the new UV map has been opened it can be treated like any other 2D photograph or image. If there are issues with the texture (typically issues can be things like cracks or areas where the scanner did not reach), the tools like content aware patch, clone heal, spot heal etc, can be used to fix it (the same as a regular photograph). Once the fixed texture has been saved it will then be applied to the mesh. The important element to remember here is the the 2D layer can incorporate any activity that might be applied to a normal image, including, Smart Objects, Additional layers, Filters, Adjustment layers etc etc. And any new activity will be stored as non destructive texture and will not be rasterised (unless this is performed on purpose). This means that in the future the texture is fully editable (as long as the original file is saved as a PSD or a TIFF).

In another example of using cross sections, it may be required to print a cross section of this milk bottle

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Create a cross section as in the above examples then choose the scene from the 3D panel. To see the cross section, check ‘cross section’ on in the properties panel. Then move the cross section into position using the X,Y and Z modifiers. In the following example, the colour of the intersection has been changed to blue and the plane has been removed from the display. The blue lines of where the cross section is made can be seen on the main view.

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Once the print settings menu is selected from menu item 3D / 3D Print Settings, a real time render will be shown in a wire frame box (this resembles the 3D printers print chamber).

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Once the print button has been pressed, a view of the printed model will be displayed. At this point the 3D model will be a cross section and can be exported using the export button. The file that is created, will be suitable for the printer chosen in the 3D Printer Settings. In this example the MakerBot Replicator 2.

 

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We hope you have fun with cross sections in Photoshop

 

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