STEP and IGES in Solidly

Solidly opens STEP (.step, .stp) and IGES (.iges, .igs) files. Both formats store a part as exact surface geometry, such as planes, cylinders, and trimmed NURBS surfaces. To draw a part, Solidly reads that geometry with the Open CASCADE kernel and converts each surface into triangles, at a level of detail set by the file's complexity.

While a STEP or IGES file is being read, the viewport's loading overlay reports the progress of each phase. A large assembly can take several seconds. Selecting another model before the read finishes stops it.

STEP and IGES are viewing formats

Solidly does not write to STEP or IGES files. The editing controls available for glTF and FBX are absent, the Node tab is read only, and Save and Save As are disabled.

The changes Solidly allows affect only the view of the model: which parts are shown, the complexity level, the model's orientation through Edit ▸ Rotate Model, and the section and exploded views. None of these changes marks the file as edited, and none of them is written to it.

Where a model is described

Parts in these formats carry display colours and no material objects, so STEP and IGES files have no Materials tab. They have no Resources tab either.

The tree

The sidebar's Hierarchy mode is named after what the file contains. A STEP file's tree is labelled Assembly, even when the file holds a single part, and so is an IGES file that defines assemblies. An IGES file without assembly structure is labelled Bodies, because its surfaces are joined into separate solids and surface bodies; see Joined IGES surfaces.

A part the file leaves unnamed is listed as Part 1, Part 2, and so on. Each row has an eye button that hides or shows it. Hiding a part changes only the view, so it records no undo step and does not mark the file as edited.

The Info tab

The Summary for these formats is described in The Info tab. The sections below it are specific to STEP and IGES.

Exporters often fill header fields they have no value for with placeholders such as "Unknown". Those placeholders are left out, so the File section lists only what the file actually records.

Volume is measured from closed solids. A part that is an open surface contributes no volume, and a file with no closed solids shows a dash in the Volume row.

A STEP file that stores its names in a regional encoding, such as GBK, Big5, or Shift JIS, instead of UTF-8 adds a Text Encoding row naming the encoding Solidly detected. The encoding is inferred from the names in the file, so a file with only one or two short names can occasionally be read in the wrong encoding.

The Node tab

Selecting a row describes it in the Node tab, which is read only. It shows three cards:

Joined IGES surfaces

Most programs write IGES files with each trimmed surface as a separate entity and nothing joining the surfaces together. For an IGES file without assembly structure, Solidly prepares the geometry the way CAD import usually does:

• Faces are joined along their shared edges, within the tolerance the file states.
• Each closed shell becomes a solid, and open shells remain surface bodies.
• Geometry the author marked as hidden stays hidden.
• Face colours are kept.
• Free curves are kept in a part named Curves, which is drawn only as edge lines.

The resulting bodies are listed as Solid 1, Solid 2, and so on, and as Surface Body 1 and onward for open shells. The Info tab's Surfaces section reports how many faces were joined and how many solids were formed. An IGES file that already contains solids, or that defines subfigure assemblies, keeps its own structure.

The toolbar

For a STEP or IGES file, the left of the toolbar holds three buttons in place of the transform tools, and a fourth button in a group of its own beside them.

The buttons are shown in every sidebar mode, are hidden in Photo Mode, and become available once the model has finished loading.

The projection button starts from the projection set in Settings ▸ Viewport ▸ Camera and does not change that setting. Selecting another model, or changing the setting, returns the button to the setting's value.

The exploded view cannot be open together with the measuring tool or the section view. Opening the exploded view turns measuring off and closes the section view, and turning on either of the other two puts the model back together. The measuring tool and the section view work together.

Escape

Esc steps back out of these tools one stage at a time, in this order:

1. Closes an open measurement label.
2. Clears the measuring tool's picks.
3. Removes the section plane, leaving the section view waiting for a new click.
4. Closes the section view.
5. Closes the exploded view.
6. Turns the measuring tool off.

With none of the tools open, Esc clears the selection, as it does for every format.

The grid and overall dimensions

A STEP or IGES file states its unit, so its lengths are real. While the sidebar shows the tree, the viewport draws two aids in the file's own units. Neither is drawn in Models mode or in Photo Mode.

The grid is laid out on the file's coordinates, with cells sized at a power of ten of the display unit, such as 1 mm or 10 mm, and a stronger line every tenth cell. The X and Z axis lines pass through the file's origin. The grid becomes finer as you zoom in. Grid Size in Settings ▸ Viewport ▸ Grid still sets how dense it is.

The overall dimensions give the width (X), height (Y), and depth (Z) of the box around what is shown, which is the whole model, or the selected rows and everything beneath them. Hidden parts are left out. The dimensions are drawn in the scene like a drawing's dimension lines, so they keep their place while the camera moves and are hidden by parts in front of them. A label is marked "≈" when its side could not be measured exactly. The dimensions use the unit chosen in the measuring tool, and they fade out as the exploded view opens.

The same grid and overall dimensions are drawn for 3MF files, whose units are also real.

Orientation

Many CAD programs use Z as the up axis, while Solidly uses Y, so a part can open lying on its side. The Info tab's Orientation section and Edit ▸ Rotate Model turn the model in quarter turns. For a STEP or IGES file the turn changes only the view, and the overall dimensions follow it. See Rotating the whole model.

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