Show the Working

glTF to OBJ

Your model never leaves your browser, and nothing is downloaded to make this work. A mesh is a list of points and a list of triangles; reading one and writing the other is arithmetic your browser can already do.

…or drop one anywhere on this page.

Into the format nearly everything reads, as plain text you can open and look at.

A .gltf is JSON, readable in a text editor, with its numbers either inline or in .bin files beside it. Only the self-contained kind can be read here, because a browser is handed one file and the others are not in it — and it says so plainly rather than producing an empty model.

None of these formats has units

This is the single most common reason a model comes out wrong, and no converter can fix it, because there is nothing in the file to read. Both of these formats are lists of numbers. Whether 10 means ten millimetres, ten inches or ten of something else is a convention between whoever exported the file and whatever opens it next — it is not written down. That is why models arrive at twenty-five times their intended size, or a twenty-fifth.

So the dimensions are shown above as three bare numbers. If your object is meant to be about 60 long and it says 2.36, somebody wrote it in inches and something else is about to read it as millimetres. The numbers are yours to recognise; nothing here can do it for you. If you want the size recorded in the file, convert it to 3MF instead — that is the one format here that stores it.

A glTF is a scene, not a mesh

The numbers are each part's own coordinates. Where that part actually sits is recorded separately, on the node pointing at it — as a matrix, or as a translation, a rotation and a scale. Nodes nest, so a wheel's position is its own transform times its axle's times the car's.

A reader that ignores all that produces a perfectly valid mesh, with exactly the right number of triangles and vertices, in which every part of the model is stacked at the origin. Nothing about the file looks wrong. There is a second catch as well: a .gltf may keep its numbers in .bin files beside it, and a browser is handed one file. When that happens this page says so and names the missing file, rather than handing you an empty model.

Both of these store a corner once

Both formats keep a list of points and refer to them by number, so a corner where six triangles meet is stored once and pointed at six times. That is what an STL cannot do, and it is why these conversions do not change the size of a file very much.

Duplicate corners are still welded on the way through, in case the model passed through an STL earlier in its life — but only bit-for-bit identical ones. Welding within a tolerance, which most tools do, quietly fuses surfaces somebody meant to keep apart. The number actually welded is reported above.

What does not come across

Materials, colours and texture coordinates. An OBJ can refer to materials, but they live in a separate .mtl file — a second file, which is not written here. Vertex colours have nowhere to go. Normals are not written: they are derivable from the three corners, and a wrong stored normal is worse than none.

Whether it is watertight is reported above, because that is what a slicer is about to ask: a closed solid has every edge shared by exactly two triangles, and one that does not leaves the slicer guessing what is inside. This is measured after welding — straight off an STL no two triangles share a corner by index, so every model on earth would look full of holes. Nothing here repairs a mesh; it tells you, and repairing is a different job.

Nothing is rotated. glTF is Y-up by its specification, COLLADA records an up axis of its own, and STL, OBJ and PLY say nothing at all — in practice arriving Z-up from CAD tools. What a file declares is reported rather than acted on, because a silent rotation is far worse than a model you can turn yourself. If your object arrives lying on its side, that is why, and it is one rotation in whatever opens it next.

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