Blender

Blender is an open source 3D content creation suite that has built-in support for the file format COLLADA that the HPL2 engine uses to import 3D models.

Tutorial - the basics

1. When Blender launches it creates a default cube.
2. Save the Blender project (Ctrl + S; File → Save).
3. Select the default cube and click on the Materials button in the Properties panel. Make sure a material is applied to the mesh (default material will work fine).
4. Click on the Textures button in the Properties panel. If there is no texture applied to the material, click on New or the default texture from the list if any. Set Type to Image or Movie, and in the Image menu click Open and select a texture (the .dds/.jpg/.tga/.png not the .mat file). It is best if the texture is in the same directory as the .blend file. In the Mapping menu, make sure UV is set for Coordinates.
5. With the cube selected, press TAB to go to Edit Mode. Make sure all the faces of the mesh is selected. You can do so by pressing A on the keyboard. Press U on the keyboard and select Unwrap (Reset if you haven't created seams for the default cube).
6. With all the faces still selected, convert all quads to tris either by Ctrl + T or Mesh → Faces → Quads to Tris.
7. Go to the UV/Image Editor and in the pop-up menu to the right of the UVs menu, select the previously loaded texture from the list.
8. Go back to 3D View, change draw mode (i.e. Viewport Shading) to Textured and you can see that the model is textured. You'll know it is ready for exporting if both Textured draw mode and the rendered image both show the cube textured.
9. Delete the default camera and default light.
10. Go to File → Export → COLLADA (.dae). In the left panel, at the bottom, there will be export options for COLLADA, specifically texture options. If you're using external textures, then you'd want to make sure Include UV Textures is checked. If you're using internal textures, then you'd want to make sure Include Material Textures is checked. If you want to copy (or overwrite) the textures over to the save location (this should be done at least once), make sure Copy is checked.
11. Save the file as “filename.dae” in a folder within one of Amnesia's directories (preferably the custom_stories directory).
12. Using the model viewer you should now see the newly created cube fully textured.

Additional Information about the above steps

The above is intended to quickly give a sure step by step process to successfully export an object and get it to display within the game engine. It is not intended to be a lengthy tutorial or be suited for advanced, novice or any specific type of user. It's just a quick how to.

Important information about images

Textures(.dds, .jpg. tga and other general image formats) and material files(.mat, the engine specific file) - The material file is an XML file that contains information on what images is used in the material to create the texture and other properties. When the game loads a 3D model it searches for the name of the image used as a texture for the model(the .dds etc) and then replaces that name with the extension .mat to know what material file to use. Because of this it is important that the texture used has the same name as the material file, with the exception of the extension.

Blender to COLLADA, How to get it right

This section was written by yasar11732 in forums.

First thing to take into account when exporting is, correctly U.V mapping the texture. Adding texture to material without U.V mapping will NOT work, it will just crash your model viewer.

Also, before adding texture to your models, save your model (blender file) to where textures resides (It is better if they all in one place). And use relative paths when adding textures instead of absolute paths, then export your .dae to same place. So that you can easily port your models from one place to another (Always keep .dae and textures together). If you don't do that, you will probably fail at some point when trying to move your models from one place to another.

Sometimes, when you export your model and see it in model viewer, you may see that texture is shown “inside” of the model, not the outside (look really weird)! That is because normals of your model is mixed up. You should go back to blender and flip the normals and export again. This should solve your problem.

And also, one should remember always to use textures with sizes of exponents of 2 (256×256, 512×512, 128×1024 etc.) Otherwise it will either crash your model viewer, or just look awkward.

One last point to remember is that, materials you create might be mixed up when you create it and one material accidentally get linked against 2 different geometries (or datablocks, or vertex groups… whatever one calls it). If this happens without proper care (accidentally), your model will not export correctly. Will look funny in model view.

You can export rotation/location/scale animations without problem, however, I couldn't export shape animations correctly into .dae format (because blender's collada export sucks! Not actively developed.) If anyone can do it and post the method, it would be great to learn about it.

And also, do not try to export armature/bones, because as far as I can say, that doesn't work either. If someone could do it, I also want to hear about how to do it.

Notes on Blender 2.8

Blender 2.8 introduces major UI changes, so here's a redux of the tutorial:

1. Make sure to remove all non-mesh components, like lights and cameras.
2. Make sure that your model has unwrapped UVs. Alternatively, you can unwrap your model after adding the texture.
3. Very important! HPL* doesn't like when there are orphaned textures and materials. You must be very careful not to add any unnecessary ones, so follow carefully. If your model crashes for no apparent reason, this is likely the case. Make sure to delete the duplicate and orphaned data before exporting.
4. Also keep in mind that HPL allows only one material with one texture per mesh (object). No more, no less.
5. Select your mesh and go to the materials tab: (image)
6. Click on the circle next to the “Base Color” setting.
7. Pick “Image Texture” from the pop-up menu.
8. Click on “Open” and pick your texture file: (image)
9. From now on, if you need to open the texture somewhere (e.g. for UV unwrapping), click on the square icon with a triangle and a dot and select the texture from the drop menu. This way you won't be cloning the texture. If you need to apply the created material/texture to another object or reapply it to the current one, make sure to use this method as well.
10. Make sure that in export settings the “Geometry→Triangulate” option is marked. You might also need to set the up axis to be the Y axis (Blender uses Z as up and HPL uses Y).

PS. As of yet I haven't found a way of properly exporting animations from Blender. I've tried multiple COLLADA plugins and countless export settings (including .fbx). Following this tutorial will make the animated model load in-game, but most likely the armature will have errors and the animation will be corrupted.

*This tutorial is applicable both for HPL1 and HPL2. HPL3 was not tested but most likely will work too.