You will need to change the settings in Sketchfab for the transmission/opacity texture.
Unfortunately, the Sketchfab API does not allow for this being done via a script. It must be done manually.
Format for the texture must be Luminance, and Invert Texture must be turned on. See here.
"Bake maps to target" allows you to specify a different mesh (the "Target Object") to the mesh(s) with the materials you are baking (the "Source Object(s)").
The Target Object is typically a low poly version of the Source Object(s), allowing you to essentially transfer the materials from the latter to the former. This is a common workflow for game development. The end result is one object (the Target Object) with the materials of the Source Object(s) projected onto it.
This process uses Selected to Active baking in Blender. The Target Object and the Source Object(s) must share the same space. You must also specify a Ray Distance and (optionally) a cage. You can think of it as rays reaching out from all points of the Target Object (at the distance you specify) and "finding" the Source Object(s). When a ray hits, it returns the part of the Source Object(s) material that it found, and that gets projected onto the Target Object.
Once the bake is complete, your textures will be available within Blender. You can see them by going to the UV Image editor, and selecting one of them as the displayed image.
In common with other images created in Blender, your texture will not be saved when you save your blend file. Belnder only saves data that is being "used" somewhere and, straight after a bake, the baked textures are not being "used" anywhere.
SimpleBake includes an option to automatically export your textures on creation. In this case, your textures will be saved into a folder called "SimpleBake_Bakes" created in the same place as your blend file is saved. Personally, I never turn this option off, as I always want my baked textures saved somewhere.
If you have selected the "Copy objects and apply baked textures" option, you will find an object in the root collection with your baked materials applied.
It is important to appreciate that SimpleBake does not change the materials on your original objects (to ensure your hard work is not ruined!). After a bake, your original object(s) should look the same in the viewport. If you want to use the baked textures that you just created on your original objects, you will need to change your materials to refer to them (i.e. plug them all into a Principled BSDF), and make sure you have the correct UVmap selected. This is either your original UVmap, or the one created by SimpleBake if you checked the "Generate UVs" option. This is basically what the "Copy objects and apply baked textures" feature mentioned above does for you.
If you checked the box for "Export Mesh" when baking, a copy of your object will be saved externally (in the same folder mentioned above). This copy of your object is ready to accept your textures. It includes only the UV map you used for baking, and only one placeholder (blank) material per object. You could re-import this into Blender if you wished (and set up your baked textures around a Principled BSDF node in each material), but it's more common to take that exported mesh and baked textures to another program or service (e.g. uploading to Sketchfab).
Start by baking at low resolution (128x128 or 512x512). Check that everything looks OK before moving on to the higher resolution bake.
Look at SimpleBake's output in the system console. This gives you an idea of what's happening step by step. If something is happening that you don't expect, you can usually see why from what's being written to the console.
Don't forget the memory usage option (in SimpleBake's panel). This is especially important if you are using a low powered computer (my laptop will crash baking complex objects if I don't reduce this).
Check back often for updates! I'm always fiddling with SimpleBake, squashing bugs and making it better. Updates are frequent.
Baking with Blender (or, more accurately, with Cycles) will never be instant. It depends a lot on what you are baking and speed of your computer.
SimpleBake makes a number of adjustments to your settings to try and streamline the baking process. That said, if it's still slow, there are some things to consider:
- For CyclesBake, your render sample count matters! - Unlike PBRBake, CyclesBake uses your render sample count. Just like when rendering an image, you need to find a balance between sample count (and so quality) and time taken. If CyclesBake is too slow, try reducing your render sample count. Note this has no relevance to PBRBake.
- Texture size always matters - At the risk of insulting your intelligence, baking a set of 8k textures is going to take considerably longer than baking a set of 1k textures... consider the resolution that you really need. This applies whatever type of baking you are doing.
- Don't forget the memory usage option - At the bottom of the SimpleBake panel you have an option for memory usage. The higher you set this, the faster the baking will be. That said, set it too high, and Blender will fall over.
The primary purpose of SimpleBake is to bake PBR maps from materials created around the Principled BSDF and Emission shaders (only those shaders, or it's not really PBR). This is what a PBR Map Bake is, and it allows you to bake PBR compatible image maps such as Diffuse, Metal, Normal, Transmission etc. Other programs and services (like Sketchfab) are set up to work with those kinds of PBR maps.
However, SimpleBake can also bake the more "traditional" bake modes that Blender offers. You can see these in the Bake panel in Blender, usually the one above SimpleBake itself. SimpleBake calls this a "Cycles Bake".
When you click Cycles Bake in SimpleBake, your bake will be based upon the settings in the default bake panel. For example, if this is set to combined, your bake will be a combined bake taking into account lighting, shadows, reflection etc. You will only get one map. SimpleBake will handle all the node setup, image creation and general configuration needed for a Cycles Bake, making it a one-click solution, and not the colossal pain it usually is to set up in Blender.
These maps you get from a Cycles bake are not PBR maps, but they can still be useful. I don't do any game development, but I think materials with lighting and shadows baked in are used for game development. These are the baking modes that Blender has offered since pretty much forever, and they do have their uses.
After hitting bake, Blender will "lock up" (freeze) until it is complete. Unfortunately, there is no way to prevent this. This is usually only brief but, if you are baking lots of stuff at high resolutions, it could also be for quite a long time (especially if you have a slow computer).
During the bake, SimpleBake will output various messages to the system console. These messages give you an idea of what is going on and how things are progressing. For this reason, I would strongly recommend always starting Blender from the console. If you start it from the Start Menu (or your operating system's equivalent), there is no way to see what it going on.
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