Will I need permission from the work owner of a 2D sketch to make it a 3D model? This is the situation: I have a booklet that contains sketches (2D drawings) of human teeth. I would like to use them as reference materials to build 3D models (using blender) and publish the models on Sketchfab and a mobile App. I'm not sure about it since I see people, for example, making car models and selling it on Sketchfab, for branded cars, like Mercedes or Corvette etc... Did they need permission to make this work from the original designers? What are my legal responsibilities in this aspect? Is this considered a copyright infringement? Thanks in advance for the advice.
What real human teeth look like is a fact, and not subject to copyright. But some particular designer's or artist's image of human teeth would be protected by copyright, unless it is too old -- (prior to 1923 under US law), or unless it lost copyright in some other way. To create a derivative work based on such a copyrighted work would require permission from the copyright holder. Otherwise it is copyright infringement, and the holder could sue. As to the car models, the original designs would be protected by copyright, and a maker of a model should have permission in some form, or again the copyright holder could sue. Of course, a copyright holder is not required to sue, and if the value of the work is too small a suit might not pay. But that is not a matter of legal right, but of the holder's choice and business decision. A copyright holder could always change such a decision at a later time, when perhaps the value had increased. The exact laws on how to file copyright suits, and exceptions that might apply, vary by country. The question does not indicate what county it applies to.) But the basic principals are the same in all countries that adhere to the Berne Copyright Convention, which is all but a very few countries in the world. See this Wikipedia article for more details.
Here's where you went wrong legally: Suppose I legally obtain some digital image created by somebody else (e.g., by downloading from a public website). That, right there, is copyright infringement- unless the copyright owner has granted permission or the image is public domain you cannot copy it - this breaches "the right to make reproductions". By posting it on the web (assuming that it isn't itself an infringing copy) they have given implied permission for you to look at it in a web browser but not to copy it into a presentation even if that presentation is never presented. If it is presented then that makes the infringement worse - it adds breaches of "the right to communicate to the public" and "the right to use the work as a basis for an audiovisual work". How is this different from the computer wallpaper? It isn't. If you are using the one of the defaults that shipped with the OS then the license gives you permission. If you are using someone else's copyright without permission then it's a breach. There are defenses to copyright infringement but these are quite nationally variable - search this or other sites for "fair dealing" and "fair use".
You can take pictures of public buildings and use them in your game, if you want. You cannot copy pictures (of anything) taken by someone else without the copyright owner's permission, so you need Google's permission to copy their photographs. The public building exception is specifically about architectural works and does not include e.g. murals drawn on buildings, or billboards. In such a case, you would need permission from the copyright holder (the artist, or employer). You might hope for a fair use exception (again, assuming you took the pictures yourself), which has a better chance of succeeding if this is a free game.
Unless you hire your friend cameraman, he will own the copyright. So sign a contract with him: you give him $1, he disclaims and/or grants all the copyright to you. Make sure this is actually in writing because, otherwise, presumption of no intention to create legal relations applies as you are friends.
In my opinion, you are totally free to publish the information. There are two areas of law that can be cosidered - private and public law. In the private law area, you can be liable for revealing trade secrets, but only if you agreed to keep them by a contract. Trade secrets do not exist by themselves (there are minor exceptions, eg. in competition law, but those do not concern us), they must be protected by contracts. Another private limitations, like libel laws, won't apply here. This is not uncommon, but not in cars - you can find clauses like these in software license agreements. Then there is the public area. Is there any regulation, any policy of the state, that prevents you from publishing it? I am not aware you whole legal code of your state, but I doubt there is. It would be a harsh limitation of freedom of speech. Even if the modification could lead to illegal effect (like, modifying toy weapon to kill by rising its power...) it would be only illegal under very rare circumstances. To conclude it - freedom of speech can be limited only if there is sufficient public interest to do so, and I don't see any.
Let me give some nuance, if not a definitive answer, since I'm not a lawyer. First, ArXiv holds only a license, not the underlying copyright. You need to contact the copyright holder to get permission (and a license). That may be the authors themselves, or a journal if they have assigned the copyright to it. However, it might be more complicated, especially for commercial use. Since the authors of papers frequently use images created by others, sometimes correctly and sometimes not, you should explore a bit more. Perhaps the paper itself will have a citation of the figure and an indication of its source, and hence of its copyright holder. In that case, the authors may not be able to help you, since, at best, they probably also hold just a license. However, for some academic purposes and in some jurisdictions an academic work can be permitted to use a figure (or other things) without regard to copyright. This is called fair use and it varies around the world. But a commercial work won't be able (probably) to depend on the same exception to copyright law and you still have to find the copyright holder. Moreover, even if the authors have a proper license (explicitly), that license (a) may not be transferrable and (b) might be for non-commercial use only. Academic copyright holders often (not universally) issue their work under a fairly permissive license but that is restricted to non-commercial use. Since they hold the copyright they can also issue additional licenses against it if they like, but need to do so explicitly. Ultimately, to use the work of others you have to deal with the copyright question. It is further complicated by the fact that a figure may be considered under the law as a "complete work" even if it appears within another work. So, replicating it is less likely to fall under fair use rules - though they vary. And, there is one other question you should ask yourself. Is your talk really a commercial use? If you are an academic, it may be a subtle question, or not.
I am not a lawyer, and none of the following should be seen as legal advice. While it is always best to assume every image has a copyright.... In your scenario... traditionally if you are selling a product, there's generally no harm in using images of that product to assist in the sale. But even then photographer copyrights should be considered. Images of products may not only contain copyrighted material within the photo, but the photo itself is probably also copyrighted by the photographer. Just blankety taking images from other web sites is a poor practice in general and will customarily just get you into trouble. However, many manufacturers or distributors will actually provide resellers with product images. You can check the product manufacturer's web site for a "press" or "media' section. There are often downloads provided in those areas. I don't know hairdressing.. but as an example, General Motors has a special web site known to GM car dealers where the dealers can download high resolution images of the cars and products for ads, etc. I've done work in the past for a GM dealer who provided me with the web site and log in details so I can get product imagery. In addition, few manufacturers will take umbrage that you are using their images to sell their products. They want their products to look as good as possible wherever they may be displayed. In many cases, they may prefer you use supplied images rather than use your own. Customarily you would include a disclaimer in the footer somewhere: The product names, company names and product images used on this web site are for identification purposes only. All trademarks and registered trademarks are the property of their respective owners. Note, I am referring to images from the manufacturer's web site, not from competing businesses. If you are building a site for "Bob's Hair Styling" it's unethical to take images from "Kate's Hair Dressing" for your use. Stick to the manufacturer... if selling Paul Mitchell products, check the Paul Mitchell web site for available product images.
German perspective: In German law, there is the concept of Schöpfungshöhe (threshold of originality), which is required for authorship rights (Urheberrecht) to apply to a work. Basically, the idea is that a minimum of creativity is required for something to be protected. However, that bar is rather low. Thus, for example: Literary works are protected practically always. Maps are generally protected, even though you might argue they "slavishly copy nature", because the act of choosing what to show and what not is already creative. However, a faithful photographic reproduction of a painting is not eligible for Urheberrecht to apply (LG Berlin, AZ 15 O 428/15) So yes, a "slavish copy" of a work would not qualify for protection if there is no creativity involved. Note, however, that other types of protection apart from Urheberrecht might apply, such as Sui generis database right.