I'm trying to wrap my head around the concept of UV unwrapping in Blender. My goal is to make a model, unwrap it and use the UV to texture in PhotoShop for use in Poser. I've looked at tutorials: create seams, unwrap, adjust UV islands, save out the UV as jpg (what does UV stand for anyway?).

What I don't understand is how I can just move the islands around and resize or reshape them and have Poser or another program understand my modifications when I import my obj. The texture map has only image information right? Does the exported Obj from Blender have some kind of data attached so it knows which pixels a certain set of vertices correspond to? If I understood the logic a bit better I could more easily figure out how this is supposed to be done.

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Geometry of object (mesh) is defined in 3D modeling space with Cartesian coordinates x, y and z.
UV texturing (UV mapping) permits polygons that make up a 3D object to be painted with color from an image. This process transforms the 2D image (called texture map) onto the 3D object in a way similar to decorative gift wrapping. In contrast to x, y and z Cartesian coordinates, which are the coordinates for the original 3D object, another set of (2D) coordinates is required to describe the surface of the mesh, so the letters "u" and "v" are used to designate those coordinates.

UV map is just an ordinary image in any popular format. Term "map" is used because process in which pixels of 2D picture are associated to points on surface of mesh resembles what cartographers do when they use flat paper to represent points on spherical surface of the globe.

as for the other questions all 3d apps have some design features in common that include the x,y,z and for the 2d part u,v

Everytime you move the uv islands around or make any change to your uv map in Blender you will need to export a new .obj for importing into your other program.

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Thanks for the info unbroken-fighter and keppel. I do have a better understanding but for some reason still having issues. I'm either saving out the obj with the wrong settings or I set something wrong in the file by clicking around randomly (don't really want to do that in Blender). I'll either create a clean file with a simple mesh to work out the process or try to clean up the file I'm working with and start the UV process over again.

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All your options for exporting are on the "Export OBJ" panel in the bottom left of the export screen, make sure you have "Include UVs" checked. If you miss something on that panel you won't get it in the .obj. Also, helpful to make sure you have selected the object[s] you plan to export and use "Selection Only" otherwise Blender wants to export every item in the scene.

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My export options, saved as a preset:

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

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Cool, a preset. Thanks all.

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flipping heck RV that theme is awefull...................its making my eyes bleed

Better? ;-)

I just did a quick [high-light region] and [save] using Linux Mint's Screenshot in the last one... it was quick to do. This time, I had to save the entire desktop, which spans two screens, then bring up the saved png in GIMP and crop. As an aside, I do wish GIMP would stop with the You-Can-Only-Save-As-XCF nonsense... it's arrogant and silly and definitely NOT industry standard. Now, you have to Export As, then when you close you still get the GIMP asking you whether you want to save changes, although you just did. Ahhh, but you didn't save as .XCF!   :-/

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
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Not to derail the thread op, but the UI talk has me wondering, does anyone here use Blender Sensei?  I started using it, and wow, it really helps make Blender a lot more usable, and is more sensible than the default layout and hotkeys.  I wonder why it's not very popular with Blender users?  It's like how I wished Blender was when I first started.  I probably would have learned it so much faster.  Instead, it seems blender sensei is ridiculed and bashed by the Blender community in the forums that I saw.  Lots of haters out there I guess.  Anyone else agree?

Ok, UV export worked. Another step forward. Might have been Robynsveil's export presets or maybe just cleaning up the file and trying again. What is the purpose of the .mtl file though? It won't open in any of my software.

Regarding Blender Sensei. I saw one tutorial where the guy was using it. I didn't really watch it as it functioned differently than my regular version of the program. Wouldn't installing it nullify all the tutorials that use plain old Blender?

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never seen any ill feeling to blender sensei or this variant ~ fluid designer, its up to the user to find the one that suits THEM

.mtl file is just the material list for an object file, can read it with notepad (any text editor) poser really don't care about them ;-) some software does  but its only basic stuff anyway no "fancy" shaders allowed

Haven't used Blender Sensei, although it looks like a good idea, particularly if it works for you! It's a bit like using Linux Mint vs Arch vs Ubuntu vs RedHat vs SuSE vs Android... interfaces and needs vary, but everyone is using Linux (yes, Android is Linux). AFAIK, the .mtl file isn't used by Poser, but don't delete it, because the .obj file does refer to it.

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Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks

Like heddheld said, its just going to be basic material values, color, specular values [that may or may not be correct for the software you're importing to], etc, you don't really NEED the .mtl file if you're going to be re-surface the model in another software package anyway. Some software reads .mtl files better than others, some add things on export that other software won't read on import. I've found .mtl to be a waste of energy most of the time. If you don't plan to use it, you can uncheck the "write materials" option on export, and that'll prevent it from creating an .mtl when you export.

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Thanks for the replies about Sensei.  I was just curious if anyone here was using it.  I won't keep talkin bout it after this post, because I don't wanna derail the thread.  False1, all Sensei is is a deeply customized UI and hotkey design created by someone. It doesn't do anyhthing that vanilla ol Blender can't do, but the way it's designed really clicked with me, and helped me get around in it much easier. It's just a really clean design, with visible icons and stuff for shortcuts that regular blender only has hotkeys for, etc., etc.

heddheld, there's some ill feelings toward it, because I remember seeing some really nasty posts in a thread at blenderartists.org one time.  I can't find the thread now, so I guess they removed it, which goes to say how bad it was.  There's still some comments on their videos by users that are unnecessarily mean if you ask me, and some posts you can find, wihich are again unnecessarily mean toward the creator.  Stuff like this quote:

"I took a close look at the work this guy has done for his promos and tutorials. I notice that the only character animation he has is of the monkey head guy. The head is 'Suzanne', one of the Blender object primitives. Why is he using Suzanne instead of a head he modeled himself? My guess is he sucks at character modeling. Except for his promo, all his other animations are simple fly bys or use the physics engine. Looks slick, but it's not the sort of thing that will actually get you into the nuts and bolts of animation. Take a close look at his castle and his spaceship. My guess is this is the best he can do. Is this the guy you want to take on as your teacher?"

Who says stuff like that about someone in a public forum?  Especially someone they don't know personally.  Oh well.  There's mean spirits floating around anywhere you look I suppose.

lol as you say theres always one but to be fair I seen worse in some forums ;-) , I was lucky to find the shortcuts easy to learn and I hate the pie menus but it takes all sorts and I have no problems with anyone using a dif proggy or method or woteva