Skinning Assetto Corsa Cars from Nothing using Mudbox and Photoshop

[Russell Sobie]

This Tutorial was originally posted over on the simracingsystem.com forums where I do all of my skinning. I was constructing it AS I was learning the process, so obviously some of this is going to be "not the best way" to do certain things. I was going to make a post just linking the original tutorial, but after asking permission to the folks here at RD, it was suggested that I just post the tutorial in its entirety here! That said, let's get started!

NOTE: I have added notices before or during the parts where it is SRS specific, like making sure your skin is under 10MB (a limitation of the SRS Skin Transfer App used to automatically share custom skins just before races start). Follow the red instructions so you aren't doing extra work where you don't need to!

Software Used

• Adobe Photoshop (with NVidia Texture Tools for Photoshop Installed) (or... with Intel Graphics' version of a similar Plugin that works better for more recent versions of PS)
• Autodesk Mudbox
• kn5conv.exe
• Content Manager
• Notepad++
• Greenshot

The Goal
Since it is almost the end of the current season and I don't know what cars are going to be used next season, I figured I'd learn on something that I figure will EVENTUALLY come up. So the McLaren 650 GT3 is what I'll be doing. The goal is to replicate my typical Red/Black two tone design, complete with a skull on the hood using the most efficient means... rather than just fumbling around with a wireframe texture in Photoshop like I have been doing.

Disclaimers

• I'm not a professional artist. I have a background in graphic design and some 3d modeling but make no claims to know the best way to do any of this stuff. This is just what I've learned jumping in head first.
• Also, Mudbox crashes. A lot. Save often!

Extracting 3D Model
I started by using a program called 3DSimED3. Unfortunately it only has a 20 day trial, and today it ran out. Normally in this situation I would just purchase a licence and call it a day, but I was curious if there was a better way to just get a 3D model out of the .kn5 files that Kunos includes in a normal installation of Assetto Corsa. My googling lead me to this youtube video: Assetto Corsa KN5 Converter & Download

I was unable to find the origin of kn5conv.exe, but I ran a few scans on it and it turned up nothing, so I gave it a try. Works great. Here's how _I_ would recommend extracting the model (as opposed to how the youtuber above does it):

1. Create a Workspace for your new skin outside of your Assetto Corsa file structure. Here's mine: S:\GameFiles\AssettoCorsaStuff\cars\ks_mclaren_650_gt3\workspace
2. Copy the mclaren_650_gt3.kn5 file from your Assetto Corsa installation into this folder (you can find it here: ...\Steam\steamapps\common\assettocorsa\content\cars\ks_mclaren_650_gt3.kn5)
3. Place an instance of kn5conv.exe in your workspace folder.
4. Drag the KN5 file onto kn5conv.exe and watch the magic happen.

The result is that in your workspace folder you will now have a number of file types that can be understood by a number of 3D software packages, such as Mudbox and Blender. I gave Blender my best shot, but couldn't figure it out and there was not a good tutorial I could find that would clue me into how it works. I did find one that was enough help so that I could get going in Mudbox, so that's what I'll be doing.

If interested, here's the youtube video I found that got me going:

Cleaning Model For Painting
Once you have an .FBX file for the car you want to paint, you can import it into Mudbox.

1. Open Mudbox.
2. File>Import...
3. Navigate to your workspace folder and import mclaren_650_gt3.fbx
4. Save the scene as a .mud file (mclaren_650_gt3.mud) in your workspace folder.

This is ALL of the geometry and a bunch of other nodes/objects that you will not need for the purposes of creating a livery. So we must now delete all of the stuff we don't care about. This will speed up performance within Mudbox, as well as avoid Mudbox wanting you to add additional materials every time you attempt to paint on a different mesh. In other words, we want to avoid painting on the windshield when what we want to do is paint on the car's body. Here's what I did to go through everything:

1. Change to the Object List.
   
   
   
   
2. Hide everything in the Object List
   
   
   
   
3. Show only Geometry
   
   
   
   
4. Switch to Object Selection, and turn on Mirror on the X axis.
   
   
   
   
5. Select geometry in the scene view that you know you will not be painting. Windows, tires, brake discs, windshield wipers, whatever. All that should be left are the panels of the car you want to put your design on.
6. Save your .mud file. You may want to save it as a different .mud file than the original, in case you realize later you deleted some part of the geometry you actually wanted!

The end result should look a bit like this:

...continued next post...

 

[Juttie205]

I have a strange issue as I make a shape in mudbox and then import it to photoshop and all looks ok until I say add a stroke to it as you can see from the pictures the stroke goes around all the parts that have been deleted and not visable? Ever had that before? Also on another note when I export to photoshop all ok I make changes then save and mudbox say there are changes so re import but nothing happens? Cannot see what I'm doing wrong

 

[Russell Sobie]

Photoshop outlines and outer glows and inner glows (basically all the Effects) don't typically translate well imported back into Mudbox. What I typically do if I want my outlines to come back to Mudbox is rasterize the Effects on the layer before re-importing. Then what you see is what you get.

I have no idea on the second question about re-importing changes. Work flow for me is:
1) Make changes in Mudbox until I'm happy.
2) Export to Photoshop.
3) Make changes in Photoshop.
4) Either finish completely or continue to...
5) Re-import Photoshop changes back into Mudbox.
6) Go to #2.

 

[Juttie205]

Do you export channel or layer to photoshop? Also do I need to save changes in photoshop or will mudbox see changes in real time?

 

[Russell Sobie]

1. Be on the Layers Tab.
2. Right click on any layer.
3. Select Export Channel to PSD...

https://imgur.com/tLWsJVO

Mudbox looks at the file it exported and sees when it changes (i.e. you save in Photoshop). And then Mudbox will show a "Re-import" dialog that you have to say "yes, please bring in the changes I made in Photoshop". I isn't going to update the skin live.

 

[Juttie205]

Ok that's what I've been doing strange does not work every time

 

[Juttie205]

This is problem I have I make a shape and export to photoshop and as you can see first picture looks fine the moment I try and put a affect or put on a channel in photoshop it shows all what ive painted before I deleted anything??

 

[Russell Sobie]

Yeah I think I have seen that before. You'll probably have to fool Mudbox into forgetting where you painted and then removed paint. One idea might be to duplicate the layer in Photoshop before you add the effect, save the file, re-import back to Mudbox, delete the OLD layer and keep the new one, then BACK to Photoshop and apply the effect to the new mudbox layer. Might work, might not. Don't know!

 

[Juttie205]

Will try that it's a strange one really frustrating

 

[Juttie205]

Have sorted it I have to select objects then inverse and delete which gets rid of strange outline then it's ok bit of a pain but at least it's working low

 

[xDama]

Thank you very much for all the information and the Youtube clips Russell! I've been practising and designing some skins the last couple of months and this has helped me out massively!

There's just 1 thing I can't seem to properly understand, or figure out: the artifacts or low quality transfer of stencils on to the car. Sometimes it's ok, and then on other moments even a simpel square is transfered as if it's a low quality image.

• I design a stencil in Photoshop of 4096x4096 pixels, use basic vector shapes and colors and save it in .TIFF with the mentioned presets.
• I increase the 3D model to about the size of 2 squares in the Mudbox grid

I do zoom in and out a lot on the 3D model and I'm also constantly resizing the stencil while painting. Does it have something to do with that?

I know that it's not possible to draw perfect vector shapes, since the stencil gets applied in a 'pixel-format', but even with a big and smooth straight line, it often creates very 'jaggered' lines. I added a screenshot to show you what I mean: some lines are ultrasmooth, others are jaggered.

This is my first experience with Mudbox and I've been looking for many ways why this happens, but I just can't figure it out.

 

[Russell Sobie]

3D models have UV coordinates assigned that can't be changed (without making changes to the model itself, which AC won't be okay with). If the UVs are really low resolution on a particular section of the model that's all you get to work with. The bigger the area on the overall texture for a portion of the car surface, the smoother the stencil application will be, as far as I can tell.

You can also play with the Falloff settings on your brush when you paint over the stencil which MIGHT help?

The good news is that going 150+ mph, no one will be able to tell you have jaggies all over your car. Just don't take any up close screenshots to show off your liveries and your secret is safe.

 

[xDama]

Thank you for your quick reply! I'll see if the falloff settings make a difference!

Btw: as a small token of appreciation - this is one of the first skins I made thanks to your tutorial and clips!

 

[LePirate]

Hi there @Russell Sobie ! Thanks A LOT for that tutorial, well made and explicit video so even a frenchie like me could understand!

One thing I'm struggling with tho, some parts of the car just won't apply on the skin as they are exported through a different psd file from the rest of the skin. Here's what I mean by that:

As you can see on the image:

-Tail light frame is supposed to have a decal on it, or at least be the color of the car;
-Windows frames are supposed to have the skull decal continuation on it, as if the layer was applied over the paint and window;
-License plate decal is from the original skin, but can't seem to edit that either. (Same deal as the tail lights frame)
-Rear window spoiler is supposed to be stickerbomb;
-Rims are supposed to be black & blue, not chrome & blue.

That's probably an issue resulting from Mudbox exporting those parts as a new psd file for each part of the car that is not the "main skin".
I assume I'm doing something wrong but can't seem to find what. Could you please help me?

 

[Russell Sobie]

Typically "trim" such as windows, window borders, or spoilers and the like are not mapped on the main skin, as you said. Instead, they are mapped with a very small and tiled texture, typically just a small square of a solid color. Since you cannot change the UVs for those parts of the model, you are stuck only being able to change that tiny texture's patch of color. You won't be able to put any details on them.

What I typically do now is find out what all parts of the car are part of the main skin files, and only leave those pieces of geometry in MudBox when I'm painting. Some exceptions might be where an auto emblem on the truck might be or the like... but I'll lock those pieces of geometry to prevent applying paint to them. Once I have culled out everything that shouldn't have paint touching it, I'll delete unused shaders.

 

[LePirate]

I found the squares you were talking about! Thank you for that! I already had a couple of them edited, but I edited the other ones and managed to fix all the colors issues I had regarding the trims!

Still having a problem with the license plate tho.. Seems to be stuck here even tho I completely removed the assets from the car's folder.

Is the another location in which I have to change assets to get the change I want?

 

[Russell Sobie]

Some stuff they make you hunt for. Use the CM Showroom to click on the thing that you want to change and then look for the Diffuse name assigned to it. Then look in the unpacked textures folder that unpacking the kn5 gave you for that file. Just copy that over to your skin folder and edit it there. Or do whatever and save over that file from that point on.

 

[maximkozirev]

Hello, I have some problem with painting custom model (ag_hondansx_gt500_98)
As you see there are artifats in model. I cannot fully paint model.
Can you tell how you solved this problem?

 

[Russell Sobie]

When the UV mapping isn't perfect, my typical workflow is to create a separate layer called "FIXME" in Mudbox. Then I grab some bright green paint with the Brush tool and paint all around the locations with the stretched texturing. Once I have it properly painted out, I'll export to PSD so I can have a look at the problem area in Photoshop. Once there, I'll just paint a solid color throughout those locations so I don't get any strange banding like you are seeing.

Downside of course is that you can't put any detail in those locations. Not really anything you can do about it. Except maybe notify the creator of the car model and ask them to adjust the UV mapping in problem spots? Who knows... maybe they'll release an update. I wouldn't hold my breath though.

 

[maximkozirev]

When the UV mapping isn't perfect, my typical workflow is to create a separate layer called "FIXME" in Mudbox. Then I grab some bright green paint with the Brush tool and paint all around the locations with the stretched texturing. Once I have it properly painted out, I'll export to PSD so I can have a look at the problem area in Photoshop. Once there, I'll just paint a solid color throughout those locations so I don't get any strange banding like you are seeing.

Downside of course is that you can't put any detail in those locations. Not really anything you can do about it. Except maybe notify the creator of the car model and ask them to adjust the UV mapping in problem spots? Who knows... maybe they'll release an update. I wouldn't hold my breath though.

Thank you for advice. I advise everyone to use 4K Layers.
Here is the result of my first livery. Honda NSX JGTC Castrol-Mugen 1998. Unfortunately, there are some noticeable shortcomings, but they appeared due to a bad UV mapping

 

[Juttie205]

Hi mate just been watching your video of the martini porsche and was wondering what the procedure is that you did for the stripes when you went into photoshop and drew them on as couldn't quite see what you did in menus as was going to fast.
I have been taking a screen shot and drawing on that etc but your way seems to keep car model same etc.
Also I have my falloff set to max but I'm getting a weird ghosting when I paint and use the eraser and normally have to to multiply passed before ghost dissappear wondering if there is a setting im missing as cannot see anything wrong in mudbox