Objective-C Scenekit: Import OBJ-File externally and color it

Question

I am new to this. And there comes my first problem.

I build an object loader with SceneKit. I got the path, the Object is available, but I have no clue how I can color the shown object.

ScnScene *testScene = [SCNScene sceneWithURL:url options:nil error:NULL];
testScene.background.contents = [UIImage imageWithName:@"color.png"];

[self.mainView.scene.rootnode addChildNode:testScene.rootNode];

This didn't work. I also tried with:

SCNMaterialProperty *testColor = [SCNMaterialProperty materialPropertyWithContents [UIImage imageNamed:@"color.png"]];
testScene.rootnode.geometry.materials = testcolor;

Or:

SCNMaterial *testColor = [SCNMaterial material];
testColor.diffuse.contesnts = [UIColor redColor];
testScene.rootnode.geometry.firstMaterial = testColor;

Nothing works. When I start the app, every object is displayed. So far the OBJ-Loader works out very well. But everything is still gray. I totally have no clue how to color the displayed objects. Has anyone a hint/idea/solution for me?

Btw. I want to avoid that I have to build a geometry out of the OBJ-Informations manually. So I try to solve this by the SCNScene.

Answer 1

The major problem was that the import by a SCNScenedoes not work that way. So the right solution is to import the obj.file into a SCNNode, add a SCNMaterial with the chosen color (or an image) to the SCNNode and give the SCNNodeto the SCNScene. To load the obj.file, you need to import that by Model IO Framework.

I will give some code how I made it colorful.

#import <SceneKit/SceneKit.h>
#import <ModelIO/ModelIO.h>
#import <SceneKit/ModelIO.h>

...

@property (nonatomic) SCNView* mainView;

....

MDLAsset *asset = [[MDLAsset alloc] initWithURL:url];     
SCNScene *scene = [SCNScene scene];
SCNNode *node = [SCNNode nodeWithMDLObject:[asset objectAtIndex:0]];

SCNMaterial *material = [SCNMaterial material];
material.diffuse.contents = [UIColor colorWithHue:0 saturation:0.1 brightness:0.5 alpha:1];
node.geometry.firstMaterial = material;

[scene.rootNode addChildNode:node];
[self.mainView.scene.rootNode addChildNode:scene.rootNode];

Alternately you can add a color by this:

material.diffuse.contents = [UIImage imageNamed:@"farbe.png"];

Now you can import any obj.file externally (from any chosen folder you like) and color it.

Thanks to SGlindemann, cashmash and Hal Mueller, who helped out to find that solution.

---

UPDATE (29.1.2017)

Somehow the way above does not work anymore. I did not figure out yet what changed. But I made another code that makes loading for 3D files possible (from mainBundle, not externally). Here I start from a SCNNode class which is called by the ViewController.m. The SCNScene is setup in the ViewController. Following there is the code which I wrote for the SCNNode class.

Before you start, put the .obj and .mtl file (both with same name) into your Xcode project. You don't need to convert it to a scene.

#import <ModelIO/ModelIO.h>
#import <SceneKit/ModelIO.h>

...

@property (nonatomic) SCNNode *objectNode;

...

NSString* path = [[NSBundle mainBundle]
            pathForResource:[NSString stringWithFormat:@"name of the obj.file"]
            ofType:@"obj"];
NSURL *url = [NSURL fileURLWithPath:path];
MDLAsset *asset = [[MDLAsset alloc]initWithURL:url];

// Create the Block
self.objectNode = [SCNNode nodeWithMDLObject:[asset objectAtIndex:0]];


[self addChildNode: self.objectNode];

return self;

This returned self has to be added into your view.

[self.view.scene.rootNode addChildNode:returnedObj];

The MDLAssetloads the .obj-file with the corresponding .mtl-file and png-File. I used this code to load objects from MagicaVoxel (this exports obj+mtl+png at once). I did not get deep into it yet.

I did NOT try this code with external loadings or typing in colors manually via SCNMaterial. So there is no statement whether this works or not. I did not try.