Samples that demonstrate sparse accessors

[khronosgroup]

Samples that demonstrate sparse accessors

[lexaknyazev]

@javagl master branch supports now glTF 2.0. Hosted online version was also updated.

[javagl]

It's probably not a "good" model, unless good==simple, but I started creating a "SimpleSparseAccessors" model.

EDIT: Updated based on KhronosGroup/glTF-Validator#18 - The model now passes 2.0 validation

The embedded version is here:

{
  "accessors" : [ {
    "bufferView" : 0,
    "byteOffset" : 0,
    "componentType" : 5123,
    "count" : 36,
    "type" : "SCALAR",
    "max" : [ 13 ],
    "min" : [ 0 ]
  }, {
    "bufferView" : 1,
    "byteOffset" : 0,
    "componentType" : 5126,
    "count" : 14,
    "type" : "VEC3",
    "max" : [ 6.0, 4.0, 0.0 ],
    "min" : [ 0.0, 0.0, 0.0 ],
    "sparse" : {
      "count" : 3,
      "indices" : {
        "bufferView" : 2,
        "byteOffset" : 0,
        "componentType" : 5123
      },
      "values" : {
        "bufferView" : 3,
        "byteOffset" : 0
      }
    }
  } ],
  "asset" : {
    "version" : "2.0"
  },
  "buffers" : [ {
    "uri" : "data:application/gltf-buffer;base64,AAAIAAcAAAABAAgAAQAJAAgAAQACAAkAAgAKAAkAAgADAAoAAwALAAoAAwAEAAsABAAMAAsABAAFAAwABQANAAwABQAGAA0AAAAAAAAAAAAAAAAAAACAPwAAAAAAAAAAAAAAQAAAAAAAAAAAAABAQAAAAAAAAAAAAACAQAAAAAAAAAAAAACgQAAAAAAAAAAAAADAQAAAAAAAAAAAAAAAAAAAgD8AAAAAAACAPwAAgD8AAAAAAAAAQAAAgD8AAAAAAABAQAAAgD8AAAAAAACAQAAAgD8AAAAAAACgQAAAgD8AAAAAAADAQAAAgD8AAAAACAAKAAwAAAAAAIA/AAAAQAAAAAAAAEBAAABAQAAAAAAAAKBAAACAQAAAAAA=",
    "byteLength" : 284
  } ],
  "bufferViews" : [ {
    "buffer" : 0,
    "byteOffset" : 0,
    "byteLength" : 72,
    "target" : 34963
  }, {
    "buffer" : 0,
    "byteOffset" : 72,
    "byteLength" : 168
  }, {
    "buffer" : 0,
    "byteOffset" : 240,
    "byteLength" : 6
  }, {
    "buffer" : 0,
    "byteOffset" : 248,
    "byteLength" : 36
  } ],
  "meshes" : [ {
    "primitives" : [ {
      "attributes" : {
        "POSITION" : 1
      },
      "indices" : 0
    } ]
  } ],
  "nodes" : [ {
    "mesh" : 0
  } ],
  "scenes" : [ {
    "nodes" : [ 0 ]
  } ]
}

Rendering it should look as in this screenshot:

As soon as it can be validated and the kinks are ironed out, I'd add it as a PR, together with a description.

[lexaknyazev]

@javagl
Validator supports sparse accessors. Unrelated crash has been fixed.

[javagl]

OT: @lexaknyazev You mentioned this once, and I wanted to validate it yesterday, but the dev branch was gone, and I saw KhronosGroup/glTF-Validator#17 , so just deferred this for now. Did you use the .snapshot?

[donmccurdy]

I think there may be a mistake in the sample:

In the screenshot above, note that the values array (right column) contains zeros for all of the x positions. The original bufferView has non-zero x positions for these indices, and so after the sparse values are applied to the accessor, y is right but x is reset to zero. In three.js (work-in-progress implementation) I see this:

[donmccurdy]

Work in progress for three.js: donmccurdy/three.js@3b504ef

[javagl]

@donmccurdy Adding a quick debug output for me gives

Sparse:
[(   1.00000,    2.00000,    0.00000), 
 (   3.00000,    3.00000,    0.00000), 
 (   5.00000,    4.00000,    0.00000)]

which seem to be the right values.

A quick standalone test that only decodes the given buffer and prints the relevant values...

public static void main(String[] args)
{
    String s = "AAAIAAcAAAABAAgAAQAJAAgAAQACAAkAAgAKAAkAAgADAAoAAwALAAoAAwAEAAsABAAMAAsABAAFAAwABQANAAwABQAGAA0AAAAAAAAAAAAAAAAAAACAPwAAAAAAAAAAAAAAQAAAAAAAAAAAAABAQAAAAAAAAAAAAACAQAAAAAAAAAAAAACgQAAAAAAAAAAAAADAQAAAAAAAAAAAAAAAAAAAgD8AAAAAAACAPwAAgD8AAAAAAAAAQAAAgD8AAAAAAABAQAAAgD8AAAAAAACAQAAAgD8AAAAAAACgQAAAgD8AAAAAAADAQAAAgD8AAAAACAAKAAwAAAAAAIA/AAAAQAAAAAAAAEBAAABAQAAAAAAAAKBAAACAQAAAAAA=";
    byte data[] = Base64.getDecoder().decode(s);
    ByteBuffer bb = ByteBuffer.wrap(data).order(ByteOrder.nativeOrder());
    bb.position(248);
    FloatBuffer fb = bb.asFloatBuffer();
    for (int i=0; i<fb.limit(); i++)
    {
        System.out.println("At "+i+" have "+fb.get(i));
    }
}

also seems to be right.

Note that before the EDIT in the above post, the model contained values that might cause the effect that you observe. I'm not sure where the difference might come from, but will have another look ASAP.

[donmccurdy]

Oops, you're right — I had an old version of the file. Looks good now:

[CentaurMare]

I implemented sparse accessors today in my viewer, this model was critical! Thanks!
The documentation for sparse accessors is a bit, well, sparse. Probably needs some pretty diagrams. It took me days reading and re-reading (off and on) the document and I couldn't understand it, once my brain clicked it was only half an hour to implement.

[emackey]

@CentaurMare This model did get merged into the sample models repo. The README file has some diagrams. Are more/better diagrams needed?

Since this issue is in the samples repo and not the main glTF repo, I'm tempted to say this should be closed, would you agree? If more official documentation/diagrams are needed, issues or pull requests could be filed to the appropriate place(s) for that.

[CentaurMare]

@emackey, yes, this can be closed. Better diagrams and/or wording are definitely needed in the spec, I will attempt to write something up and file against the spec.