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Units, tolerance and precision

Numbers have no unit​

A length in Bitbybit is a plain number. The kernels do not know millimetres from metres: a box of width 4 is 4 of whatever unit you decide on. Pick one for a model and keep to it, and say which one when you export: a STEP file or a 3D printer will read the numbers in a unit of its own, often millimetres. Angles, wherever a function takes one, are in degrees.

The scene does not care either, but its camera, lights and shadows are set up for models of a few units to a few hundred. A model in millimetres of a whole building may need the camera moved further out and the shadows told how large the scene is.

Tolerance and precision are different things​

Two numbers decide how close is close enough, and they are easy to confuse:

  • Tolerance belongs to the kernel. It is how far apart two points may be and still count as the same point when OpenCascade joins edges, sews faces or checks a solid. The default suits most models; it matters when you import geometry from elsewhere or build at a very small or very large scale.
  • Precision belongs to drawing. A curved surface is drawn as triangles, and precision is how far those triangles may stray from the true surface. A smaller number gives smoother curves and more triangles.

Precision never changes the shape. The disc below is the same exact cylinder twice; the left one is drawn with a coarse precision of 0.5 and without its edges, so its round rim becomes a polygon, the right one with the default:

Bitbybit Platform

One disc, two precisions in Rete

rete logoRete
Script Source (rete)
{
  "id": "rete-v2-json",
  "nodes": {
    "coarse": {
      "id": "coarse",
      "name": "bitbybit.occt.shapes.solid.createCylinder",
      "customName": "cylinder",
      "async": true,
      "drawable": true,
      "data": {
        "genericNodeData": {
          "hide": true,
          "oneOnOne": false,
          "flatten": 0,
          "forceExecution": false
        },
        "radius": 2,
        "height": 1,
        "center": [
          -3,
          0,
          0
        ],
        "direction": [
          0,
          1,
          0
        ],
        "angle": 360,
        "originOnCenter": false
      },
      "inputs": {},
      "position": [
        40,
        40
      ]
    },
    "options": {
      "id": "options",
      "name": "bitbybit.draw.optionsOcctShape",
      "customName": "options occt shape",
      "async": false,
      "drawable": false,
      "data": {
        "genericNodeData": {
          "hide": false,
          "oneOnOne": false,
          "flatten": 0,
          "forceExecution": false
        },
        "faceOpacity": 1,
        "edgeOpacity": 1,
        "edgeColour": "#ffffff",
        "faceColour": "#ff0000",
        "vertexColour": "#ff00ff",
        "edgeWidth": 2,
        "vertexSize": 0.03,
        "drawEdges": false,
        "drawFaces": true,
        "drawVertices": false,
        "precision": 0.5,
        "angularDeflection": 0.5,
        "relativeDeflection": false,
        "drawEdgeIndexes": false,
        "edgeIndexHeight": 0.06,
        "edgeIndexColour": "#ff00ff",
        "drawFaceIndexes": false,
        "faceIndexHeight": 0.06,
        "faceIndexColour": "#0000ff",
        "drawTwoSided": true,
        "backFaceColour": "#0000ff",
        "backFaceOpacity": 1,
        "edgeArrowSize": 0,
        "edgeArrowAngle": 15,
        "keepMeshData": false,
        "allowQualityDecrease": true,
        "forceFaceDeflection": false,
        "drawIsoCurves": false,
        "isoCurvesU": 5,
        "isoCurvesV": 5,
        "isoCurvesColour": "#808080",
        "surfaceAnalysis": "none",
        "draftDirection": [
          0,
          1,
          0
        ]
      },
      "inputs": {},
      "position": [
        40,
        380
      ]
    },
    "draw": {
      "id": "draw",
      "name": "bitbybit.draw.drawAnyAsync",
      "customName": "draw any async",
      "async": true,
      "drawable": true,
      "data": {
        "genericNodeData": {
          "hide": false,
          "oneOnOne": false,
          "flatten": 0,
          "forceExecution": false
        }
      },
      "inputs": {
        "entity": {
          "connections": [
            {
              "node": "coarse",
              "output": "result",
              "data": {}
            }
          ]
        },
        "options": {
          "connections": [
            {
              "node": "options",
              "output": "result",
              "data": {}
            }
          ]
        }
      },
      "position": [
        460,
        40
      ]
    },
    "fine": {
      "id": "fine",
      "name": "bitbybit.occt.shapes.solid.createCylinder",
      "customName": "cylinder",
      "async": true,
      "drawable": true,
      "data": {
        "genericNodeData": {
          "hide": false,
          "oneOnOne": false,
          "flatten": 0,
          "forceExecution": false
        },
        "radius": 2,
        "height": 1,
        "center": [
          3,
          0,
          0
        ],
        "direction": [
          0,
          1,
          0
        ],
        "angle": 360,
        "originOnCenter": false
      },
      "inputs": {},
      "position": [
        460,
        520
      ]
    }
  },
  "modules": {}
}

Measure the two and they give the same volume, and a STEP export of either is exactly round. Only the picture differs. Lower the precision when a model is large and curved and the drawing is slow; raise it for close-ups and renders.

The rest of the conventions​

Which way is up in each kernel, colours from 0 to 1, and the drawing defaults are on Conventions.