examples/editor_draw/editor_draw.js
function mapCanvasToData() {
    for (let i = ijkBBox[0]; i < ijkBBox[1] + 1; i++) {
      for (let j = ijkBBox[2]; j < ijkBBox[3] + 1; j++) {
        for (let k = ijkBBox[4]; k < ijkBBox[5] + 1; k++) {
          // ijk to world
          // center of voxel
          let worldCoordinate = new THREE.Vector3(i, j, k).applyMatrix4(stack2._ijk2LPS);
          // world to screen coordinate
          let screenCoordinates = worldCoordinate.clone();
          screenCoordinates.project(camera);

          screenCoordinates.x = Math.round(((screenCoordinates.x + 1) * canvas.offsetWidth) / 2);
          screenCoordinates.y = Math.round(((-screenCoordinates.y + 1) * canvas.offsetHeight) / 2);
          screenCoordinates.z = 0;

          let pixel = context.getImageData(screenCoordinates.x, screenCoordinates.y, 1, 1).data;
          if (pixel[3] > 0 && i >= 0 && j >= 0 && k >= 0) {
            // find index and texture
            let voxelIndex = i + j * stack2._columns + k * stack2._rows * stack2._columns;

            let textureSize = 4096;
            let textureDimension = textureSize * textureSize;

            let rawDataIndex = ~~(voxelIndex / textureDimension);
            let inRawDataIndex = voxelIndex % textureDimension;

            // update value...
            let oldValue = stack2.rawData[rawDataIndex][inRawDataIndex];
            let newValue = cursor.value;

            if (oldValue != newValue) {
              // update raw data
              stack2.rawData[rawDataIndex][inRawDataIndex] = newValue;

              // update texture that is passed to shader
              textures2[rawDataIndex].image.data = stack2.rawData[rawDataIndex]; // tex;
              textures2[rawDataIndex].needsUpdate = true;

              // update stats
              editorStats[oldValue] -= 1;
              editorStats[newValue] += 1;
            }
          }
        }
      }
    }
  }
function mapCanvasToData() {
    for (let i = ijkBBox[0]; i < ijkBBox[1] + 1; i++) {
      for (let j = ijkBBox[2]; j < ijkBBox[3] + 1; j++) {
        for (let k = ijkBBox[4]; k < ijkBBox[5] + 1; k++) {
          // ijk to world
          // center of voxel
          let worldCoordinate = new THREE.Vector3(i, j, k).applyMatrix4(stack2._ijk2LPS);
          // world to screen coordinate
          let screenCoordinates = worldCoordinate.clone();
          screenCoordinates.project(camera);

          screenCoordinates.x = Math.round(((screenCoordinates.x + 1) * canvas.offsetWidth) / 2);
          screenCoordinates.y = Math.round(((-screenCoordinates.y + 1) * canvas.offsetHeight) / 2);
          screenCoordinates.z = 0;

          let pixel = context.getImageData(screenCoordinates.x, screenCoordinates.y, 1, 1).data;
          if (pixel[3] > 0 && i >= 0 && j >= 0 && k >= 0) {
            // find index and texture
            let voxelIndex = i + j * stack2._columns + k * stack2._rows * stack2._columns;

            let textureSize = 4096;
            let textureDimension = textureSize * textureSize;

            let rawDataIndex = ~~(voxelIndex / textureDimension);
            let inRawDataIndex = voxelIndex % textureDimension;

            // update value...
            let oldValue = stack2.rawData[rawDataIndex][inRawDataIndex];
            let newValue = cursor.value;

            if (oldValue != newValue) {
              // update raw data
              stack2.rawData[rawDataIndex][inRawDataIndex] = newValue;

              // update texture that is passed to shader
              textures2[rawDataIndex].image.data = stack2.rawData[rawDataIndex]; // tex;
              textures2[rawDataIndex].needsUpdate = true;

              // update stats
              editorStats[oldValue] -= 1;
              editorStats[newValue] += 1;
            }
          }
        }
      }
    }
  }
Loop through IJK BBox and see if voxel can be mapped to screen