_getTextureCoordinate() {
    const subKernels = this.subKernels;
    if (subKernels === null || subKernels.length < 1) {
      return 'varying vec2 vTexCoord;\n';
    } else {
      return 'out vec2 vTexCoord;\n';
    }
  }
@desc Get texture coordinate string for the program
@returns {String} result