Example of COLRv1 implementation based on FreeType

colrv1_example.cc

void colrv1_start_glyph(FT_Face ft_face) {
  // [..Configure your graphics library's graphics context and prepare to paint and clip..]
  FT_OpaquePaint opaque_paint;
  opaque_paint.p = nullptr;
  uint_16_t glyph_id = 42;
  // Get initial paint object for glyph id 42:
  if (!FT_Get_Color_Glyph_Paint(ft_face, glyph_id, &opaque_paint))
    return false;

  colrv1_traverse_paint(face, opaque_paint)
}

bool colrv1_traverse_paint(FT_Face face,
                           FT_OpaquePaint opaque_paint) {
  FT_COLR_Paint paint;
  if (!FT_Get_Paint(face, opaque_paint, &paint)) return false;

  switch (paint.format) {
  case COLR_PAINTFORMAT_COLR_LAYERS: {
    FT_LayerIterator& layer_iterator = paint.u.colr_layers.layer_iterator;
    FT_OpaquePaint opaque_paint_fetch;
    opaque_paint_fetch.p = nullptr;
    while (FT_Get_Paint_Layers(face, &layer_iterator, &opaque_paint_fetch)) {
      colrv1_traverse_paint(canvas, scale, palette, face, path_maker, opaque_paint_fetch);
    }
    break;
  }
  case COLR_PAINTFORMAT_GLYPH:
    // [...create a layer in your graphics library...]
    // Traverse / draw operation will clip layer.
    colrv1_draw_paint(canvas, scale, palette, face, path_maker, paint);
    traverse_result = colrv1_traverse_paint(canvas, scale, palette, face, path_maker,
                                            paint.u.glyph.paint);
    // [...merge the layer to background in your graphics library...]
    break;
  case COLR_PAINTFORMAT_COLR_GLYPH:
    traverse_result = colrv1_start_glyph(canvas, scale, palette, face, path_maker,
                                         paint.u.colr_glyph.glyphID);
    break;
  case COLR_PAINTFORMAT_TRANSFORMED:
    // [...create a layer in your graphics library...]
    // Traverse / draw operation will apply transform.
    colrv1_draw_paint(canvas, scale, palette, face, path_maker, paint);
    traverse_result = colrv1_traverse_paint(canvas, scale, palette, face, path_maker,
                                            paint.u.transformed.paint);
    // [...merge the layer to background in your graphics library...]
    break;
  case COLR_PAINTFORMAT_ROTATE:
  case COLR_PAINTFORMAT_SKEW:
  case COLR_PAINTFORMAT_TRANSLATE:
    [...similarly apply transformations to your canvas...]
  case COLR_PAINTFORMAT_COMPOSITE: {
    traverse_result = colrv1_traverse_paint(canvas, scale, palette, face, path_maker,
                                            paint.u.composite.backdrop_paint);
    // [...create a layer in your graphics library...]
    // [...and set blend mode to paint.u.composite.composite_mode...]
      colrv1_traverse_paint(canvas, scale, palette, face, path_maker,
                            paint.u.composite.source_paint);
    // [...merge the layer to background in your graphics library...]
    break;
  }
  case COLR_PAINTFORMAT_RADIAL_GRADIENT:
  case COLR_PAINTFORMAT_LINEAR_GRADIENT:
  case COLR_PAINTFORMAT_SOLID: {
    colrv1_draw_paint(canvas, scale, palette, face, path_maker, paint);
    break;
  }
  default:
    break;
  }
  return traverse_result;
}


void colrv1_draw_paint(FT_Face face,
                            FT_COLR_Paint colrv1_paint) {
    float upem = /* units per em from font */;

    switch (colrv1_paint.format) {
        case COLR_PAINTFORMAT_GLYPH: {
            // [..clip the path for glyph shape colrv1_paint.u.glyph.glyphID on your graphics context..]
            break;
        }
        case COLR_PAINTFORMAT_SOLID: {
            // [..configure a solid color according to
            // palette[colrv1_paint.u.solid.color.palette_index] and alpha values
            // and fill with that..]
        case COLR_PAINTFORMAT_LINEAR_GRADIENT: {
            /* Retrieve color stops */

            const FT_UInt num_color_stops =
                    colrv1_paint.u.linear_gradient.colorline.color_stop_iterator.num_color_stops;
            Point line_positions[2];
            line_positions[0].fX = colrv1_paint.u.linear_gradient.p0.x / upem * scale.x();
            line_positions[0].fY = colrv1_paint.u.linear_gradient.p0.y / upem * (-scale.y());
            line_positions[1].fX = colrv1_paint.u.linear_gradient.p1.x / upem * scale.x();
            line_positions[1].fY = colrv1_paint.u.linear_gradient.p1.y / upem * (-scale.y());


            /* populate points */
            float stops[num_color_stops];
            Color colors[num_color_stops];

            FT_ColorStop color_stop;
            while (FT_Get_Colorline_Stops(
                    face, &color_stop,
                    &colrv1_paint.u.linear_gradient.colorline.color_stop_iterator)) {
                FT_UInt index = colrv1_paint.u.linear_gradient.colorline.color_stop_iterator
                                        .current_color_stop -
                                1;
                stops[index] = color_stop.stop_offset / float(1 << 14);
                FT_UInt16& palette_index = color_stop.color.palette_index;
                colors[index] = ColorSetARGB(
                        palette[palette_index].alpha * AlphaToFloat(color_stop.color.alpha),
                        palette[palette_index].red, palette[palette_index].green,
                        palette[palette_index].blue);
            }

            printf("Gradient linear: pos[0] (%f,%f) pos[1] (%f,%f), col[0] %u "
                   "col[1] %u, "
                   "stops[0] "
                   "%f stops[1] %f, num: %d\n",
                   line_positions[0].x(), line_positions[0].y(), line_positions[1].x(),
                   line_positions[1].y(), colors[0], colors[1], stops[0], stops[1],
                   num_color_stops);
            // [..Configure a linear gradient based on line_positions, colors and stops..
            //  .. and fill the drawing region with that..]
            break;
        }
        case COLR_PAINTFORMAT_RADIAL_GRADIENT: {
            // [..Do the same analogously as in LINEAR_GRADIENT..]
        }
        case COLR_PAINTFORMAT_TRANSFORMED:
        case COLR_PAINTFORMAT_ROTATE:
        case COLR_PAINTFORMAT_SKEW:
            // [...Apply the required transformations to your graphics context
          // based on colrv1_paint.u.transformed, colrv1_paint.u.rotate, colrv1_paint.u.skew etc....]
            break;
        }
        default:
          break;
    }
}