Joint Stroke Tracing and Correspondence for 2D Animation - TOG & SIGGRAPH 2024

[Paper] | [Project Page]

This code is used for producing stroke tracing and correspondence results, which can be imported into an inbetweening product named CACANi for making 2D animations.

Outline

• Dependencies
• Quick Start
• Vector Stroke Correspondence Dataset
• Citation

Dependencies

• cudatoolkit == 11.0.3
• cudnn == 8.4.1.50
• pytorch == 1.9.0
• torchvision == 0.9.0
• diffvg
• Krita: for making reference vector frame
• CACANi: for making inbetweening and 2D animation

Quick Start

Model Preparation

Download the models here, and place them in this file structure:

models/
    quickdraw-perceptual.pth
    point_matching_model/
        sketch_correspondence_50000.pkl
        transform_module/
            sketch_transform_30000.pkl
    stroke_tracing_model/
        sketch_tracing_30000.pkl

Create Reference Vector Frames with Krita

Our method takes as inputs consecutive raster keyframes and a single vector drawing from the starting keyframe, and then generates vector images for the remaining keyframes with one-to-one stroke correspondence. So we have to create the vector image for the reference frame here.

Note: We provide several examples for testing in directory sample_inputs/. If you use them, you can skip step-1 and step-2 below and execute step-3 directly.

1. Our method takes squared images as input, so please preprocess the images first using tools/image_squaring.py:

python3 tools/image_squaring.py --file path/to/the/image.png

2. Follow tutorial here to make vector frames as a reference in svg format with Krita.
3. Place the svg files in sample_inputs/*/svg/. Then, convert them into npz format using tools/svg_to_npz.py:

cd tools/
python3 svg_to_npz.py --database ../sample_inputs/rough/ --reference 23-0.png

Execute the Main Code

Perform joint stroke tracing and correspondence using sketch_tracing_inference.py. We provide several examples for testing in directory sample_inputs/.

python3 sketch_tracing_inference.py --dataset_base sample_inputs --data_type rough --img_seq 23-0.png 23-1.png 23-2.png

• --data_type: specify the image type with clean or rough.
• --img_seq: specify the animation frames here. The first one should be the reference frame.
• The results are placed in outputs/inference/*. Inside this folder:
  • raster/ stores rendered line drawings of the target vector frames.
  • rgb/ stores visualization (with target images underneath) of the vector stroke correspondence to the reference frame.
  • rgb-wo-bg/ stores visualization without target images underneath.
  • parameter/ stores vector stroke parameters.

Create Inbetweening and Animation with CACANi

1. Convert the output npz file (vector stroke parameters) into svg format using tools/npz_to_svg.py:

cd tools/
python3 npz_to_svg.py --database_input ../sample_inputs/ --database_output ../outputs/inference/ --data_type rough --file_names 23-1.png 23-2.png

• The results are placed in outputs/inference/*/svg. There are two kinds of results:
  • chain/: the svg files store stroke chains, defining each path as a chain.
  • separate/: the svg files store separated strokes, defining each path as a single stroke. Note that the automatic inbetweening in CACANi relies on this format.

2. Follow tutorial here to generate inbetweening and 2D animation with CACANi.

More Tools

• tools/vis_difference.py: visualize difference between the reference image and the target one. The results are placed in sample_inputs/*/raster_diff/

cd tools/
python3 vis_difference.py --database_input ../sample_inputs --data_type rough --reference_image 23-0.png --target_image 23-1.png

• tools/make_inbetweening.py: visualize the inbetweening in a single image or a gif file.

Vector Stroke Correspondence Dataset

We collect a dataset for training with 10k+ pairs of raster frames and their vector drawings with stroke correspondence. Please download it here. We provide a reference code dataset_utils/tuberlin_dataset_util.py showing how to use the data.

Citation

If you use the code and models please cite:

@article{mo2024joint,
  title={Joint Stroke Tracing and Correspondence for 2D Animation},
  author={Mo, Haoran and Gao, Chengying and Wang, Ruomei},
  journal={ACM Transactions on Graphics},
  year={2024},
  publisher={ACM New York, NY}
}