Pencil drawing is an important artistic style which is widely used in draft sketch and finished rendering. It is non-trivial if we can automatically generate the pencil drawing more precisely in order to ease the human efforts. In this paper, we propose flow-guided sketch and accentuated-tone adjusting techniques to transform the pencil drawing from natural images. This work is extended from the existing pencil drawing framework where authors combine sketch and tone together for penciling purpose. Through an edge tangent flow guided classification and convolution, we can obtain more coherent and smooth strokes, especially for portraits. In addition, we also introduce a hierarchical approach to generate strokes. According to saliency map, we can adjust the tone of images automatically. The experimental results indicate the generated toned pencil drawings by our approach are of fewer artifacts and could even solve the images with dark background
This paper proposes a fast and accurate algorithm for indirect illumination. It uses volumes of different resolutions to sample and cache the geometric information and the secondary lights. By dividing the irradiance into two parts, it treats the lights coming from the far-field and that coming from the near-field differently. For the far-field ones, it propagates sphere harmonic represented lights on coarse voxels. For the near-field ones, it shoots rays and collects their contributions on fine voxels. By doing this, the algorithm in this paper avoids using many rays to march long distance. In the experiments, it renders about ten times faster than the VGI algorithm to get the same image qualities, especially for the large and complex scenes. Meanwhile, it further accelerates the rendering by inventing an incremental multi-resolution gathering. The experiments illustrate fast and accurate indirect light effects.