1, modeling ability, that is, drawing what looks like.
2. There are three relationships and five tones in the sketch, namely, black, white, gray, light and dark boundary, projection, reflection, dark side and bright side.
3. Tone. Tones are generally divided into warm tones, cool tones, gray tones and high tones. The most basic thing is to understand the concepts of lightness, hue, chroma, contrast, similarity and so on.