Hi Haider, the matrix is the design pattern created from your tonal areas. Specifically how light shapes interact with dark shapes. For example if you had two connected puzzle pieces, one white and one black, the shape of those solid tonal puzzle pieces would make up the matrix. The black puzzle piece could have a set of tonal values that fall into the dark tones category, and the white piece could have tones that fall into light tonal range as you develop the drawing.
Or, if you wanted to group a row of trees into the same value, that shape and its border with the surrounding tonal areas (the sky and ground) would form the matrix. It is simply the shape of your tonal areas.
In short, he is asking you to design grouped patterns comprised of darks, mid-tones and lights
In the second chapter Bill explains this better than I do 🙂
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