line-of-action.com
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Line of Action is an online practice tool for artists with a large library of reference photos for figure drawing and anatomy. It offers timed pose sessions and built-in shading/value practice to help you study shading techniques.
More resources on Shading Techniques
Draw a Box
Free lessons on shading forms and organic shading.
Ctrl+Paint
Free library of digital painting videos including shading.
Figure Drawing Fundamentals
Master figure drawing shading techniques with Stan Prokopenko's comprehensive course! Learn to sculpt form and create realistic figures.
proko.com
Proko.com is an art-education site offering comprehensive drawing tutorials, anatomy lessons, and shading techniques through step-by-step video demonstrations by professional artist Stan Prokopenko. It hosts both free and premium courses on figure drawing, portraiture, and other foundational techniques.
drawabox.com
Drawabox is a free online drawing course focused on fundamentals, especially shading and value, with structured lessons, practical exercises, and illustrated examples to help learners build solid form and shading technique.
Drawing Light & Shade
Drawing Light & Shade - A book resource