drawabox.com
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Drawabox is a structured drawing fundamentals resource offering free lessons, drills, and assignments that teach basic sketching from lines and shapes, covering construction, perspective, and form. It provides instructional videos and written guides to practice shading and improve accuracy.
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Composition Library
Master advanced composition techniques with Matt Kohr's Composition Library! Learn to create compelling art through proven visual principles.
line-of-action.com
Line of Action is a free drawing-practice site that provides a pose generator and photo references to help you practice gesture drawing. It offers timed sessions with customizable content—from human figures and hands to faces and animals—so you can build speed and accuracy.
Sketching from the Imagination
Anthology of sketchbook pages from fifty concept artists and illustrators, each accompanied by the artist's comments on process, tools and inspiration. Shows how professionals generate characters, creatures and environments from imagination, offering approaches readers can borrow for their own sketchbook practice.