Gradient Mesh is a grid-based painting technique in Illustrator that enables advanced colouring of vector objects with smooth and malleable colour transitions.
If you’re reasonably familiar with Illustrator’s Pen tool and other path-drawing, selection, and colour tools, you can use the Gradient Mesh feature to add realistic colouring, lighting, and 3D characteristics to a flat vector object. Ultimately, gradient mesh can give you the effect of photo-realistic painting with all the benefits and freedom of resolution-independent vector artwork.
The Illustrator Create Gradient Mesh dialog.
Gradient mesh vs. standard gradient: Standard gradients are extremely limited. You can choose between linear gradients, which transition from one colour to the next in a straight line, and radial gradients, in which colours transition between concentric circles. A gradient mesh can transition colors in any direction, in any shape, and can be controlled with the precision of anchor points and path segments.
Gradient mesh vs. object blend: Blending objects in Illustrator involves selecting two or more objects and creating intermediary objects that morph into each other. For example, the blend of a red triangle in front of a blue square would create additional objects between the two that gradually morph the shape and colour of the triangle into those of the square -- at the exact midpoint you get a purple shape that is half triangle, half square. Object blending is an effective way to create irregularly shaped color transitions and gradients, but it’s labour intensive. Each new gradient must be created as two objects -- the starting and ending color object -- and blended individually. A gradient mesh, on the other hand, is a single object inside of which different color areas transition into one another with the effect of a blended object.
Left to right: Colour transitions in a gradient mesh object, an object filled with a standard (radial) gradient, a blended object, and an image coloured in Photoshop
Gradient mesh vs. painting in Photoshop or Corel Painter: Painting in Photoshop or Painter is far easier than creating a gradient mesh. But such painting means working with fixed-resolution pixels. Conversely, a gradient mesh in Illustrator is a resolution-independent vector object, which means you can scale it up or down to infinity and maintain its original quality. Pixel-based imagery loses quality as it’s scaled up. Moreover, colours in a gradient mesh object can be perpetually adjusted and altered, whereas adjusting a painting in Photoshop requires re-painting, potentially destroying data.