The maths isn't hard, but you do need a decent grasp of vectors and (optionally) 3D transformation matrices, and it's pretty opaque until you figure out the terminology.
I haven't done GPU raymarching myself but I've used the same techniques on the CPU side in order to develop a whole-planet procedural renderer (in Ada, of course); the raymarching was relatively straightforward but I got horribly bogged down trying to make a volumetric atmosphere work.
(Also known as iq, he's responsible for a number of amazing demos, including the 4kB demo _Elevated_: https://www.youtube.com/watch?v=jB0vBmiTr6o He also appears to have written ShaderToy. Search for iq to see his stuff omfg ShaderToy lets you write shaders to produce _music_ now? https://www.shadertoy.com/view/ldXXDj)
I haven't done GPU raymarching myself but I've used the same techniques on the CPU side in order to develop a whole-planet procedural renderer (in Ada, of course); the raymarching was relatively straightforward but I got horribly bogged down trying to make a volumetric atmosphere work.
One resource I found really useful was Iñigo Quilez' website; in particular, his article on basic terrain raymarching explains most of the concepts: http://www.iquilezles.org/www/articles/terrainmarching/terra...
(Also known as iq, he's responsible for a number of amazing demos, including the 4kB demo _Elevated_: https://www.youtube.com/watch?v=jB0vBmiTr6o He also appears to have written ShaderToy. Search for iq to see his stuff omfg ShaderToy lets you write shaders to produce _music_ now? https://www.shadertoy.com/view/ldXXDj)