= Canvas 3D Context =<i>[[VladVukicevic]], work in progress</i>Contents ==
== Rationale == Since <tt><canvas></tt> provides basically a rectangular immediate-mode drawing surface, extending it to programmatic 3D rendering is straightforward. * [[httpCanvas:3D//www.opengl.org/ OpenGL] is the natural accepted cross-platform DevDayBoston0307|Notes from 3D API to follow; it has a well-defined and rigorous specification that would be well beyond my ability to recreate. However, OpenGL itself (2.0 in its latest incarnation) is extremely sprawling, including many redundant ways and now-obsolete methods. Instead, I plan to follow [http://www.khronos.org/opengles/ OpenGL ES], which is a version of the OpenGL spec pared down for implementation in embedded systems. It removes much of the dead weight of OpenGL, e.g. rendering via <tt>glBegin</tt>session at Boston Devloper Day, <tt>glVertex[234][dfisMarch 2007][v]</tt>, etc.; rendering quads and polygons (since they can be emulated through triangles trivially); and similar. * [http://www.khronos.org/opengles/1_X/ OpenGL ES 1.1.4] is the current plan, which presents a traditional fixed-function OpenGL API. I would like to, at some point, support [httpCanvas://www.khronos.org/opengl/2_X/ OpenGL ES 2.0], which provides maps today's modern programmable hardware; however, implementing 2.0 can be done an incremental step over implementing the fixed-function API. One of the difficulties of OpenGL is that it is extremely low level, and in many ways looks strange when exposed as a web API. Where appropriate, the addition of helper objects and functions will be done to allow for speed and ease of use -- however, as with OpenGL on the desktop, it should be possible to create a richer API on top of OpenGL in whatever language is being used to interact with the 3D canvas context (e.g. JavaScript, Python, etc.). == Security Considerations == The OpenGL API is complex and stateful; much of the API, out of permance requirements, deals with pointers to arbitrary data types. For example, the function <tt>glVertexPointer(GLint size, GLenum type, GLsizei stride, const GLvoid *ptr)</tt> can take bytes, integers, floats, doubles, etc. as valid <tt>ptr</tt> types, based on the given <tt>type</tt>. After a vertex pointer is set, calling <tt>glDrawArrays(GLenum mode, GLint first, GLsizei count)</tt> to actually draw the given vertex/texture/etc. pointer arrays will require a different size of array depending on whether one is drawing TRIANGLES, TRIANGLE_STRIPS, TRIANGLE_FANS, etc. -- for example, assuming the appropriate types are used, TRIANGLES will need 3*count floats from vertices, 2*count floats from texcoords (assuming they're specified), 4*count floats from a color array, etc. OpenGL implementations usually handle an error by simply crashing; no size is passed when the various pointer arrays are set Historical|Historical notes (the <tt>size</tt> parameter refers to the number of coordinates per element, e.g. 3 for a 3D vertex). This is obviously not acceptable for a 3D original Canvas Context executing within a web browser; thus the implementation must be extrmely careful to track array sizes and the like and to check them before calling in to any underlying OpenGL implementation. == Performance == Initial testing reveals that performance can be quite good; a pbuffer context is created for the canvas, and copying into the front buffer is performed when SwapBuffers() is executed. Because all drawing is done through arrays, there isn't a lot of method call overhead for drawing operations. == Implementation Notes == - vertex/color/texcoord array data- matrix manipulation (load/push/pop/translate/rotate/scale/ortho/frustum/lookAt/matrixmode)- drawing via drawarrays or drawelements (indexes)- texturing from <img> (2D POT textures only, no TEXTURE_RECTANGLE or 1D/3D supporteddocumentation)]]
== Future Directions ==
