MS RFC 50: OpenGL Rendering Support¶
Toby Rahilly, Jonathan Bourke
toby.rahilly at gmail.com. bourke.jf at gmail.com
- Last Edited
This RFC proposes the addition of an Opengl Rendering module to MapServer for faster rendering of images.
Presently MapServer supports AGG rendering for high quality anti-aliased images. Although AGG is capable of high quality images, and is significantly faster than GD rendering, it is still not fast enough to suit desirable use cases such as live on-demand rendering.
Opengl is seen as a solution that can provide high performance rendering by making use of hardware specially designed for real-time rendering. Current Opengl prototypes are capable of rendering maps an order of magnitude faster than AGG with similar image quality.
The Opengl rendering module interface and path will be very similar to the current AGG rendering module. Like AGG, the Opengl module will make use of GD for I/O of the results. However due to the nature of Opengl there are a few key implementation differences that are worth noting.
Opengl can make use of pre-rendered textures. All symbols can be pre-rendered and stored as an Opengl texture, when they are used in rendering a map, the rendered texture is simply copied onto the image buffer with transformations. This can hugely increase the performance of complex and heavily repeated symbols, such as train tracks, because not only do they need to be only rendered once, Opengl also handles the repetition and transformations which it has been heavily optimised for.
Labels also benefit from this method greatly, as an entire alphabet can be pre-rendered and stored as textures. When a curved label is to be drawn, the Opengl module simply places the rotated textures of the characters.
Pre-rendered textures of symbols can also be shared between different map rendering instances. For this reason it is suggested that the rendering path be modified slightly, such that when a mapfile with opengl rendering is first loaded, all the symbols are pre-rendered and stored on the map object. This drastically increases the performance of batch rendering.
Another notable difference between AGG and Opengl is that Opengl is incapable of rendering into main memory, and instead renders into a inaccessible buffer in video memory. This means the image must be rendered into video memory in OpengGL, then copied to the GD image buffer before saving the result.
C API Changes¶
As the Opengl module follows very closely to the AGG implementation, very little change has to be made to the core of MapServer. Minor changes are made to the following files to add a rendering path for Opengl:
No new functionality is added to these files, rather just else-if blocks.
The bulk of Opengl functionality will be found in mapogl.cpp.
An output block like this will trigger Opengl rendering:
OUTPUTFORMAT NAME 'OGL_PNG24' DRIVER OGL/PNG IMAGEMODE RGB END
The guts of opengl rendering occurs inside of opengl drivers. Opengl drivers vary from hardware and operating system, and the rendering techniques vary with them. What this means is that maps rendered on different computers can have different results, such as color shades, anti-aliasing quality and performance.
Although Opengl is cross platform, each operating system has a different method of setting up an Opengl context to render into. Currently we have an implementation for pre-vista windows.
No issues expected.