In this article, I will show how the basic OpenGL ES 2.0/3.2 techniques, allowing to utilize the power of modern GPUs, can be used to draw 2D vector nautical charts by the example of S57 format. And as you probably already have guessed, the first technique I am going to consider is using vertex buffers for storing objects that makes up the chart. I will say at once that it is not a trivial task because, for example, some area in S57 chart can be filled and outlined with specific patterns generated from so called Presentation Library and at that, the fill pattern should be aligned to the left-top corner of the screen, but not to the area itself, so when the user shift the chart, the fill pattern does not move. The other example is a point objects or symbol that should be shown in the center of the visible part of an area, so its geographical coordinates changes when the user offsets or scales the chart. Theoretically, if we have OpenGL ES 3.2, we can use geometry shaders for drawing patterns, but if we have only OpenGL ES 2.0 that does not support geometry shaders, those patterns can also be drawn with the vertex buffers that should be regenerated each time the user changes the scale. So let’s start to solve this complex task with a simple example when we have geographic area or polygon filled with blue color, at least this example will demonstrate the basic idea of using the vertex buffers.
(more…)
