The adventures of building a web renderer

RE: The adventures of building a web renderer
Geometries vs Buffer Geometries in Three.js

I found it to be a good idea to build the initial POC using geometry objects in Three.js. They offer a very simple interface to get started. However. If you have read any of the "Geometry vs BufferGeometry" threads on StackOverflow, etc., then BufferGeometries should be your choice if you want to render anything but the most basic of scenes.

I used this very "heavy" model to test the limits of my code base:

[Image: tWgxL6T.png]

It was not able to render in neither Firefox nor Chrome when using "Geometry". With "BufferGeometry" it took 10 minute to render and 1.2GB of memory!

The rendering time has since them been reduced to 6.6 seconds in Firefox and 8.8 seconds in Chrome. The memory usage has seen an even more dramatic decrease to 8.5MB!

Here are some steps and realizations that lead to this performance improvement.

Reducing the Number of Geometries

The first data model which used BufferGeometries had two "geometry" object for each part: One for the lines and one for the triangles (quads are split into triangles to simplify the data model. Besides. WebGL doesn't support quad primitives anymore anyway.)

The "Blueberry" from the first post was used to compare performance. For this data model it took 103MB of memory.

[Image: 200x200_14.png]
I changed the model to have geometries on "step"-level: Each step had a line geometry and a triangle geometry for each color of elements added in the step. This change meant nothing to the rendering of building instructions, since all parts of a step were added simultaneously.

The memory was reduced to 70MB!

The overhead of geometry objects is massive! Based on this result I started looking into other ways of reducing memory usage.

Naive Indexing

Trawling google searches for other common ways to optimize Three.js-based modeling lead me to the topic of "indexing". The idea was simple. Rather than storing points of lines and triangles as triplets of 32 bit floats (x,y,z), the points would be stored in a "static" data structure with the "dynamic" index being offsets into the static structure in order to identify points. Furthermore. If two points are identical (such as when two triangles share a corner), then points could be reused.

The data model had to be changed slightly to accommodate indexing. In the previous data model I used a simple algorithm to detect when lines had common endpoints. This algorithm had to be removed to allow for indexed lines, resulting in a render time of 27 seconds and 270MB of data.

Adding the indexing (but still no sharing of points) resulted in a reduction in render time to 2 seconds, while the memory usage was reduced to 36MB! 

This came as quite of a surprise to me. I was effectively using more memory when setting up the data model because the number of points remained the same while a list of indexes was added. The reason for the improved performance can be found in how Three.js handles attributes that are sent to shaders. Optimizing shaders will be a subject for a later post. I will leave you with this for now and continue with sharing of points in the next post.
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RE: The adventures of building a web renderer - by Lasse Deleuran - 2018-11-06, 19:26

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