Step 54 Software: Low level functions

We have made a small library of low level graphic functions.

There are three main reasons for doing this.

Memory footprint

The easiest way to address each voxel would be through a three dimensional buffer array. Like this:

unsigned char cube[x][y][z]; (char means an 8 bit number, unsigned means that it's range is from 0 to 255. signed is -128 to +127)

Within this array each voxel would be represented by an integer, where 0 is off and 1 is on. In fact, you could use the entire integer and have 256 different brightness levels. We actually tried this first, but it turned out that our eBay LEDs had very little change in brightness in relation to duty cycle. The effect wasn't noticeable enough to be worth the trouble. We went for a monochrome solution. On and off.

With a monochrome cube and a three dimensional buffer, we would be wasting 7/8 of the memory used. The smallest amount of memory you can allocate is one byte (8 bits), and you only need 1 bit to represent on and off. 7 bits for each voxel would be wasted. 512*(7/8) = 448 bytes of wasted memory. Memory is scarce on micro controllers, so this is a sub-optimal solution.

Instead, we created a buffer that looks like this:

unsigned char cube[z][y];

In this buffer the X axis is represented within each of the bytes in the buffer array. This can be quite confusing to work with, which brings us to the second reason for making a library of low level drawing functions:

Code readability

Setting a voxel with the coordinates x=4, y=3, z=5 will require the following code:

cube[5][3] |= (0x01 << 4);

You can see how this could lead to some serious head scratching when trying to debug your effect code ;)

In draw.c we have made a bunch of functions that takes x,y,z as arguments and does this magic for you.

Setting the same voxel as in the example above is done with setvoxel(4,3,5), which is _a lot_ easier to read!

draw.c contains many more functions like this. Line drawing, plane drawing, box drawing, filling etc. Have a look in draw.c and familiarize yourself with the different functions.

Reusable code and code size

As you can see in draw.c, some of the functions are quite large. Writing that code over and over again inside effect functions would take up a lot of program memory. We only have 32 KB to work with. Its also boring to write the same code over and over again ;)