Pixels, vectors, machines

Code is a list of instructions. A program reads them in order, does the arithmetic, and writes the result somewhere: to the screen, or to a file.

Most creative-coding tools give you two places to write: setup, which runs once at the start, and draw, which runs again and again, many times a second, for animation. A still image for printing usually only needs setup, then a save.

Every mark has a position: x across, y down. On screens the origin (0, 0) is the top-left corner, and y grows downwards, the opposite of school graphs.

• Units. Screens count pixels. Plotters and printers think in millimetres. Decide early which your system uses, and convert once, at export.
• Angles are usually in radians, not degrees: a full turn is about 6.283 (2π), not 360.
• Size independence. Write positions as fractions of the width and height (0.5 × width is always the middle), so the same system works on screen and on A2.

A raster image is a grid of coloured pixels. A vector image is a set of instructions: draw a line from here to there, a circle this big. Zoom in and see the difference.

A pen plotter can only follow vectors. A printer needs pixels. If you might do both, keep your system's output as vectors and render pixels from them at the size you need.

A pixel has no size until you print it. PPI (pixels per inch) says how many pixels fit into each inch of paper. Prints need about 300 PPI to look sharp up close.

One inch is 25.4 mm, so pixels = millimetres ÷ 25.4 × PPI. Your screen shows far fewer pixels than an A2 print needs, so render the print file at full size from the system, never by enlarging a screenshot.

A colour profile (ICC) describes how a device shows colour. Using the printer and paper's profile lets you preview the print on screen before you spend paper on it: see the archival guide.

The CPU is a few powerful workers that do tasks one after another. The GPU is thousands of simple workers that each do the same small job at the same moment, which is exactly what filling millions of pixels needs.

• Most of your system runs on the CPU: reading data, following rules, building paths. That's fine for drawings of thousands of lines.
• The GPU matters for pixels: live animation, shaders, very large images, 3D previews.
• Slow isn't broken. A complex system may take minutes to render one A2 file. Render a small preview while you work, and the full size only for export.

Rendering is the work of turning your system's instructions into a finished image or file.

Keep the two apart in your code: a preview mode that's quick, and an export mode that's exact. The same seed must give the same result in both.

Computers can't really be random. They use a formula that produces numbers that look random, starting from a number called the seed. Same seed, same sequence, same artwork.

That's what makes a generative work reproducible: the seed and the parameters are its recipe. Name every output by its seed; the system worksheet shows how. Noise, the smooth kind of randomness behind flow fields, is explained with a live sketch in Techniques.

A mesh describes a 3D form as many small triangles joined at their corners. For resin printing it must be watertight: closed all round, with no gaps, so the printer knows what's inside and outside. See the resin guide.