Part 1 and part 2 described CHR data, nametable based backgrounds, sprites, and scrolling. Combined, these cover nearly everything a basic NES cart can do without using additional hardware. To go any further will require a quick tangent to discuss, in detail, how rendering happens.
Scanline based rendering, with a pause for vblank
Like any older computer, the NES was designed to work with CRT TVs. They draw scanlines to the screen, one at a time, left to right, top to bottom, using an electron gun that physically moves to point at the screen where it draws these lines. Once the bottom corner is reached, a period called “vertical blank” (or vblank) happens, wherein the electron gun moves back to the top left to prepare to draw the next frame. On the NES, the PPU (Picture Processing Unit) does this scanline based rendering automatically, every frame, while code running in the CPU does whatever work the game needs to do. Vblank gives the software an opportunity to change data in the PPU’s memory, as otherwise that memory is being used for rendering. Most of the time, changes to the PPU’s nametable and palettes have to occur during this small window.
In Star Versus’s two player mode, the first thing players see is a menu to select their ships, handicaps, and arenas. Although straight forward in concept, this menu actually maxes out the NES’s graphics in multiple ways. Specifically, it uses each available palette, requires all 64 sprites, and places 8 sprites onto the same scanline. Only by using a couple of clever tricks can it be rendered at all.
Part 1 discussed the components of NES background graphics – chr, nametable, palettes, and attributes. But this was only half of the story.
For starters, there are actually two nametables . They each have their own attributes for determining color, though they share the same chr. Cartridge hardware determines their position, either side-by-side or stacked on top. Shown here are examples of games using each orientation, Lode Runner (1984) and Bubble Bobble (1988).
Released in 1983, the Nintendo Entertainment System (NES) home console was a cheap, yet capable machine that went on to achieve tremendous success. Using a custom designed Picture Processing Unit (PPU) for graphics, the system could produce visuals that were quite impressive at the time, and still hold up fairly well if viewed in the proper context. Of utmost importance was memory efficiency, creating graphics using as few bytes as possible. At the same time, however, the NES provided developers with powerful, easy to use features that helped set it apart from older home consoles. Understanding how NES graphics are made creates an appreciation for the technical prowess of the system, and provides contrast against how easy modern day game makers have it with today’s machines.
The background graphics of the NES are built from four separate components, that when combined together produce the image you see on screen. Each component handles a separate aspect; color, position, raw pixel art, etc. This may seem overly complex and cumbersome, but it ends up being much more memory efficient, and also enables simple effects with very little code. If you want to understand NES graphics, knowing these four components is key.
This document assumes some familiarity with computer math, in particular the fact that 8 bits = 1 byte, 8 bits can represent 256 values, and how hexadecimal notation works. However, even those without a technical background can hopefully find it interesting.
Here is an image from the opening scene of Castlevania (1986), of the gates leading to the titular castle. This image is 256×240 pixels, and uses 10 different colors. To represent this image in memory we’d want to take advantage of this limited color palette, and save space by only storing the minimum amount of information. One naive approach could be using an indexed palette, with 4 bits for every pixel, fitting 2 pixels per byte. This requires 256*240/2 = 30720 bytes, but as you’ll soon see, the NES does much a better job.