Beginner's Guide to Egfx

by Bret Victor

Egfx is cool. I swear. When I first wrote it, I thought it was kinda neat, nothing special... but when I actually used it for the first time on my ditch day stack, I had an enormous amount of fun. I made a maze game where the maze was many times the screen size and the screen panned around the maze, "glowing" animations with things fading in and out, and lots of other stuff. Egfx made it all really easy.

Egfx can be a little complicated at first (especially if you didn't actually write it yourself), so hopefully I can explain how to use it...

What is Egfx?

Egfx is what you use if you want to draw pictures on the LED screen, instead of just text. It supports multiple, arbitrarily-sized drawing buffers, multiple "colors" (shades of intensity) and real drawing objects such as lines and rectangles. All the cool kids use Egfx, and that's why they are constantly surrounded by beautiful scantily-clad women (or men, depending on their tastes). Shouldn't you use Egfx too?

Starting Up

When you start up Egfx with Egfx_SetMode , you specify how many colors you want (numcolors) and what the screen refresh rate should be. There are some trade-offs involved, because if the product of these two numbers is too high, the processor will be slowed down or the screen will flicker horribly. The refresh rate though should be above 50 Hz or there will be a noticable flicker. The general rule is to use the fewest colors and slowest refresh rate that still looks good. I think that at 60 Hz, seven or eight colors is about the max, but you can experiment.

Buffers and Colors

When you draw in Egfx, you draw into a "buffer", which is not necessarily connected to the physical screen. The buffer is created with Egfx_CreateBuffer and can be of arbitrary size. A one-byte value (the "pixel color") is associated with each pixel in the buffer. The intensity of the pixel on the screen is determined by its color as well as the baseline and numcolors. baseline is a parameter that you can set which determines what color is considered "off" or "black". All colors that are baseline or less do not light up. Colors that are (baseline + numcolors - 1) or greater are at full intensity. Colors in between are proportionally dimmer. You can play neat tricks with the baseline parameter, such as slowly increasing or decreasing it to make the screen "fade out" or "fade in". baseline starts out set to zero to make things simple, but it is recommended that you set it to a value closer to midrange such as 128 (and then Wipe) to give yourself more freedom.

Cursors and Drawing

The "cursor" is the position within the buffer where the next drawing command will start drawing. There are a number of commands that deal with moving the cursor around. In addition, some of the drawing commands themselves move the cursor while they draw. You often want the cursor to move in some direction after a drawing command completes; for this, the automove parameter can be used to move the cursor in one of the four directions (or execute a graphics script resource) after each drawing command. Whenever the cursor is told to go past the edge of the buffer, it wraps around to the other side.

A drawing command changes the pixel color of the pixels it draws on. How exactly it changes them depends on the drawing mode. A drawing mode might use the color parameter, the delta parameter, or neither.

Putting It Onscreen

In order for anyone to see what's in the buffer, the buffer (or part of it) has to be mapped to the physical screen. The easiest way to do this is with ScreenSet, which maps the entire screen to a chunk of the buffer. Note that the buffer appears to "wrap around" in the horizontal direction; if the chunk is too close to the right edge of the buffer, part of the left edge of the buffer will be shown on the screen. And if the buffer's width is less than the width of the screen, you will see identical pictures side-by-side. Note also that there is no wrap around in the vertical direction -- you should avoid mapping the screen off the bottom edge of the buffer.

You can also map a chunk of the buffer to only part of the screen, using RangeSet. This command lets you set a range of onscreen columns instead of all of them.

Once your buffer is mapped to the screen, you can make it appear it move around using the Pan commands. These commands shift the buffer mappings according to the pan parameter. For example, if the screen is mapped so that the top-left corner is position (0,0) in the buffer and you pan right, the top-left corner will now be at position (1,0) and the picture will appear to have shifted left. You can implement quite a number of impressive effects just by panning around. Many types of animations can be implemented with panning or screen/range setting, and it is much quicker than redrawing.

Multiple Buffers

You can call Egfx_CreateBuffer as much as you like to allocate more drawing buffers. Use SetBuffer to set which buffer you are drawing to, and use ScreenSet or RangeSet to map it to the screen or part of the screen. The stored cursor positions are properties of the current drawing buffer, and change when you switch buffers. All other parameters stay the same though, including the cursor position (although it might "wrap around" if it is out of the range of the buffer being switched to). When a Pan command is given, each range of the screen pans through whichever buffer it's been set to.

Does this mean you can do "double-buffered" animations? Of course! Also, some pretty interesting effects can be implemented by mapping differently sized buffers to different parts of the screen and panning through them.


All Egfx commands are given in "scripts", which simply consist of the command numbers, followed by their parameters if any, and ending with a zero byte. Libcoke may generate graphics scripts for you, if it exists. You can execute a graphics script directly with Egfx_ScriptInline, or you can upload a script as a resource. Resource scripts can be executed with Egfx_ScriptResource, called from within other scripts with the ScriptResource graphics command, used for AutoMove, or executed periodically automatically using Egfx_StartAutoScript.

Gfx and Egfx

Neither Gfx nor Egfx erase their buffers when you turn them on. Therefore, it is conceivable to be, say, scrolling some text with Gfx while displaying an animation with Egfx, and switching between them by calling Gfx_SetMode and Egfx_RestoreMode. Note that when you enable one graphics mode, it automatically turns the other one off, so you don't have to worry about both of them accidently being enabled together.

Have fun!

As always, feel free to ask me anything that's on your mind: And of course, if you feel like adding anything to Egfx or implementing the commands that haven't been implemented yet, that would be great.