The "passes option" ([X] options screen or "passes=" parameter) selects one of the single-pass, dual-pass, triple-pass, solid-guessing (default), solid-guessing after pass n, boundary tracing, tesseral, or synchronous orbit modes. This option applies to most fractal types.
Single-pass mode ("1") draws the screen pixel by pixel.
Dual-pass ("2") generates a half-resolution screen first as a preview using 2x2-pixel boxes, and then generates the rest of the dots with a second pass. Dual-pass uses no more time than single-pass.
Triple-pass ("3") generates the coarse first pass of the solidguessing mode (see "g" below), then switches to either "1" (with low resolution video modes) or "2" (with higher resolution video modes). The advantage of "3" vs "2" is that when using high resolution modes, the first pass has a much lower resolution (about 160x120) and is therefore much quicker than the first pass of the passes=2 mode. However, with the "2" mode, the first pass does not represent wasted time. The "3" mode wastes the effort of generating the coarse first screen.
The single, dual, and triple pass modes all result in identical images. These modes are for those who desire the highest possible accuracy. Most people will want to use the guessing mode, described next.
Solid-guessing ("g") is the default. It performs from two to four visible passes - more in higher resolution video modes. Its first visible pass is actually two passes - one pixel per 4x4, 8x8, or 16x16 pixel box is generated, and the guessing logic is applied to fill in the blocks at the next level (2x2, 4x4, or 8x8). Subsequent passes fill in the display at the next finer resolution, skipping blocks which are surrounded by the same color. Solid-guessing can guess wrong, but it sure guesses quickly!
Solid-guessing stop after pass n ("g1" through "g6") are a variation on the guessing mode in which the algorithm stops after the nth pass. This facility is for exploring in low resolution when you'd rather see a low resolution image with large blocky pixels filling the whole screen than a small low resolution image such as you get with the [v] (View Windows) command. Note that on the [x] screen you can't directly type g1 or g2. Press g repeatedly until you get the option you want, or else use the left or right cursor keys.
Boundary Tracing ("b"), which only works accurately with fractal types (such as the Mandelbrot set, but not the Newton type) that do not contain "islands" of colors, finds a color boundary, traces it around the screen, and then "blits" in the color over the enclosed area.
Tesseral ("t") is a sort of "super-solid-guessing" option that successively divides the image into subsections and colors in rectangles that have a boundary of a solid color. It's actually slower than the solid-guessing algorithm, but it looks neat, so we left it in. This mode is also subject to errors when islands of color appear inside the rectangles.
Diffusion Scan ("d") is a draw type based on dithering techniques. It scans the image spreading the points evenly and to each point it paints a square of the appropriate size so that the image will be incrementally enhanced. This method calculates ALL the points in the image being a good substitute for Single/Dual/Triple pass and presents a quick visualization even in the slowest fractals. With "fillcolor=0" (below) the squares are not painted and the points are spread over the image until all have being calculated (sort of a "Fade In").
Synchronous orbits ("s") is an experimental mode using the "fractal witchcraft" algorithm based on the Almondbread implementation by Michael Ganss. This algorithm optimizes deep zooms by calculating parallel orbits starting at different points, and subdividing when the orbits break formation. Michael's implementation had to be extensively modified to work with Fractint's DOS medium memory model environment.
Synchronous orbits (also known as SOI) has some limitations. SOI is loosely coupled with fractint and most options don't work with it. Only types mandel and julia are implemented. SOI is only useful for very deep zooms, but only up to the limit of double precision. Within this narrow magnification range, SOI can result in tremndous speedups. If you invoke fractint with "debug=3444" on the command line, a long double (rather than double) version will be used, which allows zooming about 1000 times deeper. SOI really needs to be ported to fractint's arbitary precision. This will likely happen only after Fractint is moved to a better programming environment.
The "fillcolor=" option in the [X] screen or on the command line sets a fixed color to be used by the Boundary Tracing and Tesseral calculations for filling in defined regions. The effect of this is to show off the boundaries of the areas delimited by these two methods.