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The Epson Stylus Color/Photo printers don't have memory to print using all of the nozzles in the print head. For example, the Stylus Photo 700/EX has 32 nozzles. At 720 dpi, with an 8" wide image, a single line requires @math{(8 * 720 * 6 / 8)} bytes, or 4320 bytes (because the Stylus Photo printers have 6 ink colors). To use 32 nozzles per color would require 138240 bytes. It's actually worse than that, though, because the nozzles are spaced 8 rows apart. Therefore, in order to store enough data to permit sending the page as a simple raster, the printer would require enough memory to store 256 rows, or 1105920 bytes. Considering that the Photo EX can print 11" wide, we're looking at more like 1.5 MB. In fact, these printers are capable of 1440 dpi horizontal resolution. This would require 3 MB. The printers actually have 64K-256K.
With the newer (740/750 and later) printers it's even worse, since these printers support multiple dot sizes; of course, the even newer 2880x720 printers don't help either.
Older Epson printers had a mode called MicroWeave (tm). In this mode, the host fed the printer individual rows of dots, and the printer bundled them up and sent them to the print head in the correct order to achieve high quality. This MicroWeave mode still works in new printers, but in some cases the implementation is very minimal: the printer uses exactly one nozzle of each color (the first one). This makes printing extremely slow (more than 30 minutes for one 8.5x11" page), although the quality is extremely high with no visible banding whatsoever. It's not good for the print head, though, since no ink is flowing through the other nozzles. This leads to drying of ink and possible permanent damage to the print head.
By the way, although the Epson manual says that microweave mode should be used at 720 dpi, 360 dpi continues to work in much the same way. At 360 dpi, data is fed to the printer one row at a time on all Epson printers. The pattern that the printer uses to print is very prone to banding. However, 360 dpi is inherently a low quality mode; if you're using it, presumably you don't much care about quality. It is possible to do microweave at 360 DPI, with significantly improved quality.
Except for the Stylus Pro printers (5000, 5500, 7000, 7500, 9000, 9500, and when it's released the 10000), which can do microweave at any resolution, printers from roughly the Stylus Color 600 and later do not have the capability to do MicroWeave correctly in many cases (some printers can do MicroWeave correctly at 720 DPI). Instead, the host must arrange the output in the order that it will be sent to the print head. This is a very complex process; the jets in the print head are spaced more than one row (1/720") apart, so we can't simply send consecutive rows of dots to the printer. Instead, we have to pass e. g. the first, ninth, 17th, 25th... rows in order for them to print in the correct position on the paper. This interleaving process is called "soft" weaving.
This decision was probably made to save money on memory in the printer. It certainly makes the driver code far more complicated than it would be if the printer could arrange the output. Is that a bad thing? Usually this takes far less CPU time than the dithering process, and it does allow us more control over the printing process, e.g. to reduce banding. Conceivably, we could even use this ability to map out bad jets.
Interestingly, apparently the Windows (and presumably Macintosh) drivers for most or all Epson printers still list a "microweave" mode. Experiments have demonstrated that this does not in fact use the "microweave" mode of the printer. Possibly it does nothing, or it uses a different weave pattern from what the non-"microweave" mode does. This is unnecessarily confusing, at least for people who write drivers who try to explain them to people who don't.
What makes this interesting is that there are many different ways of of accomplishing this goal. The naive way would be to divide the image up into groups of 256 rows (for a printer with 32 jets and a separation of 8 rows), and print all the mod8=0 rows in the first pass, mod8=1 rows in the second, and so forth. The problem with this approach is that the individual ink jets are not perfectly uniform; some emit slightly bigger or smaller drops than others. Since each group of 8 adjacent rows is printed with the same nozzle, that means that there will be distinct streaks of lighter and darker bands within the image (8 rows is 1/90", which is visible; 1/720" is not). Possibly worse is that these patterns will repeat every 256 rows. This creates banding patterns that are about 1/3" wide.
So we have to do something to break up this patterning.
Epson does not publish the weaving algorithms that they use in their bundled drivers. Indeed, their developer web site (http://www.ercipd.com/isv/edr_docs.htm) does not even describe how to do this weaving at all; it says that the only way to achieve 720 dpi is to use MicroWeave. It does note (correctly) that 1440 dpi horizontal can only be achieved by the driver (i. e. in software). The manual actually makes it fairly clear how to do this (it requires two passes with horizontal head movement between passes), and it is presumably possible to do this with MicroWeave.
The information about how to do this is apparently available under non-disclosure agreement (NDA). It's actually easy enough to reverse engineer what's inside a print file with a simple Perl script, which is supplied with the Gimp-Print distribution as tests/parse-escp2. In any event, we weren't particularly interested in the weaving patterns Epson used. There are many factors that go into choosing a good weaving pattern; we're learning them as we go along. Issues such as drying time (giving the ink a few seconds more or less to dry can have highly visible effects) affect the quality of the output.
The Uniprint GhostScript driver has been able to do weaving for a long time. It uses patterns that must be specified for each choice of resolution and printer. We preferred an algorithmic approach that computes a weave pattern for any given choice of inputs. This obviously requires extensive testing; we developed a test suite specifically for this purpose.
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