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+The second thing you’ll notice is that exquisite shading effects are
+possible in 360x480 256-color mode; adjacent lines blend together
+remarkably smoothly, even with the default palette. The VGA allows you
+to select your 256 colors from a palette of 256K, so you could, if you
+wished, set up the colors to produce still finer shading albeit with
+fewer distinctly different colors available. For more on this and
+related topics, see the coverage of palette reprogramming that begins in
+the next chapter.
+
+The one thing you may not notice right away is just how much detail is
+visible on the screen, because the blending of colors tends to obscure
+the superior resolution of this mode. Each of the four rectangles
+displayed measures 180 pixels horizontally by 240 vertically. Put
+another way, each *one* of those rectangles has two-thirds as many
+pixels as the entire mode 13H screen; in all, 360x480 256-color mode has
+2.7 times as many pixels as mode 13H! As mentioned above, the resolution
+is unevenly distributed, with vertical resolution matching that of mode
+12H but horizontal resolution barely exceeding that of mode 13H—but
+resolution is hot stuff, no matter how it’s laid out, and 360x480
+256-color mode has the highest 256-color resolution you’re ever likely
+to see on a standard VGA. (SuperVGAs are quite another matter—but when
+you *require* a SuperVGA you’re automatically excluding what might be a
+significant chunk of the market for your code.)
+
+Now that we’ve seen the wonders of which our new mode is capable, let’s
+take the time to understand how it works.
+
+### How 360x480 256-Color Mode Works {#Heading5}
+
+In describing 360x480 256-color mode, I’m going to assume that you’re
+familiar with the discussion of 320x400 256-color mode in the last
+chapter. If not, go back to that chapter and read it; the two modes have
+a great deal in common, and I’m not going to bore you by repeating
+myself when the goods are just a few page flips (the paper kind) away.
+
+360x480 256-color mode is essentially 320x400 256-color mode, but
+stretched in both dimensions. Let’s look at the vertical stretching
+first, since that’s the simpler of the two.
+
+#### 480 Scan Lines per Screen: A Little Slower, But No Big Deal {#Heading6}
+
+There’s nothing unusual about 480 scan lines; standard modes 11H and 12H
+support that vertical resolution. The number of scan lines has nothing
+to do with either the number of colors or the horizontal resolution, so
+converting 320x400 256mode to 320x480 256-color mode is a simple matter
+of reprogramming the VGA’s vertical control registers—which control the
+scan lines displayed, the vertical sync pulse, vertical blanking, and
+the total number of scan lines—to the 480-scansettings, and setting the
+polarities of the horizontal and vertical sync pulses to tell the
+monitor to adjust to a 480-line screen.
+
+Switching to 480 scan lines has the effect of slowing the screen refresh
+rate. The VGA always displays at 70 Hz *except* in 480-scan-line modes;
+there, due to the time required to scan the extra lines, the refresh
+rate slows to 60 Hz. (VGA monitors always scan at the same rate
+horizontally; that is, the distance across the screen covered by the
+electron beam in a given period of time is the same in all modes.
+Consequently, adding extra lines per frame requires extra time.) 60 Hz
+isn’t *bad*—that’s the only refresh rate the EGA ever supported, and the
+EGA was the industry standard in its time—but it does tend to flicker a
+little more and so is a little harder on the eyes than 70 Hz.
+
+#### 360 Pixels per Scan Line: No Mean Feat {#Heading7}
+
+Converting from 320 to 360 pixels per scan line is more difficult than
+converting from 400 to 480 scan lines per screen. None of the VGA’s
+graphics modes supports 360 pixels across the screen, or anything like
+it; the standard choices are 320 and 640 pixels across. However, the VGA
+*does* support the horizontal resolution we seek—360 pixels—in 40-column
+*text* mode.
+
+Unfortunately, the register settings that select those horizontal
+resolutions aren’t directly transferable to graphics mode. Text modes
+display 9 dots (the width of one character) for each time information is
+fetched from display memory, while graphics modes display just 4 or 8
+dots per display memory fetch. (Although it’s a bit confusing, it’s
+standard terminology to refer to the interval required for one display
+memory fetch as a “character,” and I’ll follow that terminology from now
+on.) Consequently, both modes display either 40 or 80 characters per
+scan line; the only difference is that text modes display more pixels
+per character. Given that graphics modes *can’t* display 9 dots per
+character (there’s only enough information for eight 16pixels or four
+256-color pixels in each memory fetch, and that’s that), we’d seem to be
+at an impasse.
+
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+* * * * *
+
+Graphics Programming Black Book © 2001 Michael Abrash