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VGA TEXT ATTRIBUTES AND SPECIAL EFFECTS Written by Chris Dunford Edited by Jim Fowler This document is an attempt to explains how VGA text attributes work, and how the effects in FXCOLOR are possible. It also provides tips for creating your own special effects. What's different with the VGA? ------------------------------ The CGA implemented a direct relationship between a 4-bit attribute and a color. Each attribute mapped to one color, and the color could not be changed. That changed with the EGA (which had palette registers to play with), and changed again, radically, with the VGA. The attribute number specifies the displayed color very indirectly; it is combined with bits and pieces of 5 different internal VGA registers before it appears on the screen as one of the 262,144 possible colors. There are, therefore, several ways to change a character's color other than by changing its attribute. And, of course, if you change the way an attribute is displayed, all characters with that attribute are affected. Thus, it is possible to "design" attributes with interesting effects, and to alter the way entire screen fields are displayed very efficiently. Mapping attributes to colors ---------------------------- Before we start, let me note that we are working here with 4-bit attributes. The 8-bit attribute that's associated with a character is, of course, two 4-bit attributes: one for the foreground, and one for the background. This document discusses how these 4-bit attributes become colors; everything here applies equally to foreground and background attributes (thus it is possible to use blinking backgrounds, etc.) Screen colors in text modes are controlled by the VGA's attribute controller. In one of its two modes (the one that the PS/2 BIOS sets by default), here is how a color is selected by an attribute: AND attribute |-------| color plane enable register | | selects | palette register 0-15 color select register | | | | | bits 0-5 bits 6-7 | +------------+------------+ | | AND +-------+--------video DAC mask register | | selects | video DAC color register 0-255 This appears complex, but it can be simplified. First, notice the two logical AND operations. Since both the color plane enable register and the video DAC mask register normally contain all ones, they tend to drop out of the picture, resulting in: attribute 0-15 | | selects | palette register 0-15 color select register | | | | | bits 0-5 bits 6-7 | +------------+------------+ | | selects | video DAC color register 0-255 The 4-bit attribute (0-15) selects one of the 16 palette registers. The palette register contains a number. This number is combined with the contents of the color select register to form another number. For example, suppose: attribute = 13 palreg[13] = 1Bh CSR = 3 Attribute 13 selects palette register 13, which contains 1Bh. The color select register contains 3. We put the contents of the color select register in bits 6-7 of the final number, and the contents of palreg[13] in bits 0-5: 3 1Bh -- ------ 11 011011 The resulting number is 11011011 binary, 219 decimal. This number selects one of the 256 video DAC color registers. The color that will be displayed is the color defined by the selected video DAC color register (DAC stands for Digital-to-Analog Converter). Each video DAC register is an 18-bit register that contains three 6-bit values: one for each of the red, green, and blue primary colors. Each value specifies the intensity of that primary color on the screen. A zero for a primary means that it's not included in the color; 63 is maximum intensity. Suppose, in our example, that video DAC register 219 contains the numbers 0, 3Fh, and 3Fh (it's convenient to think of the register as three 6-bit values rather than one 18-bit value). This means that red will be off, and green and blue will be at maximum intensity. The color displayed by attribute 13 will be bright cyan. Notice that you could change the color of a character with FG attribute N in four different ways: 1. Change the attribute 2. Change the contents of palreg[N] 3. Change the contents of the video DAC register specified by palreg[N] and the color select register 4. Change the contents of the color select register These all have different effects on the total screen display. Changing an attribute affects a single character; changing a palette register affects all characters with that attribute; changing a video DAC register affects any character whose attribute, when combined with color select, selects that register -- this could be several attributes; changing the color select register could very well affect everything on the screen. Another way to visualize color selection ---------------------------------------- There is a simpler way to look at the process. The two bits used from the color select register always end up in the high two bits of the video DAC color register number. Thus, the selected DAC color register will always be one of: CSR=0: 00xx xxxx (register range 0-63) CSR=1: 01xx xxxx (64-127) CSR=2: 10xx xxxx (128-191) CSR=3: 11xx xxxx (192-255) The remaining 6 bits come directly from the selected palette register. If we break the 256 video DAC color registers into four palettes of 64 colors each, as follows: DAC registers Palette 0-63 0 64-127 1 128-191 2 192-255 3 then the color select register selects one of the four 64-color palettes, and the palette register (which is selected by the attribute) selects one of the 64 colors of the CSR-selected palette. Thus, it's simply stated: the attribute selects one of the 64 colors available from the palette selected by the color select register. Mode 1 ------ I mentioned that the above represents one of the two modes supported by the attribute controller; call it "mode 0". The other mode (mode 1) is very similar; the only difference is in which bits are combined from the palette register and the color select register. The simplified diagram is as follows: attribute 0-15 | | selects | palette register 0-15 color select register | | | | | bits 0-3 bits 4-7 | +------------+------------+ | | selects | video DAC color register 0-255 The difference is that there are four bits each from the palette register and the color select register (instead of 6 and 2 under the mode 0). The effect of mode 1 is that the 256 video DAC color registers are broken down into sixteen 16-color palettes instead of four 64-color palettes: DAC registers Palette 0-15 0 16-31 1 32-47 2 ... 240-255 15 The palette register (still selected by the attribute) selects one of the 16 colors available in the palette selected by the current color select register. The more complex effects (fading, pulsing, etc.) of FXCOLOR are generated under mode 1. This is because all 256 colors defined by the video DAC color registers can be accessed by simply changing the contents of the color select register, which is very efficient (it takes less than 1/100th of a second). Under mode 0, only 75% of the available colors can be accessed by just changing the color select register (this is because there are 64 colors per palette, but only 16 attributes). To get to the remaining colors, you have to change the palette registers. But there are 16 palette registers; if you need to alter more than one displayed color, this is less efficient than using the color select register. A mode 1 technique ------------------ As mentioned, there are a number of ways to alter displayed colors. I will concentrate on one technique: creating a series of palettes and then switching palettes by systematically changing the contents of the color select register. This document concentrates on mode 1; some of the same effects (such as periodic intensification and simulated blinking) can be obtained in mode 0. Note that the use of mode 1 largely dictates that you must reinitialize the palette registers. The VGA BIOS initializes some of these registers with values greater than 15; these will be masked in mode 1 to the low 4 bits, yielding values in the range 0..15. These values will probably not result in the colors you intend (two attributes will yield red foreground, for example). The simplest solution is to just fill the palette registers sequentially with numbers from 0..15; this largely causes the palregs to drop out of the picture: attribute N will map directly to color N within the palette selected by the color select register. Assuming that the palette registers are set as described, create a "base" palette in palette 0. That is, fill palette 0 (video DAC color registers 0-15) with 16 base color definitions. It makes sense to set the standard color combinations for the 16 CGA-like colors that people will expect to see (color intensity values in hex): DAC regs Attr R G B Color 0-2 0 0 0 0 Black 3-5 1 0 0 2A Blue 6-8 2 0 2A 0 Green 9-11 3 0 2A 2A Cyan 12-14 4 2A 0 0 Red 15-17 5 2A 0 2A Magenta 18-20 6 15 2A 0 Brown 21-23 7 2A 2A 2A White 24-26 8 15 15 15 Grey 27-29 9 0 0 3F Bright Blue 30-32 10 0 3F 0 Bright Green 33-35 11 0 3F 3F Bright Cyan 36-38 12 3F 0 0 Bright Red 39-41 13 3F 0 3F Bright Magenta 42-44 14 3F 3F 0 Yellow 45-47 15 3F 3F 3F Bright White (In FXCOLOR, 35h is used in place of 3Fh. This leaves some room to pulse even the bright colors by intensification.) The second step is to duplicate palette 0 into palettes 1-15, and then vary the color definitions for attributes of interest in some systematic way. For example, let's specify that attribute 1 is not going to be "blue"; it's going to be a green that continuously cycles from normal intensity to high intensity and back. The effect is similar to blinking text, but more subtle. To do this, set the colors for attribute 1 in each of the palettes as follows (RGB values in hex): Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 2A 2C 2E 31 33 35 36 37 38 39 3A 3B 3C 3D 3E 3F B 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Note that the intensity of the green primary slowly increases from normal (2Ah) in palette 0 to maximum intensity (3Fh) in palette 15. Also note that attribute 1 no longer has anything to do with blue -- it's green. If these values are loaded into the video DAC color registers, and you then sequence through the palettes in a cyclical pattern (0->15->0), characters with attribute 1 will slowly "pulse" from green to bright green. The effect is more subtle - and readable - than simply switching the characters off and on. The palette change is easily and efficiently accomplished by setting the color select register -- say, in a timer tick intercept routine. Using a timer intercept is an ideal way to accomplish this sort of special effect. It is both efficient and flexible: the speed of the effect can be adjusted by altering the values of either the incremental rate of the palette changes and/or the interval period of timer ticks before a change is made. Increasing the increment value for palette changes causes the pulse rate to speed up by skipping palettes; increasing the interval period between timer ticks before making a change slows the pulsing by skipping clock ticks. Effects ------- Many "special effects" are possible using this technique. PERIODIC INTENSIFICATION Pulsing is a gradual change from a low intensity color to a high intensity color. The effect can be hardened (resulting in blinking) by using the normal color for half of the palettes and an intensified version for the other half. For example, the following palettes blink cyan to bright cyan (RGB values in hex): Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 2A 2A 2A 2A 2A 2A 2A 2A 3F 3F 3F 3F 3F 3F 3F 3F B 2A 2A 2A 2A 2A 2A 2A 2A 3F 3F 3F 3F 3F 3F 3F 3F [ fx_Blink( "bg", "bg+" ) ] The effect can be varied by altering the ratio of low intensity palettes to high intensity palettes ("beacons", below, are simply extreme cases of periodic intensification). COLOR CHANGES Another effect is to blink text from one color to another. This set of palettes blinks bright green to bright red: Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 3F 3F 3F 3F 3F 3F 3F 3F 0 0 0 0 0 0 0 0 B 0 0 0 0 0 0 0 0 3F 3F 3F 3F 3F 3F 3F 3F [ fx_Blink( "g+", "r+" ) ] GRADED COLOR CHANGES An interesting effect is to "grade" (fade) one color into another. Rather than simply blinking green to red, change it gradually by fading out the green primary and fading in the red (RGB values in hex): Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 2 4 6 9 0C 0F 12 15 18 1B 1E 21 24 27 2A G 2A 27 24 21 1E 1B 18 15 12 0F 0C 9 6 4 2 0 B 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 [ fx_Fade( "g", "r" ) ] This attribute will still change from green to red, but the change is not instantaneous; it slowly fades from one color to the other, passing through various other unnamed colors (all combinations of red and green) on the way. SIMULATED BLINKING Regular hardware blinking can be simulated via a variant of the above scheme; simply use the background color for half of the palettes (or the foreground color, if you want to flash the background) (RGB values in hex): Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 3F 3F 3F 3F 3F 3F 3F 3F 0 0 0 0 0 0 0 0 G 3F 3F 3F 3F 3F 3F 3F 3F 0 0 0 0 0 0 0 0 B 0 0 0 0 0 0 0 0 2A 2A 2A 2A 2A 2A 2A 2A [ fx_Blink( "gr+", "b" ) ] The example flashes yellow text if displayed on a blue background. SOFTENED BLINKING By placing the background attribute in half of the palettes (as for flashing) and fading out the foreground in the other half, a softer version of blinking is possible (RGB values in hex): Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 2A 24 1E 19 14 0F 0A 5 0 0 0 0 0 0 0 0 B 2A 24 1E 19 14 0F 0A 5 0 0 0 0 0 0 0 0 [ fx_Blink( "bg", "n", .T. ) ] The example shows softened blinking of cyan text on a black background. If any of the background primaries are also components of the foreground color, do not fade those (RGB values in hex): Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 2A 24 1E 19 14 0F 0A 5 0 0 0 0 0 0 0 0 B 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A [ fx_Blink( "bg", "b", .T. ) ] The example shows softened blinking of cyan on blue. FADING This is an even softer version of blinking text. It fades the foreground over the full 16 palettes by grading the foregound color into the background color (RGB values in hex): Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 2A 27 24 21 1E 1B 18 15 12 0F 0C 9 6 4 2 0 B 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 [ fx_Fade( "g", "n" ) ] The example fades a green foreground into a black background. If the background is a color other than black, a graded color change from the FG color to the BG color is used. STROBES AND BEACONS These are two other effects (not in the demo). A strobe is a color that's invisible most of the time (BG=FG) but flashes briefly to maximum intensity. The following palettes strobe yellow when used with a blue background (RGB values in hex): Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3F G 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3F B 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 0 A beacon is a low intensity color that briefly flashes to maximum intensity. The following palettes create a blue beacon (RGB values in hex): Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 B 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 3F IMPORTANT: Note that only palettes that are a multiple of the palette incremant rate are ever used. E.g., if the palette increment rate is 2, palette 15 will not be used; put the strobe or beacon color in palette 14 instead. Advantages/disadvantages ------------------------ Creating special effects in this manner has several advantages over using hardware effects (such as blinking), and some disdvantages. Among the advantages are: - MANY more effects are possible; I have mentioned only a few of the possibilities. - Transition rates are under your control; the hardware blink rate is fixed. - Effects can be mixed on one screen (one attribute could be a strobe, another a graded color change). - Any effect can be applied to background as well as foreground. Among the disadvantages are: - A background timer routine has some effect on system efficiency (but it's small). - Effects are associated with attribute numbers, not with bit positions. The last one bears some explanation. Hardware blinking is associated with a single bit (bit 7) of the FG/BG attribute byte. Thus, you have 16 FG colors, all of which can be blinked. Software-controlled special effects, however, are associated with attributes. If you set, say, attribute 6 to be pulsed green, you no longer have a brown attribute available. Thus, the number of effective "base" colors is reduced for each special effect attribute you create. Mode 0 ------ I have covered attribute control mode 1 in some detail, but some of the same effects can be accomplished in mode 0. The primary difference is that you have only four palettes to work with. The simpler effects such as color changes, periodic intensification, and simulated blink can easily be accomplished in mode 0 by putting the second color in palette 1, and then alternating between palettes 0 and 1. More complex effects would require periodic alterations to the palette registers and/or color plane enable register. Programming ----------- The programming required to accomplish these special effects is relatively straightforward. The VGA BIOS provides the necessary services; but register-level programming is used in FXCOLOR to eliminate distracting "blips" caused when RGB values or palettes are changed. The linkable object module -------------------------- FXCOLOR.OBJ(ASM) provides a linkable object module containing callable functions for Clipper 5.01 that can be used to generate all of the special effects described above, and others. Be sure to call fx_Enable() when you start and fx_Disable() when you exit. The demo program (FXDEMO.PRG) demonstrates use of the commands. Author and reference -------------------- The original document is by: Chris Dunford CompuServe ID# 76703,2002 The original document is copyright (C) 1989 by Chris Dunford. Permission is granted to distribute freely by electronic or other means, but it may not be republished without permission. This document is an edited version of the original and is presented here with permission. Edited by: Jim Fowler CompuServe ID# 73340,3325 Clipper is a registered trademark of Nantucket Corp.