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VKON / TVision-0.20 / tvision.git / examples / palette / README.md

Understanding & Using Turbo Vision's Palette

TI813C.txt   Understanding & Using Turbo Vision's Palette.
Category    :General
Platform    :All
Product     :Borland C++  3.1

7/2/98 10:39:22 AM

Description

The palette system in Turbo Vision is designed to make it easy for either the programmer or the user to customize the colors of an application. The system uses an object oriented approach which relies on an implementation where the colors of a particular view are dependent on the colors of the owner view. So, if there is a view X that can appear inside either of two TWindow objects A and B, the colors seen by the user will depend on whether it is a child of A or B at that moment.

There are two ways of thinking about colors in Turbo Vision: One is to consider a palette entry as a particular color. The other is to envision it as representing the color currently used for a particular type of object, like selected text or normal text. Which of these is used will depend upon the nature of the object being drawn. The latter method should be employed when considering the palettes for pre-defined Turbo Vision objects.

When the programmer is writing a draw function for a view, he/she will want to be able to select a particular color or style for drawing. It may be desirable to have similar components of unrelated views to be drawn in the same color. It may also be desirable to give the user a method of changing at runtime the colors used for the application. The palette system in Turbo Vision will allow both of these possibilities.

A further benefit of the Turbo Vision palettes is that the system automatically detects the type of display being used (color, black and white, or monochrome) and sets up the palette accordingly. If the programmer chooses to modify the system palette, this should be taken into account when the new palette is designed. See the Construction section for further details.

How it works

So, how does one actually get a particular color from the palette inside the draw member function of a view? The answer lies in understanding in greater detail how a given palette is related to its parents.

Every class derived from TView (which means every visible Turbo Vision object) has a color palette. The palette may be inherited (like TApplication) or it may be NULL (like TDeskTop) but all views do have one. The virtual function getPalette() is used to supply the palette for each view. It is important to realize that this particular function is never called explicitly by the programmer; it will be called by Turbo Vision when necessary.

The member functions of TView that actually do write to the view, with one exception, all take a color index as one of their parameters. This parameter must be thought of as the value of an index to a style, not the actual color.

For example, When using TView::writeStr(), one parameter specifies a color index. Inside of writeStr() the following procedure is applied to convert this index into an actual color value. The current view's palette is examined at the specified index entry and a value X is found there. Next, the view's owner's palette is retrieved and X is used as an index into this palette where another value Y resides. This process continues until the view being examined no longer has an owner. At this time, the value at the current index is returned and interpreted as a standard PC color attribute byte.

Example

Here are the palettes for the classes in the enclosed example and how they map into each other:

                          x01   x02   x03   x04   x05   x06
   ┌────────────────────┬─────┬─────┬─────┬─────┬─────┬─────┐
   │TTestView           │ x09 │ x0A │ x0B │ x0C │ x0D │ x0E │
   └────────────────────┴─────┴─────┴─────┴─────┴─────┴─────┘
                           ▼     ▼     ▼     ▼     ▼     ▼
                 x01-x08  x09   x0A   x0B   x0C   x0D   x0E
   ┌────────────┬───────┬─────┬─────┬─────┬─────┬─────┬─────┐
   │TTestWindow │  ...  │ x88 │ x89 │ x8A │ x8B │ x8C │ x8D │
   └────────────┴───────┴─────┴─────┴─────┴─────┴─────┴─────┘
                           ▼     ▼     ▼     ▼     ▼     ▼
                 x01-x87  x88   x89   x8A   x8B   x8C   x8D
   ┌────────────┬───────┬─────┬─────┬─────┬─────┬─────┬─────┐
   │TTestApp    │  ...  │ x3E │ x2D │ x72 │ x5F │ x68 │ x4E │
   └────────────┴───────┴─────┴─────┴─────┴─────┴─────┴─────┘

                                Table I

All numbers used in the palettes in this document are in hexadecimal (it is easier to understand attributes in that base).

Here is a concrete case of a "palette" walk, taken from the code supplied with this document. Suppose there is a view of type TTestView inserted inside a window of type TTestWindow, itself inserted in the desktop. The palettes for these views are shown above. To draw something with color 0x01, simply use 0x01 as the parameter to the write functions used in TTestView::draw(). TTestView's palette contains the number 0x09 at index 0x01 and TTestView's owner is TTestWindow. Index 0x09 in TTestWindow's palette contains 0x88. TTestWindow's owner is TDesktop which has a NULL palette and is skipped (see notes). TDesktop's owner is TTestApp and it's palette contains 0x3E at index 0x88. So the color that will be used is 0x3E or yellow on cyan.

The writeXXX functions in TView all take color index values in the current palette except one, writeLine(..., TDrawBuffer). A TDrawBuffer is a buffer for an entire line. Once constructed, it is drawn into the view using writeLine(). TDrawBuffer's member functions for drawing are quite similar to TView's with one exception. They do NOT use color index values. They use attribute bytes to determine the colors used. What this means is that in order to use the color palettes, one must obtain the color attribute for a member of the current palette by hand. This is done with the function getColor(). Pass it the index and it performs the "palette walk" and returns the actual attribute represented. Use this value in TDrawBuffer's write functions.

Note that any attribute byte can be used with a TDrawBuffer. getColor() need not be the source of the value used.

Palette construction

Creating a new palette for a given view is quite simple, though deciding what indexes to use may take some thought. It requires inheriting from the view and overriding the getPalette() member function. This function has the following prototype:


            

              
              
TPalette& TTestView::getPalette() const;

The actual palette is a character string where the bytes contain the appropriate reference values. These bytes are normally written out in hex when the string is created. For example,


            

              
              
#define cpTestView "\x9\xA\xB\xC\xD\xE"

is the definition of the palette for TTestView. The cpTestView symbol is then used in TPalette constructor like this:


            

              
              
TPalette palette( cpTestView, sizeof(cpTestView)-1 );

The subtraction of 1 from sizeof() is to remove the terminating NULL that all C++ literal strings have by default. Also, since a reference to this palette is returned by getPalette(), it must exist from the first call to getPalette onward and is normally made a static local variable to function (to avoid polluting the global name space). So the entire function looks like this:


            

              
              
#define cpTestView "\x9\xA\xB\xC\xD\xE"
TPalette& TTestView::getPalette() const;
{
    static TPalette palette(cpTestView, sizeof(cpTestView)-1);
    return palette;
}

What if a user has a black and white display? A palette that has been designed with the benefit of color will usually look terrible when viewed in either black and white or monochrome mode. For this reason, Turbo Vision has three completely distinct system palettes: cpAppColor, cpAppBlackWhite, and cpAppMonoChrome. At startup, the program will detect what kind of display is attached and use the appropriate settings. So, when modifying the system palette, one needs to modify all three of the basic system palettes.

The enclosed example addresses this issue, as well as a similar one involving windows, since they also have three palettes (for a different reason). The color choices for the alternate color palettes can be select from the lists below:

        cpAppBlackWhite                     cpAppMonochrome
    0x07    Light Grey on Black         0x07    White on Black
    0x0F    White on Black              0x70    Black on White
    0x70    Black on Light Grey         0x09    White on Black
                                                Underlined
    0x78    Dark Grey on Light Grey             (not recommended)
    0x7F    White on Light Grey

One final note on TWindow. This view has three palettes, like TProgram. However, this is so that it is easy to have three different color schemes for windows used in an application, such as yellow on blue for an editor but black on grey for a dialog box. Extending each of these palettes is done in a similar fashion to the three palettes for TProgram with the exception that TWindow's palettes are not in a header file and thus must be included by the programmer in the application along with the extensions. See the example for further details.

Notes

  1. In some of the example programs and the User's Guide, you will see getColor() being called with a value greater than 255. In this case, both bytes of the word passed are mapped into colors and returned as a word with the attributes stored in the high and low bytes. This is required if one is uses TDrawBuffer::writeCStr to display strings with highlighted characters (see documentation on writeCStr() for more details).

  2. The missing entries in table I can be found in the Turbo Vision User's Guide by looking under TWindow and TProgram.

  3. If at any time the index being used is out of range for the palette being examined, the error attribute is returned immediately. The error attribute is Flashing White on Red.

  4. If at any time the current palette has no entries (NULL palette), then the owner's palette is examined directly. TDeskTop is one view that has a NULL palette.

  5. The top of the chain will always be the application object, for it is the only view that will not have an owner. The palette for the application is inherited from TProgram, but can be changed by overriding getPalette() in the application object (the class derived from TApplication).

  6. Using the TDrawBuffer object because of its ability to completely bypass the palette system is sometimes desirable, but can produce unexpected side effects. For example, running such a program on a VGA system running video mode 2 (BW80) will still produce colors, even though Turbo Vision itself will be running black and white. (This example will display this behaviour so try typing mode BW80 at the DOS prompt and then running the demo).