| GrayAdjust | Function (Macro)

gray.h

void GrayAdjust (short adjustment);

Adjusts grayscale support to make it flickerless.

Note: This function has become more or less obsolete since the effect is hardly visible with the new HW2 grayscale support.
This function is introduced to improve grayscale support on HW2 calculators, i.e. to make it more flickerless (it can be used on HW1 calculators too, but HW1 grayscale support is usually satisfactorily flickerless by default). Namely, if the plane switching frequency is not well synchronized with the LCD refresh frequency, the flickering may be too ugly. Unfortunately, it is not possible to use hardwired values because this frequency drifts with the battery strength, so a value which is good if the batteries are full is not good if the batteries are worn out, and vice versa. So the only solution is to make the frequency ratio adjustable. This trick was used in Universal OS by Julien Muchembled to produce quite flickerless grayscale support on HW2 calculators (many thanks to him for telling to me about this). His grayscale support allows adjusting the LCD refresh frequency (by changing the logical height of the display) using the keys DIAMOND+LEFT/RIGHT.
This solution was a bit unflexible to me, so I decided to use slightly modified variant of Julien's method. I introduced the function GrayAdjust for fine adjusting of the grayscale quality. This function does exactly the same as pressing DIAMOND+LEFT/RIGHT in Universal OS, i.e. it adjusts the LCD refresh frequency. The default value for adjustment is 0, which means "factory settings". Values less than 0 increases and values greater than 0 decreases the LCD refresh frequency. Legal values for adjustment range from -28 to 127 on TI-89 and from 0 to 127 on TI-92+ (although only slight variations around 0 are meaningful, for example from -10 to +10). Note that values less than 0 are not allowed on the TI-92+, else strange things would happen with the screen (use macros from compat.h to check the calculator model).
So how would one use this function? You can put an option into your program which displays a grayscale picture, and ask the user to adjust the quality. Here is a simplified example (called "Adjust Grayscale") of the program which displays the full screen filled with dark gray, then allows adjusting the quality using the `'+'` and `'-'` keys (use `'ESC'` for exit):

    // Adjust grayscale quality using + and - keys.

    #define USE_TI89              // Compile for TI-89
    #define USE_TI92PLUS          // Compile for TI-92 Plus
    #define USE_V200              // Compile for V200

    #define OPTIMIZE_ROM_CALLS    // Use ROM Call Optimization
    #define MIN_AMS 100           // Compile for AMS 1.00 or higher
    #define SAVE_SCREEN           // Save/Restore LCD Contents

    #include <tigcclib.h>         // Include All Header Files

    // Main Function
    void _main(void)
    {
      int key, value = 0;
      if (!GrayOn ())
        return;
      GrayAdjust (value);
      memset (GetPlane (DARK_PLANE), 255, LCD_SIZE);  // Fill the dark plane and
      memset (GetPlane (LIGHT_PLANE), 0, LCD_SIZE);   //  clear the light plane
      while ((key=ngetchx ()) != KEY_ESC)
        {
          if (key== '+' && value < 127)
            value++;
          if (key== '-' && value > (TI89 ? -28 : 0))
            value--;
          GrayAdjust(value);
        }
      GrayOff ();
    }

This program does not have to be embedded in your program: as the LCD refresh frequency is kept in the hardware register, it will still be valid after exiting from the program, so this example may be used as a standalone adjusting program. However, the factory settings are restored each time the calculator is turned on. If you embed the adjustment code in your program, it is not a bad idea to use the same adjustment key as used in Universal OS (DIAMOND+LEFT/RIGHT), due to conformance. These keys may be checked easily using pseudoconstants from the compat.h header file, as in

    if (key == KEY_DIAMOND + KEY_LEFT) ...

Note: Changing adjustment also has influence to the lightness of the display, but you always can change the contrast the usual way. Increasing adjustment makes the display lighter, and decreasing it makes the display darker. Anyway, do not use this function for adjusting the display lightness. Its purpose is just to estabilish precise synchronization.