[ft-devel] supporting coloured bitmaps in FreeType

[Graham Asher]

Hi to all FreeTypers after a long silence on my part - too much to do in CartoType.

I have a simple proposal that would enable FreeType to support fonts which can create coloured glyphs. It is primarily intended for bitmap font formats used for emojis, emoticons and the like.

I have an immediate need for this for a client, and will have to use it whether or not it becomes part of official FreeType, but it seems like a simple and non-disruptive change that would add a small amount of value.

The idea is to add just two modes (for the moment - no need to be over-elaborate) to FT_Pixel_Mode, for premultiplied ARGB, and the same format using 8-bit indexes into a palette. These modes are convenient when creating bitmap fonts from PNG images. Thus FT_Pixel_Mode and its associated documentation comment become:

     /*************************************************************************/
  /*                                                                       */
  /* <Enum>                                                                */
  /*    FT_Pixel_Mode                                                      */
  /*                                                                       */
  /* <Description>                                                         */
  /*    An enumeration type used to describe the format of pixels in a     */
  /*    given bitmap.  Note that additional formats may be added in the    */
  /*    future.                                                            */
  /*                                                                       */
  /* <Values>                                                              */
  /*    FT_PIXEL_MODE_NONE ::                                              */
  /*      Value~0 is reserved.                                             */
  /*                                                                       */
  /*    FT_PIXEL_MODE_MONO ::                                              */
  /*      A monochrome bitmap, using 1~bit per pixel.  Note that pixels    */
  /*      are stored in most-significant order (MSB), which means that     */
  /*      the left-most pixel in a byte has value 128.                     */
  /*                                                                       */
  /*    FT_PIXEL_MODE_GRAY ::                                              */
  /*      An 8-bit bitmap, generally used to represent anti-aliased glyph  */
  /*      images.  Each pixel is stored in one byte.  Note that the number */
  /*      of `gray' levels is stored in the `num_grays' field of the       */
  /*      @FT_Bitmap structure (it generally is 256).                      */
  /*                                                                       */
  /*    FT_PIXEL_MODE_GRAY2 ::                                             */
  /*      A 2-bit per pixel bitmap, used to represent embedded             */
  /*      anti-aliased bitmaps in font files according to the OpenType     */
  /*      specification.  We haven't found a single font using this        */
  /*      format, however.                                                 */
  /*                                                                       */
  /*    FT_PIXEL_MODE_GRAY4 ::                                             */
  /*      A 4-bit per pixel bitmap, representing embedded anti-aliased     */
  /*      bitmaps in font files according to the OpenType specification.   */
  /*      We haven't found a single font using this format, however.       */
  /*                                                                       */
  /*    FT_PIXEL_MODE_LCD ::                                               */
  /*      An 8-bit bitmap, representing RGB or BGR decimated glyph images  */
  /*      used for display on LCD displays; the bitmap is three times      */
  /*      wider than the original glyph image.  See also                   */
  /*      @FT_RENDER_MODE_LCD.                                             */
  /*                                                                       */
  /*    FT_PIXEL_MODE_LCD_V ::                                             */
  /*      An 8-bit bitmap, representing RGB or BGR decimated glyph images  */
  /*      used for display on rotated LCD displays; the bitmap is three    */
  /*      times taller than the original glyph image.  See also            */
  /*      @FT_RENDER_MODE_LCD_V.                                           */
  /*                                                                       */
  /*    FT_PIXEL_MODE_ARGB_PRE ::                                          */
  /*      A 32-bit bitmap containing premultiplied ARGB pixels.            */
  /*                                                                       */
  /*    FT_PIXEL_MODE_ARGB_PRE_PALETTE ::                                  */
  /*      An 8-bit bitmap containing palette indexes. Each value           */
  /*      is an index to a palette of premultiplied ARGB color values.     */
  /*      The palette field of the FT_Bitmap structure points to the       */
  /*      base of the palette, which is an array of 4-byte values.         */
  /*                                                                       */
  typedef enum  FT_Pixel_Mode_
  {
    FT_PIXEL_MODE_NONE = 0,
    FT_PIXEL_MODE_MONO,
    FT_PIXEL_MODE_GRAY,
    FT_PIXEL_MODE_GRAY2,
    FT_PIXEL_MODE_GRAY4,
    FT_PIXEL_MODE_LCD,
    FT_PIXEL_MODE_LCD_V,
    FT_PIXEL_MODE_ARGB_PRE,
    FT_PIXEL_MODE_ARGB_PRE_PALETTE,
    FT_PIXEL_MODE_MAX      /* do not remove */
  } FT_Pixel_Mode;

Best regards,

Graham Asher