linux FreeType库字体颜色设置

在使用 FreeType 库渲染字体时，可以通过设置绘制字体时的颜色来改变字体的颜色。具体实现方式如下：

1. 定义颜色变量

FT_Error error;
FT_Library library;
FT_Face face;
FT_GlyphSlot slot;
FT_Vector pen;
FT_Matrix matrix;

int width, height, pitch;
unsigned char* image;

FT_UInt glyph_index;
FT_Bool use_kerning;
FT_UInt previous;

FT_BitmapGlyph bitmap_glyph;
FT_Bitmap bitmap;

unsigned char r = 0xff, g = 0xff, b = 0xff, a = 0xff;  // 定义颜色变量，这里以白色为例

2. 设置颜色

// 设置颜色
FT_Set_Transform(face, &matrix, &pen);
slot = face->glyph;
slot->bitmap_left = 0;
slot->bitmap_top = 0;

// 设置字体颜色
slot->bitmap.buffer = image;
slot->bitmap.width = width;
slot->bitmap.rows = height;
slot->bitmap.pitch = pitch;
slot->bitmap.num_grays = 256;
slot->bitmap.pixel_mode = FT_PIXEL_MODE_GRAY;
slot->bitmap.palette_mode = FT_PALETTE_MODE_RGB;
slot->bitmap.palette = NULL;
slot->bitmap.pixel_mode = FT_PIXEL_MODE_GRAY;

// 设置字体颜色
for (int y = 0; y < height; y++) {
  for (int x = 0; x < width; x++) {
    int index = y * pitch + x;
    unsigned char gray = image[index];
    image[index * 4] = r * gray / 255;
    image[index * 4 + 1] = g * gray / 255;
    image[index * 4 + 2] = b * gray / 255;
    image[index * 4 + 3] = a;
  }
}

这样就可以设置字体的颜色了，通过改变 `r`, `g`, `b`, `a` 参数的值可以实现不同的颜色效果。