problem.cr

class Page
  # ....
  private def render_glyph_to_texture(glyph, width, height) : Nil
    bitmap = @face.value.glyph.value.bitmap  
    buffer = Bytes.new bitmap.buffer, bitmap.width * bitmap.rows
    show_pixels bitmap.buffer, bitmap.width, bitmap.rows
    @texture.update(bitmap.buffer, bitmap.width, bitmap.rows, 0, 0, 8)
  end

  def show_pixels(pixels, width, height)
    height.times do |y|
      width.times do |x|
        pixel = pixels[x + y * width]
        char = pixel == 0 ? ' ' : pixel < 128 ? '+' : '*'
        print char
      end
      print '\n'
    end
  end
  # ....
end

class Texture
  # ....
  def update(pixels, width, height, x_dest, y_dest, bpp)
    activate do
      GL.safe_call do
        external_format = self.class.translate_bpp(bpp)
        pp x_dest, y_dest, width, height, @size
        LibGL.texSubImage2D LibGL::TEXTURE_2D, 0, x_dest, y_dest, width, height, external_format, LibGL::UNSIGNED_BYTE, pixels
      end
    end
  end
  
  def self.translate_bpp(bpp)
    case bpp
    when 8; LibGL::RED
    when 16; LibGL::RG
    when 24; LibGL::RGB
    when 32; LibGL::RGBA
    else
      raise ArgumentError.new "Invalid bits per pixel format given!(#{bpp})"
    end
  end
  # ....
end