13. How To Tutorial # 1: Outputting good quality 8-bit images

NuGraf has several built-in facilities that compress 24-bit rendered output to 256 or less colors. The most versatile and accurate of these is the Wu Quantization Algorithm which chooses a reduced color representation of an image given a fixed number of colors; the resulting 8-bit image usually looks as good as the original 24-bit image.

As an example, the following C code shows how to output an 8-bit colormapped TIFF image which was reduced with the Wu quantization algorithm:

{
	Nd_Int	num_quantize_colors = -1L;

	/* Select the TIFF file output driver */
	Ni_Output_Driver("driver1",
		Nt_FILE, Nt_TIFF, Nt_CMDSEP,
		Nt_FILENAME, "image.tif", Nt_CMDSEP,
		Nt_MODE, Nt_RGB, Nt_CMDSEP,
		Nt_OPTIONSTRING, "colormaplength=256, compression=lzw", Nt_CMDSEP,
		Nt_IMAGEQUANTIZE,
			Nt_ENABLED, Nt_ON,
			Nt_NUMCOLORS, (Nd_Int *) &num_quantize_colors,
			Nt_CMDSEP,
		Nt_CMDEND);
}

The "colormaplength=256'' option tells the TIFF driver to output the image as a 256-color colormapped image rather than a 24-bit image; this statement is not required by implicit colormapped output drivers such as the GIF file driver or the VGA frame buffer driver. The Wu quantization algorithm reduces the 24-bit rendered output down to the number of colors specified by the Nt_NUMCOLORS option, or if that value is set to -1L then it reduces the output to the maximum number of colors that the current colormapped output device can handle (which is 256 for the TIFF driver statement shown above). The quantization algorithm can compress a 24-bit image down to a minimum of 2 colors and a maximum of 256 colors.

The second method that can be used to compress output to 256-or-less colors is to map the 24-bit image's colors directly into a predefined colormap. This is the method used if the renderer determines that an output driver can only handle colormapped data (the Wu Image Quantization algorithm must also be disabled). The colormap is segmented by the renderer into a specific number of shades of red, green and blue. For example, if only 252 output colors are available then the renderer will choose 6 shades of red, 7 shades of green and 6 shades of blue (you can override the renderer's choice by specifying the Nt_RGBSHADES option to the command). To improve the appearance of the resulting 8-bit image you should also enable the Nt_DITHER option.

As an example, the following C code shows how to configure the output driver mechanism so that it reduces a 24-bit image down to 64 colors using 4 shades each of red, green and blue (as chosen internally by the renderer). Dithering is also enabled to increase the apparent number of colors in the image. The output is sent to a colormapped TIFF file.

{
	/* Select the TIFF file output driver */
	Ni_Output_Driver("driver1",
		Nt_FILE, Nt_TIFF, Nt_CMDSEP,
		Nt_FILENAME, "image.tif", Nt_CMDSEP,
		Nt_MODE, Nt_RGB, Nt_CMDSEP,
		Nt_OPTIONSTRING, "colormaplength=64, compression=lzw", Nt_CMDSEP,
		Nt_DITHER, Nt_ENABLED, Nt_ON, Nt_CMDSEP,
		Nt_CMDEND);
}