# Creating images with custom VDLs

A custom VDL (video display list) is useful if the 3DO Image Writer has to reduce the number of colors during conversion. It is especially recommended if the image shows banding after the colors have been reduced to 15-bit.

This section discusses images with custom VDLs:

- Creating an image with a custom VDL, contains step-by-step instructions.
- Standard and custom VDLs, provides some background about custom VDLs.

## Creating an image with a custom VDL

If you choose a custom VDL, the Image Writer creates a color table based on the image you are working with. This is helpful, for example, if you start out with 24-bit source art: The Image Writer can perform color reduction based on the characteristics of the picture you are dealing with. There are two options:

- The dynamic VDL is computed on a scanline-by-scanline basis.
- The fixed VDL is computed once and used for the entire image.

### How to create an image with a custom VDL

To convert your source art to an image using a custom VDL, follow these steps:

1. Resize the image if necessary (see Resizing source art).

2. From the File menu, choose Export > 3DO Image Writer.

3. Specify the name of the image. An extension .img is recommended.

4. In the dialog that appears, choose Custom in the "VDL type" box.

5. Choose between dynamic and fixed VDL.

   - If you choose Dynamic, the Image Writer creates a custom VDL on a scanline-by-scanline basis.

   - If you choose Fixed, the Image Writer uses the system VDL for the image.

6. Leave the default color reduction method: BoxPopLength sort.

7. If you expect to draw cels on top of the image, choose the CLUT bypass option.

   This option allows the parts of the image that are covered by the cel to use the standard VDL instead of the custom VDL. In addition to setting the CLUT bypass option for the image, you also need to set the high bit of the frame buffer pixel for the cel. This has to be done by the program that displays the cel.

   Note: You can have transparent and opaque cel pixels on top of an image that uses a custom VDL. Translucent cels will not work correctly even if you have turned on the CLUT bypass option, because the cel engine assumes a linear ascending CLUT.

8. Choose OK and you're done.

### For more information

The 3DO Image Writer dialog discusses additional options you have when creating images with a custom VDL.

## Standard and custom VDLs

A VDL is a list of display control commands that tell the video display list processor (VDLP) how to control the display generator. A VDL contains information about three color lookup tables (CLUTs) that the 3DO hardware uses to index the color from the 15-bit frame buffer color to the 24-bit color your TV uses.

- The standard (system-supplied) color table divides red, green, and blue into even parts, using the same 32 shades for each image.

- A custom VDL using a custom color table tries to stay as close to the original colors as possible, which is useful if they all belong to the same range (for example, 32 shades of blue).

### About custom VDLs

The standard VDL uses the standard (system-supplied) color table, which means that the 8-bit values contained in the CLUT are linear ascending. They ascend in even intervals from a low value in register 0 to a high value in register 31. For example, they may ascend in steps of 8-from 0 in register 0 to 255 in register 31-0, 7, 15, 23, 31, 39, and so on. Banding may occur because the cel engine assumes a linear palette that covers the full range of the color spectrum when it scales color values to create lighting effects. For example, if the cel engine multiplies the red, green, and blue components of a 15-bit color value by 1.5, it assumes that the higher resulting color value will result in a brighter (or darker) color, which is true if the palette is linear and is either ascending or descending.

### For more information

To understand how custom color tables work,see Understanding the Display Generator. It describes how an image is processed on its way from the frame buffer to the TV screen.