# How the 3DO hardware processes images and cels

This section explains how the 3DO hardware processes images and cels. Understanding these concepts helps you choose appropriately among image and cel types and take advantage of the 3DO hardware's special effects capabilities.

This section provides more information about images and cels, discussing the following topics:

- What are images and cels?
- What is a 3DO cel?
- Where does the color come from?
- Where do the special effects come from?
- Where does the cel's shape come from?

## What are images and cels?

Cels and images are graphics formats that take advantage of the 3DO hardware. Images and cels differ in how they are treated by the 3DO hardware:

- Images are projected directly onto the frame buffer, processed by the display generator and then displayed on the TV screen.

- Cels are first processed by the cel engine, then projected onto the frame buffer, passed to the display generator, and displayed on the TV screen.

Figure 1 shows how a cel moves through the cel engine onto the frame buffer, is processed by the display generator, and is then displayed on the TV screen.

Figure 1: How the 3DO hardware processes a cel.

As a result of the additional processing a cel receives, images and cels have different characteristics:

- Images are static, well-suited for a background. They have to start out at the size of the screen. Because there's no processing overhead involved, a relatively big image file can be displayed relatively quickly.

- Cels are dynamic. The can be moved, distorted, and zoomed, and can use special effects, as explained below.

## What is a 3DO cel?

A 3DO cel is really just another file format, like PICT or TIFF. A cel consists of data data organized in a particular way. Each cel consists of the following pieces:

- CCB (cel control block)-has information about the cel's position and distortion. These values are usually set by the program.

- Palette-contains the colors. The artist can determine the palette with one of the tools listed in Tools for 3DO Art Production.

- Bitmap.

## Where does the color come from?

Color for both images and cels comes from a color lookup table or CLUT. The CLUT is inside the hardware; it is the master palette for the display, and consists of registers filled with 15-bit values.

For images and for uncoded cels, every pixel has to have its own 15-bit value to specify the exact color in the CLUT. As a result, an image or an uncoded cel is large and uses a lot of RAM but the color is precise.

Note: On the 3DO system, an 8-bit cel has the same resolution but fewer colors than a 16-bit cel.

### Color for coded cels

Coded cels use less disk space than uncoded cels. In many cases, this happens without a visible deterioration of the visual quality. In a coded cel, each pixel has a maximum of 5 bits available to carry color information or other information. Those bits point to a Palette Lookup Table or PLUT. This PLUT acts as an intermediary between the color lookup table (CLUT) and the cel.

Figure 2 illustrates how an uncoded cel and a coded cel access the CLUT: the uncoded cel needs to provide 16 pixels of information for each 16-bit color value in the system color table. The coded cel, on the other hand, needs only 5 pixels of information. Those five pixels point to a 16-bit value in the PLUT that in turn points to the 16-bit value in the PLUT. Since one PLUT is used for all the pixels in the cel, the cel can be a lot smaller.

Figure 2: How uncoded and coded cels use the system color table.

Here's some detail on why coded cels are used frequently:

- Coded cels do allow special effects, just as uncoded cels.

- Coded cel have individual PLUTs. If you use coded cels, you can still have a lot of colors on screen since each cel has its own PLUT.

- Coded cels are smaller than uncoded cels. The smaller the cel, the faster the response of the title, the better the frame rate of the animation, and the better the quality of music you can add.

## Where does the cel's shape come from?

Most titles don't move rectangular shapes around a screen, but rather creatures, vehicles, or other objects. You therefore need to make the region around the object transparent.

The 3DO hardware lets you specify transparent regions in a cel in two ways:

- Specify transparent regions using a black Transparency mask (3DO CelWriter, 3DO Coded8 CelWriter, 3DO Animator). No true black (RGB 0-0-0) is possible for any pixel in the cel; you have to use RGB 0-0-9.

- Specify transparent regions using a special background color (3DO Animator, 3DO Coded-8 CelWriter). True black (RGB 0-0-0) can be used the cel.

For information on creating irregularly shaped cels, see the relevant sections in the manuals for the individual tools.

## Where do the special effects come from?

Special effects are possible for any kind of cel, and are determined by a part of the cel called the PIXC. In the simplest case, the PIXC determines whether each pixel in the cel is displayed as opaque (the default), with enhanced or reduced luminance, or mixed with the background.

By default, each pixel in the cel is processed using the settings in Figure 3. The figure actually shows the left half of the Effects Editor of the 3DO PostPro tool-more about the right half later.

Figure 3: PIXC settings specification.

Note how just using one set of PIXC options, you can multiply or divide the pixels in a cel by a different number (resulting in enhanced or reduced luminance) or mix it with the background picture in the frame buffer using the Pixel Source 2 pop-up menu. Consider experimenting with these settings for best results.

### Using two PIXC settings in one cel

In addition to changing the display for the whole cel, you can also change the display for only part of the cel, for example, display most of the cel as opaque and a window as translucent, or a light as more luminous.

To do that, you specify two groups of pixels in the cel. One group of uses one set of pixel settings, the other a second, different set:

- All pixels that are not masked (or don't use a special color) use the settings on the left, as explained above. This group of PIXC settings is also referred to as P-Mode B or P-Mode 1.

- All masked pixels (for example, all stained glass window pixels) use the settings on the right. This group of PIXC settings is also referred to as P-Mode A or P-Mode 0.

Figure 4 shows the two different paths each pixel in the cel has to choose from: Either the path on the left-the group of PIXC settings called P-Mode A-or the path on the right-the group of PIXC settings called P-Mode B.

Figure 4: PIXC settings for two different groups of pixels.