Transparency here refers to partial transparency—being able to see through one layer to the layer under it, but having the top layer impart some change to the way the bottom layer appears. The VDP hardware does not support such transparency (although pixels can be fully transparent), with the exception of shadow and highlight mode. The shadowed section in the following example from Mega Turrican is a form of transparency, although it is limited to just lowering the color intensity:
If a developer wants to create more varied transparency effects, they have a few options available. Perhaps the most common is the use of dithering, which is generally a technique for blending pixels of different colors using patterns to create the illusion of shading. Dithering patterns can also be used which alternate between shaded and fully transparent pixels to create the illusion of partial transparency.
Let’s look at this famous waterfall example from the first stage of Sonic the Hedgehog:
If you look closely, you can see that the waterfall is made of alternating vertical lines. One set of lines is fully transparent, and the far background can be seen through them. The other set of lines is shaded with quickly changing colors that simulate falling water. The far background cannot be seen at all through the shaded pixels—only through the fully transparent pixels. However, when the lines alternate as they do here, we have enough visual information from every other line to both reconstruct the far background and see the waterfall effect.
One reason this Sonic the Hedgehog waterfall example is famous is because it is often used to demonstrate how the appearance of dithering changes based on the video signal type. The above example is from a pixel-perfect emulator output, where the edge of each pixel is clearly defined. On certain analog video signal types, primarily composite, all of the video information is encoded on a single channel and the video quality becomes degraded, resulting in a picture that appears less sharp. Composite video output for the above scene might look something like this (this was generated by Blargg’s NTSC composite filter via emulator, so it is just a digital approximation):
The dithering pattern is no longer highly noticeable, and the transparency effect is increased. The far background is easier to see through the waterfall.
Here’s a screenshot of Yu Yu Hakusho: Makyo Toitsusen, which features a huge amount of dithering, both in the fog passing across the screen and in the shading of the foreground and background:
If we again apply the composite filter, we get this:
The pixels noticeably become less sharp due to the degradation of the signal, but the dithering is no longer visible on the screen. Again, note that these composite screenshots were created digitally using Blargg’s NTSC composite filter; the real composite output from the Mega Drive will look different (it might have rainbow banding or other artifacts, for instance). I should also point out that square pixels captured via emulation look very different from the output of a CRT display.
To get slightly off topic, there is a temptation to look at screenshot comparisons such as shown above and say that Mega Drive (and other retro console) games are meant to be played via composite; otherwise, developers would not have used such dithering. This is rather controversial, as it is difficult to assess designer intent from a single graphical effect. I suspect, in many cases, rushed designers were simply using the methods they knew with no thought of grand intent (additionally, dithering was used in many contexts where designers presumably knew it would be visible, such as on PC-98 games and handheld LCD games). We can also ask: Is it worth degrading the entire display quality for the sake of one transparency effect? Ultimately, this is an issue of personal preference. It’s further complicated by the type of display—one’s preference for composite vs. RGB output with real hardware on a CRT monitor might be different than one’s preference for composite filtered vs. unfiltered emulator output on an LCD.
There is another technique that can be used to create a transparency effect. It involves flashing tiles on and off every frame. The flashing occurs so quickly that our minds combine the visual information into one image that appears to contain transparency.
Observe the shadows in the following example from Yu Yu Hakusho: Makyo Toitsusen:
If you pause the video, you can see that there is a black circle under one character. Every frame, this circle changes location from one character to the other.
Here is another example of flashing-based transparency from Battle Mania 2:
The bottom of the truck is visible through the transparent fog, which is actually flashing on and off each frame.
There are two other techniques that achieve effects resembling transparency. These are discussed in their own sections. One involves silhouettes, and the other involves mid-frame palette swaps.
List of Effects:
- Full-Screen Scrolling
- Row / Column Scrolling
- Line Scrolling
- Multi-Jointed Characters
- Tilting / Rotation
- Shadow and Highlight
- Palette Swapping
- Vertical Scaling
- Sprite Raster Effects
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