bsnes/docs/guides/shaders.md

Most of the consoles higan emulates were designed for low resolution NTSC televisions, and their video output is chunky and blocky by today's standards. Video shaders customise how a console's video output is drawn to the computer screen, and can clean up and smooth out the original video, reproduce the scanlines and blurring of the original display, or any other visual effect.

The available video shaders are listed in the "Video Shaders" sub-menu of the Settings menu. Which shaders are available depends on the video driver higan is configured to use. Most drivers only support these shaders:

• None draws each computer pixel according to the colour of the single nearest console pixel, sometimes called "nearest neighbour" scaling. This produces unnaturally crisp and blocky images.
  • If you use aspect correction or non-integral scaling, neighbouring console pixels may be drawn with a different number of computer pixels due to rounding errors, causing a distracting rippling effect.
• Blur draws each computer pixel according to the weighted average colour of the four nearest console pixels, sometimes called "bilinear" scaling. This produces unnaturally blurry images.

In addition to those, the OpenGL driver also supports custom shaders.

Note: For technical reasons, higan's emulation of certain consoles can produce surprising behaviour in certain shaders, particularly shaders that compare each console pixel with its neighbours. See Video Shaders and TV-based consoles for details.

Where to get custom shaders

• higan includes some simple example shaders. If your copy of higan did not come with shaders, you can get them from the unofficial higan repository.
• quark-shaders contains many high-quality shaders for use with higan.
• You can write your own.

How to install custom shaders

Make sure the shader you want to install is in the correct format: it should be a folder whose name ends in .shader, it should contain a file named manifest.bml, and probably some *.fs or *.vs files.

Place the shader folder inside the Video Shaders folder of your higan installation. If you don't have a Video Shaders folder, create it beside the *.sys folders like Game Boy Advance.sys and Super Famicom.sys.

• On Windows, this is probably the folder containing higan.exe
• On Linux, this is probably ~/.local/share/higan

Launch higan, open the Settings menu, and choose "Advanced ..." to open the Advanced tab of the Settings window. Under "Driver Selection", make sure "Video" is set to "OpenGL". If it wasn't already set that way, you'll need to restart higan for the change to take effect.

Open the Settings menu again, choose the "Video Shader" sub-menu, and now the shaders you installed should be listed at the bottom of the menu.

Load a game (so you can see the results), switch between shaders to see what they do, and pick your favourite!

Notable examples

The quark-shaders repository mentioned above contains lots of carefully-crafted shaders, but some are particularly noteworthy:

• AANN implements "anti-aliased nearest neighbour" scaling. This uses anti-aliasing to hide the rounding errors often introduced by aspect ratio correction and non-integral scaling, producing an image nearly as crisp as the "None" shader, but without the distracting ripple effect.
• Gameboy emulates the squarish aspect-ratio, greenish-colours, and limited palette of the original Game Boy. At larger scales, you can even see the fine gaps between each pixel, and the shadow that dark colours would cast on the LCD background.
• NTSC performs NTSC encoding, bandwidth limiting, and NTSC decoding of the video image to recreate the colour fringing, blurring and shimmer that most game players would have seen on real televisions. Some games depend on NTSC artefacts to display colours outside the console's official palette or to create effects like transparency.

Video Shaders and TV-based consoles

Simple shaders (like "None" and the third-party "AANN" shader) just blindly scale up the images they're given, but sophisticated shaders (such as the third-party "xBR" shader) try to produce higher-quality output by recognising particular patterns, like taking three diagonal pixels and turning that into a smooth diagonal line.

These shaders assume that each pixel drawn by the game's artists becomes a single pixel in the video output they analyze. The hand-held consoles that higan emulates (and also the Famicom) can only output video at one specific resolution, so this "one pixel equals one pixel" rule holds true, and pattern-based shaders like "xBR" work just fine. Unfortunately, this is not true for most of the TV-based consoles that higan supports.

The Super Famicom's "normal" video mode draws 256 pixels across the width of the screen, but the "high resolution" mode draws 512. Since Super Famicom games can enable hi-res mode at any time (even halfway through a frame), higan always renders Super Famicom video output 512 pixels wide, just in case. This means that in "normal" mode, each pixel drawn by the game's artists becomes two pixels in the video output, breaking the assumption that pattern-based shaders are based on.

The Super Famicom has a similar issue in the vertical direction: normally, an NTSC-based Super Famicom draws about 240 rows of output every frame, sometimes referred to as "240p" video. When a game turns on "interlaced" mode, it draws the 240 odd-numbered lines of one frame, then the 240 even-numbered lines of the next, and so forth. This is sometimes referred to as "480i" video. Although interlaced mode cannot be enabled mid-frame like high-resolution mode, resolution switching is still complex, so higan always draws all 480 lines of video output. This means for a normal, non-interlaced game, each pixel drawn by the game's artists becomes four pixels in the video output (two horizontally and two vertically) making pattern-based shaders even less useful. It also breaks most scanline-emulation shaders, since they typically draw a scanline for each row of pixels in the video output.

The Mega Drive has similar problems to the Super Famicom. It has the same behaviour with interlacing, but its high-resolution mode switches from 256 pixels across to 320 pixels across. Therefore in normal mode, each pixel drawn by the game's artists becomes five pixels in the video output, while in high-resolution mode, each pixel drawn by the game's artists becomes four pixels in the video output (or 10 and 8 pixels in non-interlaced mode).

The PC Engine does not support an interlaced mode, but its horizontal resolution is much more flexible than the Super Famicom or Mega Drive, and so it has the same problems with shaders as those consoles.