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Finish describing video shader issues.

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@ -3,10 +3,8 @@ are similar in many ways,
but some of them do have particular quirks
that you should be aware of.
Video Shaders and the Super Famicom (and Mega Drive)
----------------------------------------------------
Shaders look weird on Super Famicom because of interlace/hi-res
Video Shaders and TV-based consoles
-----------------------------------
[Video Shaders](guides/shaders.md)
customize how higan scales
@ -22,46 +20,69 @@ try to produce higher-quality output
by recognising particular patterns of pixel,
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
is a single pixel in the video output they analyze.
Most of the consoles higan emulates
becomes a single pixel in the video output they analyze.
Many of the consoles higan emulates
can only output video at one specific resolution,
so this "one pixel equals one pixel" rule holds true,
and shaders like "xBR" work just fine.
and pattern-based shaders like "xBR" work just fine.
Unfortunately,
this is not the case for the Super Famicom.
The "normal" video mode for the Super Famicom
The "normal" video mode
draws 256 pixels across the width of the screen,
but it also supports a "hi-res" mode
that draws 512 pixels on each line.
Since Super Famicom games can enable hi-res mode at any time,
even halfway through a frame,
higan always renders Super Famicom video output at 512 pixels wide,
just in case hi-res mode is enabled later on.
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 pattern-based shaders are based on.
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 238 rows of output,
~60 times per second.
This is sometimes referred to as "240p" video.
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 ~480 lines of output,
~30 times a second;
first the ~240 odd-numbered lines,
then going back to the top and drawing the ~240 even-numbered lines.
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.
TODO: https://board.byuu.org/viewtopic.php?p=44758#p44758
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).
This also affects the PC Engine:
https://board.byuu.org/viewtopic.php?p=44779#p44779
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.
Music and Sound Effect Volume on the Mega Drive
-----------------------------------------------