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libretro-shader.lyx
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#LyX 2.0 created this file. For more info see http://www.lyx.org/
\lyxformat 413
\begin_document
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\pdf_title "Cg/HLSL Libretro shader tutorial"
\pdf_author "Hans-Kristian Antzen, Daniel De Matteis"
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\begin_body
\begin_layout Title
Cg/HLSL libretro shader tutorial
\end_layout
\begin_layout Author
Hans-Kristian Arntzen, Daniel De Matteis
\end_layout
\begin_layout Standard
\begin_inset CommandInset toc
LatexCommand tableofcontents
\end_inset
\end_layout
\begin_layout Section
Introduction
\end_layout
\begin_layout Standard
This document is for a (fresh) shader developer that wants to develop shader
programs for use in various emulators/games.
Shader programs run on your GPU, and thus enables very sophisticated effects
to be performed on the picture which might not be possible in real-time
on the CPU.
Some introduction to shader programming in general is given, so more experience
d developers that only need reference for the specification may just skip
ahead.
\end_layout
\begin_layout Standard
Current emulators that support the specification explained here to a certain
degree are:
\end_layout
\begin_layout Itemize
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch
\end_layout
\begin_layout Itemize
\begin_inset Index idx
status open
\begin_layout Plain Layout
SNES9x
\end_layout
\end_inset
SNES9x Win32
\end_layout
\begin_layout Standard
There are three popular shader languages in use today:
\end_layout
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\begin_inset Index idx
status open
\begin_layout Plain Layout
HLSL
\end_layout
\end_inset
HLSL (High-Level Shading Language, Direct3D)
\end_layout
\begin_layout Itemize
\begin_inset Index idx
status open
\begin_layout Plain Layout
GLSL
\end_layout
\end_inset
GLSL (GL Shading Language, OpenGL)
\end_layout
\begin_layout Itemize
\begin_inset Index idx
status open
\begin_layout Plain Layout
Cg
\end_layout
\end_inset
Cg (HLSL/GLSL, nVidia)
\end_layout
\begin_layout Standard
The spec is for the
\begin_inset Index idx
status open
\begin_layout Plain Layout
Cg
\end_layout
\end_inset
Cg shading language developed by nVidia.
It
\begin_inset Quotes eld
\end_inset
wraps
\begin_inset Quotes erd
\end_inset
around
\begin_inset Index idx
status open
\begin_layout Plain Layout
OpenGL
\end_layout
\end_inset
OpenGL and
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status open
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HLSL
\end_layout
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HLSL to make shaders written in Cg quite portable.
It is also the shading language implemented on the
\begin_inset Index idx
status open
\begin_layout Plain Layout
PlayStation3
\end_layout
\end_inset
PlayStation3, thus increasing the popularity of it.
\end_layout
\begin_layout Subsection
The rendering pipeline
\end_layout
\begin_layout Standard
With shaders you are able to take control over a large chunk of the GPUs
inner workings by writing your own programs that are uploaded and run on
the GPU.
In the old days, GPUs were a big black box that was highly configurable
using endless amount of API calls.
In more modern times, rather than giving you endless amounts of buttons,
you are expected to implement the few «buttons» you actually need, and
have a streamlined API.
\end_layout
\begin_layout Standard
The rendering pipeline is somewhat complex, but we can in general simplify
it to:
\end_layout
\begin_layout Itemize
Vertex processing
\end_layout
\begin_layout Itemize
Rasterization
\end_layout
\begin_layout Itemize
Fragment processing
\end_layout
\begin_layout Itemize
Framebuffer blend
\end_layout
\begin_layout Standard
We are allowed to take control of what happens during vertex processing,
and fragment processing.
\end_layout
\begin_layout Subsection
A Cg/HLSL program
\end_layout
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If you were to process an image on a CPU, you would most likely do something
like this:
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for (unsigned y = 0; y < height; y++) {
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for (unsigned x = 0; x < width; x++)
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out_pixel[y][x] = process_pixel(in_pixel[y][x], y, x);
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}
\end_layout
\end_inset
We quickly realize that this is highly serial and slow.
We see that out_pixel[y][x] isn't dependent on out_pixel[y + k][x + k],
so we see that we can parallelize quite a bit.
\end_layout
\begin_layout Standard
Essentially, we only need to implement process_pixel() as a single function,
which is called thousands, even millions of time every frame.
The only purpose in life for process_pixel() is to process an input, and
produce an output.
No state is needed, thus, a
\begin_inset Quotes eld
\end_inset
pure
\begin_inset Quotes erd
\end_inset
function in computer science terms.
\end_layout
\begin_layout Standard
For the Cg program, we need to implement two different functions.
\end_layout
\begin_layout Standard
main_vertex() takes care of transforming every incoming vertex from camera
space down to clip space.
This essentially means projection of 3D (coordinates on GPU) down to 2D
(your screen)
\begin_inset Foot
status collapsed
\begin_layout Plain Layout
Since we're dealing with old school emulators here, which are already 2D,
the vertex shading is very trivial.
\end_layout
\end_inset
.
\end_layout
\begin_layout Standard
Vertex shaders get various coordinates as input, and uniforms.
Every vertex emitted by the emulator is run through main_vertex which calculate
s the final output position
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status collapsed
\begin_layout Plain Layout
For our emulators this is just 4 times, since we're rendering a quad on
the screen.
3D games would obviously have a lot more vertices.
\end_layout
\end_inset
.
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\begin_layout Standard
While coordinates differ for each invocation, uniforms are constant throughout
every call.
Think of it as a global variable that you're not allowed to change.
\end_layout
\begin_layout Standard
Vertex shading can almost be ignored altogether, but since the vertex shader
is run only 4 times, and the fragment shader is run millions of times per
frame, it is a good idea to precalculate values in vertex shader that can
later be used in fragment shader.
There are some limitiations to this which will be mentioned later.
\end_layout
\begin_layout Standard
main_fragment() takes care of calculating a pixel color for every single
output pixel on the screen.
If you're playing at 1080p, the fragment shader will have to be run 1920
* 1080 times! This is obviously straining on the GPU unless the shader
is written efficiently.
\end_layout
\begin_layout Standard
Obviously, main_fragment is where the real action happens.
For many shaders we can stick with a
\begin_inset Quotes eld
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dummy
\begin_inset Quotes erd
\end_inset
vertex shader which does some very simple stuff.
\end_layout
\begin_layout Standard
The fragment shader receives a handle to a texture (the game frame itself),
and the texture coordinate for the current pixel, and a bunch of uniforms.
\end_layout
\begin_layout Standard
A fragment shader's final output is a color, simple as that.
Processing ends here.
\end_layout
\begin_layout Section
Hello World
\end_layout
\begin_layout Standard
We'll start off with the basic vertex shader.
No fancy things are being done.
You'll see a similiar vertex shader in most of the Cg programs out there
in the wild.
\end_layout
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inline false
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void main_vertex(
\end_layout
\begin_layout Plain Layout
float4 pos : POSITION,
\end_layout
\begin_layout Plain Layout
out float4 out_pos : POSITION,
\end_layout
\begin_layout Plain Layout
uniform float4x4 modelViewProj,
\end_layout
\begin_layout Plain Layout
float4 color : COLOR,
\end_layout
\begin_layout Plain Layout
out float4 out_color : COLOR,
\end_layout
\begin_layout Plain Layout
float2 tex : TEXCOORD,
\end_layout
\begin_layout Plain Layout
out float2 out_tex : TEXCOORD
\end_layout
\begin_layout Plain Layout
)
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\begin_layout Plain Layout
{
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out_pos = mul(modelViewProj, pos);
\end_layout
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out_color = color;
\end_layout
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out_tex = tex;
\end_layout
\begin_layout Plain Layout
}
\end_layout
\end_inset
\end_layout
\begin_layout Standard
This looks vaguely familiar to C, and it is.
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status open
\begin_layout Plain Layout
Cg
\end_layout
\end_inset
Cg stands for
\begin_inset Quotes eld
\end_inset
C for graphics
\begin_inset Quotes erd
\end_inset
after all.
We notice some things are happening, notable some new types.
\end_layout
\begin_layout Subsection
Cg types
\end_layout
\begin_layout Subsubsection
Float4
\end_layout
\begin_layout Standard
float4 is a vector type.
It contains 4 elements.
It could be colors, positions, whatever.
It's used for vector processing which the GPUs are extremely efficient
at.
\end_layout
\begin_layout Subsubsection
Semantics
\end_layout
\begin_layout Standard
We see various semantics.
The POSITION semantic means that the variable is tied to vertex coordinates.
We see that we have an input POSITION, and an output (out) POSITION.
We thus transform the input to the output with a matrix multiply with the
current model-view projection.
Since this matrix is the same for every vertex, it is a uniform.
Remember that the variable names DO matter.
modelViewProj has to be called exactly that, as the emulator will pass
the MVP to this uniform.
It is in the specification.
\end_layout
\begin_layout Standard
Since we have semantics for the POSITION, etc, we can call them whatever
we want, as the Cg environment figures out what the variables mean.
\end_layout
\begin_layout Standard
The transformation happens here:
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
out_pos = mul(modelViewProj, pos);
\end_layout
\end_inset
\end_layout
\begin_layout Standard
The COLOR semantic isn't very interesting for us, but the example code in
nVidias Cg documentation includes it, so we just follow along.
\end_layout
\begin_layout Standard
TEXCOORD is the texture coordinate we get from the emulator, and generally
we just pass it to the fragment shader directly.
The coordinate will then be
\begin_inset Quotes eld
\end_inset
linearly interpolated
\begin_inset Quotes erd
\end_inset
across the fragments.
More complex shaders can output (almost) as many variables they want, that
will be linearily interpolated for free to the fragment shader.
\end_layout
\begin_layout Standard
We also need a fragment shader to go along with the vertex shader, and here's
a basic shader that only outputs the pixel as-is.
This is pretty much the result you'd get if you didn't run any shader (fixed-fu
nction) at all.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
float4 main_fragment(uniform sampler2D s0 : TEXUNIT0,
\end_layout
\begin_layout Plain Layout
float2 tex : TEXCOORD) : COLOR
\end_layout
\begin_layout Plain Layout
{
\end_layout
\begin_layout Plain Layout
return tex2D(s0, tex);
\end_layout
\begin_layout Plain Layout
}
\end_layout
\end_inset
\end_layout
\begin_layout Standard
This is arguably simpler than the vertex shader.
Important to notice are:
\end_layout
\begin_layout Standard
sampler2D is a handle to a texture in Cg.
The semantic here is TEXUNIT0, which means that it refers to the texture
in texture unit 0.
This is also part of the specification.
\end_layout
\begin_layout Standard
float2 tex : TEXCOORD is the interpolated coordinate we received from the
vertex shader.
\end_layout
\begin_layout Standard
tex2D(s0, tex); simply does texture lookup and returns a COLOR, which is
emitted to the framebuffer.
Simple enough.
Practically every fragment does more than one texture lookup.
For example, classic pixel shaders look at the neighbor pixels as well
to determine the output.
But where is the neighbor pixel? We'll revise the fragment shader and try
to make a really blurry shader to demonstrate.
We now need to pull up some uniforms.
We need to know how to modify our tex coordinates so that it points to
a neighbor pixel.
\end_layout
\begin_layout Standard
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inline false
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\begin_layout Plain Layout
struct input
\end_layout
\begin_layout Plain Layout
{
\end_layout
\begin_layout Plain Layout
float2 video_size;
\end_layout
\begin_layout Plain Layout
float2 texture_size;
\end_layout
\begin_layout Plain Layout
float2 output_size;
\end_layout
\begin_layout Plain Layout
float frame_count;
\end_layout
\begin_layout Plain Layout
};
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
float4 main_fragment(uniform sampler2D s0 : TEXUNIT0,
\end_layout
\begin_layout Plain Layout
uniform input IN, float2 tex : TEXCOORD) : COLOR
\end_layout
\begin_layout Plain Layout
{
\end_layout
\begin_layout Plain Layout
float4 result = float4(0.0);
\end_layout
\begin_layout Plain Layout
float dx = 1.0 / IN.texture_size.x;
\end_layout
\begin_layout Plain Layout
float dy = 1.0 / IN.texture_size.y;
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
// Grab some of the neighboring pixels and
\end_layout
\begin_layout Plain Layout
// blend together for a very mushy blur.
\end_layout
\begin_layout Plain Layout
result += tex2D(s0, tex + float2(-dx, -dy));
\end_layout
\begin_layout Plain Layout
result += tex2D(s0, tex + float2(dx, -dy));
\end_layout
\begin_layout Plain Layout
result += tex2D(s0, tex + float2(0.0, 0.0));
\end_layout
\begin_layout Plain Layout
result += tex2D(s0, tex + float2(-dx, 0.0));
\end_layout
\begin_layout Plain Layout
return result / 4.0;
\end_layout
\begin_layout Plain Layout
}
\end_layout
\end_inset
\end_layout
\begin_layout Standard
Here we use IN.texture_size to determine the the size of the texture.
Since GL maps the whole texture to the interval [0.0, 1.0], 1.0 / IN.texture_size
means we get the offset for a single pixel, simple enough.
Almost every shader uses this.
We can calculate these offsets in vertex shader to improve performance
since the coordinates are linearily interpolated anyways, but that is for
another time ...
;)
\end_layout
\begin_layout Subsection
Putting it together
\end_layout
\begin_layout Standard
The final runnable product is a single .cg file with the main_vertex and
main_fragment functions added together.
Not very complicated.
For the icing on the cake, you should add a license header.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
/* Stupid blur shader.
\end_layout
\begin_layout Plain Layout
Author: Your friendly neighbor.
\end_layout
\begin_layout Plain Layout
License: We don't have those things!
\end_layout
\begin_layout Plain Layout
*/
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
struct input
\end_layout
\begin_layout Plain Layout
{
\end_layout
\begin_layout Plain Layout
float2 video_size;
\end_layout
\begin_layout Plain Layout
float2 texture_size;
\end_layout
\begin_layout Plain Layout
float2 output_size;
\end_layout
\begin_layout Plain Layout
float frame_count;
\end_layout
\begin_layout Plain Layout
};
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
void main_vertex(
\end_layout
\begin_layout Plain Layout
float4 pos : POSITION,
\end_layout
\begin_layout Plain Layout
out float4 out_pos : POSITION,
\end_layout
\begin_layout Plain Layout
uniform float4x4 modelViewProj,
\end_layout
\begin_layout Plain Layout
float4 color : COLOR,
\end_layout
\begin_layout Plain Layout
out float4 out_color : COLOR,
\end_layout
\begin_layout Plain Layout
float2 tex : TEXCOORD,
\end_layout
\begin_layout Plain Layout
out float2 out_tex : TEXCOORD
\end_layout
\begin_layout Plain Layout
)
\end_layout
\begin_layout Plain Layout
{
\end_layout
\begin_layout Plain Layout
out_pos = mul(modelViewProj, pos);
\end_layout
\begin_layout Plain Layout
out_color = color; out_tex = tex;
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
float4 main_fragment(uniform sampler2D s0 : TEXUNIT0,
\end_layout
\begin_layout Plain Layout
uniform input IN, float2 tex : TEXCOORD) : COLOR
\end_layout
\begin_layout Plain Layout
{
\end_layout
\begin_layout Plain Layout
float4 result = float4(0.0);
\end_layout
\begin_layout Plain Layout
float dx = 1.0 / IN.texture_size.x;
\end_layout
\begin_layout Plain Layout
float dy = 1.0 / IN.texture_size.y;
\end_layout
\begin_layout Plain Layout