Created XML shaders (markdown)

XML-shaders.md

@@ -0,0 +1,55 @@
+## History
+XML shaders were originally implemented in bSNES as single pass GLSL shaders. The extension is .shader and is marked up with XML. These shaders were written against the fixed-function pipeline and is now referred to as a legacy XML shaders by RetroArch. The prefix of `ruby` originates from bSNES' driver module, `ruby::`.
+
+### Example legacy XML shader
+    <?xml version="1.0" encoding="UTF-8"?>
+    <!--
+         Blank shader
+    -->
+    <shader language="GLSL">
+       <fragment><![CDATA[
+          uniform sampler2D rubyTexture;
+          void main()
+          {
+             gl_FragColor = texture2D(rubyTexture, gl_TexCoord[0].xy);
+          }
+       ]]></fragment>
+    </shader>
+
+RetroArch implemented this legacy shader spec to be compatible with many shaders written at the time. It is also referred to as v1.0 XML shaders.
+This specification was then extended to support multi-pass, scaling arguments, etc, which resulted in v1.1 XML shaders (spec [here](http://gitorious.org/bsnes/pages/XmlShaderFormat)). It is still legacy as it uses fixed-function features. RetroArch implements v1.1 XML shaders, and some more features to be feature equivalent with the [[Cg shader]] implementation. bSNES did not implement v1.1 and adoption of this spec was slowed down.
+
+## Modern XML shaders
+Legacy XML shaders used fixed function, and they would therefore never work with modern GL (GLES, GL3.x+).
+To fix this, RetroArch extended the XML shader spec. Fixed function cruft like `gl_ModelViewProjectionMatrix`, `gl_MultiTexCoord0` and `gl_Vertex` was replaced with uniforms and attribute streams. The modern XML shader spec in RetroArch focuses on being compatible with GLES2 (and compatible with GL 3.x+ as well).
+
+The `ruby` prefix was later deprecated and you could use `TexCoord`, `VertexCoord` etc.
+For compatibility reasons, the `ruby` prefix is still accepted.
+
+### Example modern shader
+    <?xml version="1.0" encoding="UTF-8"?>
+    <shader language="GLSL" style="GLES2">
+       <vertex><![CDATA[
+          attribute vec2 rubyTexCoord;
+          attribute vec2 rubyVertexCoord;
+          uniform mat4 rubyMVPMatrix;
+          varying vec2 tex_coord;
+          void main()
+          {
+             gl_Position = rubyMVPMatrix * vec4(rubyVertexCoord, 0.0, 1.0);
+             tex_coord = rubyTexCoord;
+          }
+       ]]></vertex>
+       <fragment><![CDATA[
+          uniform sampler2D rubyTexture;
+          varying vec2 tex_coord;
+          void main()
+          {
+             gl_FragColor = texture2D(rubyTexture, tex_coord);
+          }
+       ]]></fragment>
+    </shader>
+
+## Moving off XML shaders
+XML shaders as a whole are deprecated in RetroArch, and will not be selectable in [[RGUI]].
+You can still use them via `video_shader` config option. To use GLSL in RetroArch, the new [[GLSL shader]] format is used, which mirrors the [[Cg shader]] implementation quite well. The new GLSL shaders only support modern style, no fixed function. To convert XML shaders into straight GLSL, see [[GLSL shader]].