Update screen-reading shaders page to 3.0 shading language

tutorials/shading/screen-reading_shaders.rst

@@ -25,9 +25,11 @@ It takes as parameter the UV of the screen and returns a vec3 RGB with the color
 special built-in varying: SCREEN_UV can be used to obtain the UV for
 the current fragment. As a result, this simple 2D fragment shader:
 
-::
+.. code-block:: glsl
 
-    COLOR=textureLod( SCREEN_TEXTURE, SCREEN_UV, 0.0);
+	void fragment() {}
+		COLOR=textureLod( SCREEN_TEXTURE, SCREEN_UV, 0.0);
+	}
 
 results in an invisible object, because it just shows what lies behind.
 
@@ -45,7 +47,7 @@ special demo for *Screen Space Shaders*, that you can download to see
 and learn. One example is a simple shader to adjust brightness, contrast
 and saturation:
 
-::
+.. code-block:: glsl
 
 	shader_type canvas_item;
 
@@ -53,13 +55,15 @@ and saturation:
 	uniform float contrast = 1.0;
 	uniform float saturation = 1.0;
 
-	vec3 c = textureLod(SCREEN_TEXTURE, SCREEN_UV, 0.0).rgb;
+	void fragment() {
+		vec3 c = textureLod(SCREEN_TEXTURE, SCREEN_UV, 0.0).rgb;
 
-	c.rgb = mix(vec3(0.0), c.rgb, brightness);
-	c.rgb = mix(vec3(0.5), c.rgb, contrast);
-	c.rgb = mix(vec3(dot(vec3(1.0), c.rgb)*0.33333), c.rgb, saturation);
+		c.rgb = mix(vec3(0.0), c.rgb, brightness);
+		c.rgb = mix(vec3(0.5), c.rgb, contrast);
+		c.rgb = mix(vec3(dot(vec3(1.0), c.rgb)*0.33333), c.rgb, saturation);
 
-	COLOR.rgb = c;
+		COLOR.rgb = c;
+	}
 
 Behind the scenes
 ~~~~~~~~~~~~~~~~~
@@ -125,8 +129,10 @@ converted via the inverse projection matrix.
 
 The following code retrieves the 3D position below the pixel being drawn:
 
-::
-
-	float depth = textureLod(DEPTH_TEXTURE,SCREEN_UV,0.0).r;
-	vec4 upos = INV_PROJECTION_MATRIX * vec4(SCREEN_UV*2.0-1.0,depth*2.0-1.0,1.0);
-	vec3 pixel_position = upos.xyz/upos.w;
+.. code-block:: glsl
+	
+	void fragment() {
+		float depth = textureLod(DEPTH_TEXTURE,SCREEN_UV,0.0).r;
+		vec4 upos = INV_PROJECTION_MATRIX * vec4(SCREEN_UV*2.0-1.0,depth*2.0-1.0,1.0);
+		vec3 pixel_position = upos.xyz/upos.w;
+	}