GLSL 3.30 - OpenGL

4 Variables and Types
It is a compile-time error if a layout declaration's array size (from table above) does not match any array
size specified in declarations of an input variable in the same shader. The following are all examples of
compile time errors:
// code sequence within one shader...
in vec4 Color1[]; // size unknown
...Color1.length()...// illegal, length() unknown
in vec4 Color2[2]; // size is 2
...Color1.length()...// illegal, Color1 still has no size
in vec4 Color3[3]; // illegal, input sizes are inconsistent
layout(lines) in; // legal, input size is 2, matching
in vec4 Color4[3]; // illegal, contradicts layout
...Color1.length()...// legal, length() is 2, Color1 sized by layout()
layout(lines) in; // legal, matches other layout() declaration
layout(triangles) in;// illegal, does not match earlier layout() declaration
It is a link-time error if not all provided sizes (sized input arrays and layout size) match across all
geometry shaders in the program.
Fragment shaders can have an input layout only for redeclaring the built-in variable gl_FragCoord (see
section 7.2 “Fragment Shader Special Variables”). The layout qualifier identifiers for gl_FragCoord are
layout-qualifier-id
origin_upper_left
pixel_center_integer
By default, gl_FragCoord assumes a lower-left origin for window coordinates and assumes pixel centers
are located at half-pixel coordinates. For example, the (x, y) location (0.5, 0.5) is returned for the lowerleft-most
pixel in a window. The origin can be changed by redeclaring gl_FragCoord with the
origin_upper_left identifier, moving the origin of gl_FragCoord to the upper left of the window, with y
increasing in value toward the bottom of the window. The values returned can also be shifted by half a
pixel in both x and y by pixel_center_integer so it appears the pixels are centered at whole number pixel
offsets. This moves the (x, y) value returned by gl_FragCoord of (0.5, 0.5) by default, to (0.0, 0.0) with
pixel_center_integer. Redeclarations are done as follows
in vec4 gl_FragCoord;
// redeclaration that changes nothing is allowed
// All the following are allowed redeclaration that change behavior
layout(origin_upper_left) in vec4 gl_FragCoord;
layout(pixel_center_integer) in vec4 gl_FragCoord;
layout(origin_upper_left, pixel_center_integer) in vec4 gl_FragCoord;
If gl_FragCoord is redeclared in any fragment shader in a program, it must be redeclared in all the
fragment shaders in that program that have a static use gl_FragCoord. All redeclarations of
gl_FragCoord in all fragment shaders in a single program must have the same set of qualifiers. Within
any shader, the first redeclarations of gl_FragCoord must appear before any use of gl_FragCoord. The
built-in gl_FragCoord is only predeclared in fragment shaders, so redeclaring it in any other shader
language will be illegal.
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