GLSL and struct arrays

OpenGL Development
#1

Hello,

I want to compute multiple lights in my fragment shader. I pass the light data to the vertex shader via a uniform struct[] and then I want to do something like that:

struct Global_lights {
  ...
};

uniform ...
uniform Global_lights[] lights;

out ...
out Global_lights[] frag_lights;

void pass_lights() {
	frag_lights = lights;
}

The fragment shader:

struct Global_lights {
  ...
};

in ...
in Global_lights[] frag_lights;

If I try to compile this I get two errors:
Error compiling Shader
Vertex shader failed to compile with the following errors:
ERROR: 0:27: error(#282) Implicitly sized arrays can not be assigned to
ERROR: 0:27: error(#160) Cannot convert from: “uniform array of structure” to: “default out array of structure”
ERROR: error(#273) 2 compilation errors. No code generated

I somewhat understand the error message but I’d like some kind of source to read what is allowed and what not. Can someone offer a soluting? The exact amount of lights has to variable.
Thanks!

#2

when u want to use a uniform in the fragment-shader, you don’t have to pass it through the vertex-shader.
just declare the uniform array in the fragment shader.

When you want to use a uniform in multiple shader stages(vertex, fragment …) just declare it with the same name and type in all stages were you want to use it.

#3

Wow thanks, I didn’t konw that.
If I declare a uniform in the fragment shader because I only need it in the fragment shader, do I still have to declare it in vertex shader as well? My test suggests that but it doesn’t really makes sense.
vertex shader

#version 140

in vec4 vertex_position;
in vec4 vertex_color;

uniform vec2 camera_offset;
uniform mat4 mvp_matrix;
// if I remove this the color uniform the uniform in the frag shader is not what it should be
uniform vec4 color;

out vec4 frag_position;
out vec4 frag_colorcode;

void main() {
	frag_position = vec4(vertex_position.xy - camera_offset.xy, vertex_position.zw) * mvp_matrix;
	frag_colorcode = vertex_color;
	gl_Position = frag_position;
}

fragment shader

 #version 140
 
//@insert global_lights.struct
 
in vec4 frag_position;
in vec4 frag_colorcode;

uniform vec4 color;
uniform Global_lights[] frag_lights;
 
 void main() {
 	gl_FragColor = (color + frag_colorcode);
}

Thanks!

#4

If you only use a uniform in the frag shader, you don’t need to declare it in the vertex shader, and vice versa.

#5

Okay I found the problem! Thank you.
One followup question: Is there a way to determine the length of an implicit declared (uniform) array? .length() doesn’t work. Is there some other way to iterate over the array?

#6

There are no implicitly declared arrays in GLSL. You can pass a uniform variable for the length and only compute lighting for the relevant lights, but you need an explicit size on the uniform struct array.

#7

Hm I see … arg why is everything so uncomfortable …

Another followup question:
Lets say I have 32 fixed lights. All have a specific position in model space. I have to transform these light coords with the mvpMatrix to get my light calaulations right. It makes absolutly no sense to perform this calculation for every fragment so I wanted to do it once in the vertex shader and then pass it via the positions via an out variable to my fragment shader. Problem is: The amount of “in”-variables in the fragment shader is limited. Much more limited then the amount of uniforms.

Is there a solution to this problem?

#8

Do it on the CPU once for each light.