IOS Metal Fragment shader file compilation - ios

Question

I have fragment shader "fsh" file and I am trying to compile it, it is originally taken from Shadertoy, and it is in GLSL, I am trying to port it to METAL, and I am getting the following error:
program_source:129:12: error: program scope variable must reside in constant address space
const vec3 ro, rd;
As far as I can understand, I can not define ro and rd in global scope like this, how can I fix this ?
Thank you very much.
The code is below:
const vec3 ro, rd;
....
void main(void)
{
float t = u_time;
vec3 col = vec3(0.);
vec2 uv = gl_FragCoord.xy / iResolution.xy; // 0 <> 1
uv -= .5;
uv.x *= iResolution.x/iResolution.y;
vec2 mouse = gl_FragCoord.xy/iResolution.xy;
vec3 pos = vec3(.3, .15, 0.);
float bt = t * 5.;
float h1 = N(floor(bt));
float h2 = N(floor(bt+1.));
float bumps = mix(h1, h2, fract(bt))*.1;
bumps = bumps*bumps*bumps*CAM_SHAKE;
pos.y += bumps;
float lookatY = pos.y+bumps;
vec3 lookat = vec3(0.3, lookatY, 1.);
vec3 lookat2 = vec3(0., lookatY, .7);
lookat = mix(lookat, lookat2, sin(t*.1)*.5+.5);
uv.y += bumps*4.;
CameraSetup(uv, pos, lookat, 2., mouse.x);
t *= .03;
t += mouse.x;
// fix for GLES devices by MacroMachines
#ifdef GL_ES
const float stp = 1./8.;
#else
float stp = 1./8.;
#endif
for(float i=0.; i<1.; i+=stp) {
col += StreetLights(i, t);
}
for(float i=0.; i<1.; i+=stp) {
float n = N(i+floor(t));
col += HeadLights(i+n*stp*.7, t);
}
#ifndef GL_ES
#ifdef HIGH_QUALITY
stp = 1./32.;
#else
stp = 1./16.;
#endif
#endif
for(float i=0.; i<1.; i+=stp) {
col += EnvironmentLights(i, t);
}
col += TailLights(0., t);
col += TailLights(.5, t);
col += sat(rd.y)*vec3(.6, .5, .9);
gl_FragColor = vec4(col, 0.);
}

Answer 1

The equivalent declaration in Metal Shading Language (MSL) would be
constant float3 ro, rd;
However, you should also initialize these variables with values, since your shader functions will not be allowed to mutate them. Something like
constant float3 ro(0, 0, 0), rd(1, 1, 1);
A few more translation hints:
Metal doesn't have syntax for declaring uniforms. Instead, you'll need to pass such values in via a buffer in the constant or device address space. This includes things like your screen resolution and time variables.
Vector type names generally start with the name of their element type, followed by the number of elements (half2, float3). There are no explicit precision qualifiers in MSL.
Rather than writing to special values like gl_FragColor, basic fragment functions in Metal return a color value (which by convention is written to the first color attachment of the framebuffer, provided it passes the depth and stencil test).