Metal equivalent to OpenGL mix - ios

Question

I'm trying to understand what is the equivalent of mix OpenGL function in metal. This is the OpenGL code I'm trying to convert:
float udRoundBox( vec2 p, vec2 b, float r )
{
return length(max(abs(p)-b+r,0.0))-r;
}
void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
// setup
float t = 0.2 + 0.2 * sin(mod(iTime, 2.0 * PI) - 0.5 * PI);
float iRadius = min(iResolution.x, iResolution.y) * (0.05 + t);
vec2 halfRes = 0.5 * iResolution.xy;
// compute box
float b = udRoundBox( fragCoord.xy - halfRes, halfRes, iRadius );
// colorize (red / black )
vec3 c = mix( vec3(1.0,0.0,0.0), vec3(0.0,0.0,0.0), smoothstep(0.0,1.0,b) );
fragColor = vec4( c, 1.0 );
}
I was able to convert part of it so far:
float udRoundBox( float2 p, float2 b, float r )
{
return length(max(abs(p)-b+r,0.0))-r;
}
float4 cornerRadius(sampler_h src) {
float2 greenCoord = src.coord(); // this is alreay in relative coords; no need to devide by image size
float t = 0.5;
float iRadius = min(greenCoord.x, greenCoord.y) * (t);
float2 halfRes = float2(greenCoord.x * 0.5, greenCoord.y * 0.5);
float b = udRoundBox( float2(greenCoord.x - halfRes.x, greenCoord.y - halfRes.y), halfRes, iRadius );
float3 c = mix(float3(1.0,0.0,0.0), float3(0.0,0.0,0.0), smoothstep(0.0,1.0,b) );
return float4(c, 1.0);
}
But it's producing green screen. I'm trying to achieve corner radius on a video like so:

Answer 1

The mix function is an implementation of linear interpolation, more frequently referred to as a Lerp function.
You can use linear interpolation where you have a value, let's say t and you want to know how that value maps within a certain range.
For example if I have three values:
a = 0
b = 1
and
t = 0.5
I could call mix(a,b,t) and my result would be 0.5. That is because the mix function expects a start range value, an end range value and a factor by which to interpolate, so I get 0.5 which is halfway between 0 and 1.
Looking at the documentation Metal has an implementation of mix that does a linear interpolation.

Answer 2

The problem is, that greenCoord (which was only a good variable name for the other question you asked, by the way) is the relative coordinate of the current pixel and has nothing to do with the absolute input resolution.
If you want a replacement for your iResolution, use src.size() instead.
And it seems you need your input coordinates in absolute (pixel) units. You can achieve that by adding a destination parameter to the inputs of your kernel like so:
float4 cornerRadius(sampler src, destination dest) {
const float2 destCoord = dest.coord(); // pixel position in the output buffer in absolute coordinates
const float2 srcSize = src.size();
const float t = 0.5;
const float radius = min(srcSize.x, srcSize.y) * t;
const float2 halfRes = 0.5 * srcSize;
const float b = udRoundBox(destCoord - halfRes, halfRes, radius);
const float3 c = mix(float3(1.0,0.0,0.0), float3(0.0,0.0,0.0), smoothstep(0.0,1.0,b) );
return float4(c, 1.0);
}