Spark ar question

can somebody please help me figure out why this shader doesn’t work?
I think there is some coordinate problems

#define PI2 6.28318530717959
#define PNUM 20

vec2 filterUV1(vec2 uv, std::Texture2d iChannel1)
{
// iq’s improved texture filtering (Shader - Shadertoy BETA)
vec2 x = uv * (iChannel1.size).xy;
vec2 p = floor(x);
vec2 f = fract(x);
f = ff(3.0-2.0*f);
return (p+f)/(iChannel1.size).xy;
}

vec4 getPixel(int x, int y, std::Texture2d iChannel0)
{
return iChannel0.sample(vec2(float(x)+.5,float(y)+.5) / (iChannel0.size).xy);
}

bool isPixel(int x, int y, vec2 fragCoord, std::Texture2d iChannel0)
{
vec2 iResolution = std::getRenderTargetSize();
fragCoord = fragment(floor(std::getRenderTargetSize() * std::getVertexTexCoord()));
vec2 c=fragCoord/iResolution.xy * (iChannel0.size).xy;
return ( int(c.x)==x && int(c.y)==y );
}

vec2 readPos(int i, std::Texture2d iChannel0)
{
return getPixel(i,0,iChannel0).xy;
}

bool writePos(vec2 pos, int i, vec2 fragCoord, std::Texture2d iChannel0, inout vec4 fragColor)
{

    fragCoord = fragment(floor(std::getRenderTargetSize() * std::getVertexTexCoord()));
if (isPixel(i,0,fragCoord, iChannel0))

{
fragColor.xy=pos; return true;
}
return false;
}

vec4 getRand(vec2 pos, std::Texture2d iChannel1)
{
return iChannel1.sample(filterUV1(pos/vec2(400,300), iChannel1));
}

float dotDist(vec2 pos,vec2 fragCoord)
{
fragCoord = fragment(floor(std::getRenderTargetSize() * std::getVertexTexCoord()));
return length(pos-fragCoord);
}

// iq: Inigo Quilez :: computer graphics, mathematics, shaders, fractals, demoscene and more
float lineDist(vec2 a,vec2 b,vec2 p)
{
vec2 pa = p - a, ba = b - a;
float h = clamp( dot(pa,ba)/dot(ba,ba), 0.0, 1.0 );
return length( pa - ba*h );
}

vec4 drawDot(vec2 pos,float r, vec2 fragCoord)
{
fragCoord = fragment(floor(std::getRenderTargetSize() * std::getVertexTexCoord()));
return vec4(clamp(r-length(pos-fragCoord),0.,1.)/r*3.);
}

#define N(x) (x.yx*vec2(1,-1))

// gives a parametric position on a pentagram with radius 1 within t=0…5
// (maybe there’s more elegant ways to do this…)
vec2 pentaPos(float t)
{
float w=sqrt((5.+sqrt(5.)).5);
float s=sqrt(1.-w
w*.25);
float ang=-floor(t)PI22./5.;
vec2 x=vec2(cos(ang),sin(ang));
return -N(x)s+xw*(fract(t)-.5);
}

void mainImage( in float iFrame, in vec2 iMouse, in std::Texture2d iChannel0, in std::Texture2d iChannel1, out vec4 fragColor )
{
vec2 iResolution = std::getRenderTargetSize();
vec2 fragCoord = fragment(floor(std::getRenderTargetSize() * std::getVertexTexCoord()));
float iTime = std::getTime();

float time=float(iFrame)*1./60.;
vec2 uv=fragCoord/iResolution.xy;
float v=0.;
for(int i=0;i<50;i++)
v += iChannel0.sample(getRand(vec2(i,0), iChannel1).xy).x/50.;
fragColor = iChannel0.sample(uv);
int pnum = int(min(iResolution.y/50.0,float(PNUM-1)));
bool write=false;
for(int i=0;i<PNUM;i++)
{
    bool isMouse = (i==pnum);
    // breaking here if i>pnum didnt work in windows (failed to unloll loop)
    if(i<=pnum) {
    vec2 pos;
        
    pos=readPos(i, iChannel0);
    vec2 oldpos=pos;
    
	float ang = (getRand(pos, iChannel1)+getRand(pos+vec2(1,3)*time, iChannel1)).x*PI2;
	pos+=vec2(.7,0)
        +vec2(4,5)*vec2(cos(15.*time+float(i)),
                        .5*sin(15.*time+float(i)+.5)+
                        .5*sin(21.*time+float(i)+.5))*getRand(pos, iChannel1).x;
        //+vec2(.2,2)*vec2(cos(ang),sin(ang));
	//vec4 c = drawDot(mod(pos,iResolution.xy),2.5,fragCoord);

    if(isMouse) 
    {
        pos=iMouse.xy;
        if(iMouse.xy==vec2(0) && mod(iTime+5.,37.7)>18.)
        {
            pos=pentaPos(iTime*.5)*.45*iResolution.y+iResolution.xy*.5;
            pos+=(getRand(pos*.6+iTime*vec2(.1,1.), iChannel1).xy-.5)*7./500.*iResolution.y;
        }
    	if(length(oldpos-pos)>40.) oldpos=pos;
    }
            
    vec2 mpos=mod(pos,iResolution.xy);
    //float dd = dotDist(mpos,fragCoord);
    float dd = lineDist(mpos,oldpos-(pos-mpos),fragCoord);
	vec4 c = vec4(clamp((isMouse?5.:3.)-dd,0.,1.9),0,max(0.,1.-dd/40.),0);
    if(mpos==oldpos-(pos-mpos)) c=vec4(0.); // ignore 0-length segments
    if(getRand(pos*.3+time, iChannel1).z>.8 && !isMouse) 
        pos+=vec2(10,0);
    else
		fragColor = max(fragColor,c);        

    if(writePos(pos, i, fragCoord, iChannel0, fragColor)) write=true;
    }
}

if(!write)
{
   fragColor.z=max(-1.,fragColor.z-.002);
   fragColor.x=max(0.,fragColor.x-.003);
}
    
if(iTime<0.) 
{
    fragColor=vec4(0,0,.6,0);
    for(int i=0;i<PNUM;i++)
	{
        if(i<=pnum-int((iTime-5.0)*0.125*float(PNUM)))
        {
            vec4 rnd = iChannel1.sample(vec2(float(i)+.5,.5) / (iChannel1.size).xy);
    	    writePos(vec2(0.+rnd.x*80.,iResolution.y/float(pnum)*float(i+1)), i, fragCoord, iChannel0, fragColor);     
        }
    }
}

}

Can you share the project? It is really hard to tell what is going on just by looking at this. What is the output supposed to look like? Can you link to the original shadertoy?

Old! Horror Movie.arexport (1.3 MB)

i got everything right in this shader except the satanic scribbles that i think is happening in buffer a
is it because of wrong coordinates ?

There is so much going on in this shader, but the problem seems to be just with Buffer A. You can step through that BufferA shader on shadertoy and compare the outputs with your spark shader. Start from the beginning of the shader and add to it until you find the problem area.