I think generating rivers is a backward approach as you would need to tweak a lot of things according to their shape later on which will be hard. I would instead create random terrain height map and extract features from it (as in the real world) which is much easier and closer to reality. In the final map you ignore the height and use flat one (if you really want a flat map). Here are few things you can extract from height map:
Rivers and lakes
by seeding random high altitude point and following it downhill to sea level or edge of map.
vegetation or ground
from slope and altitude you can determine if ground is sand,dirt,rock. If there are trees, bushes, grass or whatever.
Here look at this QA: random island generator
and some river overview:
The way you tweak the terrain generation will affect also the river shapes (no need to generate just islands).
The Seeds are working also for this approach.
[Edit1] promised C++ code
This basically generate random height map and then seed and downhill follow the rivers (lakes are generated automatically if the terrain block downhill watter flow). The terrain type is also determined from slope and altitude.
//---------------------------------------------------------------------------
picture pic;
//---------------------------------------------------------------------------
void map_random(int _xs,int _ys)
{
// config
int h0=-1000,h1=3000; // [m] terrain elevation range
int h_water= 0; // [m] sea level
int h_sand=15; // [m] sand level
int h_evergreen=1500; // [m] evergreen level
int h_snow=2000; // [m] snow level
int h_rock=1800; // [m] mountine rock level
float a_rock=60.0; // [deg] mountine rock slope
float d_pixel=35.0; // [m] pixel size
int d_river_w=5; // [pixel] river max width
int d_river_l=150; // [pixel] river base length per width increase
bool _island=true;
// types
enum _cover_enum
{
_cover_none=0,
_cover_water, // sea
_cover_snow,
_covers,
_cover_shift=0,
_cover_mask=15,
};
DWORD _cover[_covers]=
{
// RRGGBB
0x00000000, // none
0x00003080, // watter (sea)
0x00EEEEEE, // snow
};
enum _terrain_enum
{
_terrain_dirt=0,
_terrain_sand,
_terrain_rock,
_terrain_water, // streams,rivers,lakes
_terrain_temp, // temp
_terrains,
_terrain_shift=4,
_terrain_mask=15,
};
DWORD _terrain[_terrains]=
{
// RRGGBB
0x00301510, // dirt
0x00EEC49A, // sand
0x006F6F6F, // rock
0x00006080, // water (streams,rivers,lakes)
0x00006080, // temp
};
enum _flora_enum
{
_flora_none=0,
_flora_grass,
_flora_hardwood,
_flora_evergreen,
_flora_deadwood,
_floras,
_flora_shift=8,
_flora_mask=15,
};
DWORD _flora[_floras]=
{
// RRGGBB
0x00000000, // none
0x007F7F3F, // grass
0x001FFF1F, // hardwood
0x00007F00, // evergreen
0x007F3F1F, // deadwood
};
// variables
float a,b,da; int c,t,f;
int x,y,z,xx,yy,mxs,mys,dx,dy,dx2,dy2,r,r2,ix,l;
int xh1,yh1; // topest hill position
int **ter=NULL,**typ=NULL;
Randomize();
// align resolution to power of 2
for (mxs=1;mxs+1<_xs;mxs<<=1); if (mxs<3) mxs=3;
for (mys=1;mys+1<_ys;mys<<=1); if (mys<3) mys=3;
ter=new int*[mys+1]; for (y=0;y<=mys;y++) ter[y]=new int[mxs+1];
typ=new int*[mys+1]; for (y=0;y<=mys;y++) typ[y]=new int[mxs+1];
// [Terrain]
for (;;)
{
// diamond & square random height map -> ter[][]
dx=mxs; dx2=dx>>1; r=(mxs+mys)<<1; // init step,half step and randomness
dy=mys; dy2=dy>>1; r2=r>>1;
// set corners values
if (_island)
{
t=-r2;
ter[ 0][ 0]=t;
ter[ 0][mxs]=t;
ter[mys][ 0]=t;
ter[mys][mxs]=t;
ter[dy2][dx2]=r+r; // top of central hill
}
else{
ter[ 0][ 0]=Random(r);
ter[ 0][mxs]=Random(r);
ter[mys][ 0]=Random(r);
ter[mys][mxs]=Random(r);
}
for (;dx2|dy2;dx=dx2,dx2>>=1,dy=dy2,dy2>>=1) // subdivide step until full image is filled
{
if (!dx) dx=1;
if (!dy) dy=1;
// diamond (skip first one for islands)
if ((!_island)||(dx!=mxs))
for (y=dy2,yy=mys-dy2;y<=yy;y+=dy)
for (x=dx2,xx=mxs-dx2;x<=xx;x+=dx)
ter[y][x]=((ter[y-dy2][x-dx2]+ter[y-dy2][x+dx2]+ter[y+dy2][x-dx2]+ter[y+dy2][x+dx2])>>2)+Random(r)-r2;
// square
for (y=dy2,yy=mys-dy2;y<=yy;y+=dy)
for (x=dx ,xx=mxs-dx ;x<=xx;x+=dx)
ter[y][x]=((ter[y][x-dx2]+ter[y][x+dx2]+ter[y-dy2][x]+ter[y+dy2][x])>>2)+Random(r)-r2;
for (y=dy ,yy=mys-dy ;y<=yy;y+=dy)
for (x=dx2,xx=mxs-dx2;x<=xx;x+=dx)
ter[y][x]=((ter[y][x-dx2]+ter[y][x+dx2]+ter[y-dy2][x]+ter[y+dy2][x])>>2)+Random(r)-r2;
for (x=dx2,xx=mxs-dx2;x<=xx;x+=dx)
{
y= 0; ter[y][x]=((ter[y][x-dx2]+ter[y][x+dx2]+ter[y+dy2][x])/3)+Random(r)-r2;
y=mys; ter[y][x]=((ter[y][x-dx2]+ter[y][x+dx2]+ter[y-dy2][x])/3)+Random(r)-r2;
}
for (y=dy2,yy=mys-dy2;y<=yy;y+=dy)
{
x= 0; ter[y][x]=((ter[y][x+dx2]+ter[y-dy2][x]+ter[y+dy2][x])/3)+Random(r)-r2;
x=mxs; ter[y][x]=((ter[y][x-dx2]+ter[y-dy2][x]+ter[y+dy2][x])/3)+Random(r)-r2;
}
if (_island)
{
// recompute middle position after first pass so there can be more central hills
if (dx==mxs) ter[dy2][dx2]=Random(r2);
// adjust border to underwatter
for (y=0;y<=mys;y+=dy2) { ter[y][0]=t; ter[y][mxs]=t; }
for (x=0;x<=mxs;x+=dx2) { ter[0][x]=t; ter[mys][x]=t; }
}
// adjust randomness
r>>=1; if (r<2) r=2; r2=r>>1;
}
// rescale to <h0,h1>
xx=ter[0][0]; yy=xx;
for (y=0;y<=mys;y++)
for (x=0;x<=mxs;x++)
{
z=ter[y][x];
if (xx>z) xx=z;
if (yy<z){ yy=z; xh1=x; yh1=y; }
}
for (y=0;y<=mys;y++)
for (x=0;x<=mxs;x++)
ter[y][x]=h0+(((ter[y][x]-xx)*(h1-h0))/(yy-xx));
// test for correctness
if (_island)
{
l=0;
for (x=0;x<=mxs;x++) { if (ter[0][x]>h_water) l++; if (ter[mys][x]>h_water) l++; }
for (y=0;y<=mys;y++) { if (ter[y][0]>h_water) l++; if (ter[y][mxs]>h_water) l++; }
if (l>1+((mxs+mys)>>3)) continue;
}
break;
}
// [Surface]
for (y=0;y<mys;y++)
for (x=0;x<mxs;x++)
{
z=ter[y][x];
// max slope [deg]
a=atan2(ter[y][x+1]-z,d_pixel);
b=atan2(ter[y+1][x]-z,d_pixel);
if (a<b) a=b; a*=180.0/M_PI;
c=_cover_none;
if (z<=h_water) c=_cover_water;
if (z>=h_snow ) c=_cover_snow;
t=_terrain_dirt;
if (z<=h_sand) t=_terrain_sand;
if (z>=h_rock) t=_terrain_rock;
if (a>=a_rock) t=_terrain_rock;
f=_flora_none;
if (t==_terrain_dirt)
{
r=Random(100);
if (r>10) f=_flora_grass;
if (r>50)
{
if (z>h_evergreen) f=_flora_evergreen;
else{
r=Random(h_evergreen);
if (r<=z) f=_flora_evergreen;
else f=_flora_hardwood;
}
}
if (r<5) f=_flora_deadwood;
}
typ[y][x]=(c<<_cover_shift)|(t<<_terrain_shift)|(f<<_flora_shift);
}
// [Rivers]
for (ix=10+Random(5),a=0.0,da=2.0*M_PI/float(ix);ix;ix--)
{
// random start around topest hill
a+=da*(0.75+(0.50*Random()));
for (l=0;l<10;l++)
{
b=Random(mxs>>3);
x=xh1; x+=float(b*cos(a));
y=yh1; y+=float(b*sin(a));
if ((x<1)||(x>=mxs)) continue;
if ((y<1)||(y>=mys)) continue;
if (typ[y][x]&0x00F==_cover_water) continue;
l=-1;
break;
} if (l>=0) continue; // safety check
for (l=0,r2=0;;)
{
// stop on map edge
if ((x<=0)||(x>=mxs-1)||(y<=0)||(y>=mys-1)) break;
// decode generated surface
r=typ[y][x];
c=(r>> _cover_shift)& _cover_mask;
t=(r>>_terrain_shift)&_terrain_mask;
f=(r>> _flora_shift)& _flora_mask;
// stop if reached sea
if (c==_cover_water) break;
// insert river dot radius = r2
dx=x-r2; if (dx<0) dx=0; dx2=x+r2; if (dx2>=mxs) dx2=mxs-1;
dy=y-r2; if (dy<0) dy=0; dy2=y+r2; if (dy2>=mys) dy2=mys-1;
for (yy=dy;yy<=dy2;yy++)
for (xx=dx;xx<=dx2;xx++)
if (((xx-x)*(xx-x))+((yy-y)*(yy-y))<=r2*r2)
if (((typ[yy][xx]>>_terrain_shift)&_terrain_mask)!=_terrain_water)
typ[yy][xx]=(typ[yy][xx]&0x00F)|(_terrain_temp<<_terrain_shift);
// step to smalest elevation neighbor
dx=x; dy=y; z=h1; typ[y][x]=(typ[y][x]&0x00F)|(_terrain_water<<_terrain_shift); xx=x; yy=y;
xx--; r=ter[yy][xx]; if ((z>=r)&&(((typ[yy][xx]>>_terrain_shift)&_terrain_mask)!=_terrain_water)) { z=r; dx=xx; dy=yy; }
yy--; r=ter[yy][xx]; if ((z>=r)&&(((typ[yy][xx]>>_terrain_shift)&_terrain_mask)!=_terrain_water)) { z=r; dx=xx; dy=yy; }
xx++; r=ter[yy][xx]; if ((z>=r)&&(((typ[yy][xx]>>_terrain_shift)&_terrain_mask)!=_terrain_water)) { z=r; dx=xx; dy=yy; }
xx++; r=ter[yy][xx]; if ((z>=r)&&(((typ[yy][xx]>>_terrain_shift)&_terrain_mask)!=_terrain_water)) { z=r; dx=xx; dy=yy; }
yy++; r=ter[yy][xx]; if ((z>=r)&&(((typ[yy][xx]>>_terrain_shift)&_terrain_mask)!=_terrain_water)) { z=r; dx=xx; dy=yy; }
yy++; r=ter[yy][xx]; if ((z>=r)&&(((typ[yy][xx]>>_terrain_shift)&_terrain_mask)!=_terrain_water)) { z=r; dx=xx; dy=yy; }
xx--; r=ter[yy][xx]; if ((z>=r)&&(((typ[yy][xx]>>_terrain_shift)&_terrain_mask)!=_terrain_water)) { z=r; dx=xx; dy=yy; }
xx--; r=ter[yy][xx]; if ((z>=r)&&(((typ[yy][xx]>>_terrain_shift)&_terrain_mask)!=_terrain_water)) { z=r; dx=xx; dy=yy; }
if ((dx==x)&&(dy==y))
{
// handle invalid path or need for a lake!!!
if (dx>mxs>>1) dx++; else dx--;
if (dy>mys>>1) dy++; else dy--;
}
x=dx; y=dy;
// increase river volume with length
l++; if (l>d_river_l*(r2+1)) { l=0; if (r2<d_river_w) r2++; }
}
// make merging of rivers possible
for (y=0;y<=mys;y++)
for (x=0;x<=mxs;x++)
if (((typ[y][x]>>_terrain_shift)&_terrain_mask)==_terrain_water)
typ[y][x]=(typ[y][x]&0x00F)|(_terrain_temp<<_terrain_shift);
}
for (y=0;y<=mys;y++)
for (x=0;x<=mxs;x++)
if (((typ[y][x]>>_terrain_shift)&_terrain_mask)==_terrain_temp)
typ[y][x]=(typ[y][x]&0x00F)|(_terrain_water<<_terrain_shift);
// [copy data] rewrite this part to suite your needs
for (y=1;y<_ys;y++)
for (x=1;x<_xs;x++)
{
float nx,ny,nz,x0,y0,z0,x1,y1,z1;
// (nx,ny,nz) = surface normal
nx=0.0; ny=0.0; nz=ter[y][x];
x0=-d_pixel; y0=0.0; z0=ter[y][x-1];
x1=0.0; y1=-d_pixel; z1=ter[y-1][x];
x0-=nx; x1-=nx;
y0-=ny; y1-=ny;
z0-=nz; z1-=nz;
nx=(y0*z1)-(z0*y1);
ny=(z0*x1)-(x0*z1);
nz=(x0*y1)-(y0*x1);
x0=1.0/sqrt((nx*nx)+(ny*ny)+(nz*nz));
nx*=x0;
ny*=x0;
nz*=x0;
// z = ambient light + normal shading
nz=(+0.7*nx)+(-0.7*ny)+(+0.7*nz);
if (nz<0.0) nz=0.0;
nz=255.0*(0.2+(0.8*nz)); z=nz;
// r = base color
r=typ[y][x];
c=(r>> _cover_shift)& _cover_mask;
t=(r>>_terrain_shift)&_terrain_mask;
f=(r>> _flora_shift)& _flora_mask;
r=_terrain[t];
if (c) r= _cover[c];
if (f){ if (c) r|=_flora[f]; else r=_flora[f]; };
// sea color is depending on depth not surface normal
if (c==_cover_water) z=256-((ter[y][x]<<7)/h0);
// apply lighting z to color r
yy=int(r>>16)&255; yy=(yy*z)>>8; if (yy>255) yy=255; r=(r&0x0000FFFF)|(yy<<16);
yy=int(r>> 8)&255; yy=(yy*z)>>8; if (yy>255) yy=255; r=(r&0x00FF00FF)|(yy<< 8);
yy=int(r )&255; yy=(yy*z)>>8; if (yy>255) yy=255; r=(r&0x00FFFF00)|(yy );
// set pixel to target image
pic.p[y][x].dd=r;
}
// free ter[][],typ[][]
for (y=0;y<=mys;y++) delete[] ter[y]; delete[] ter; ter=NULL;
for (y=0;y<=mys;y++) delete[] typ[y]; delete[] typ; typ=NULL;
}
//---------------------------------------------------------------------------
The code is based on the code from the linked Answer of mine but with added features (rivers included). I use my own picture class for images so some members are:
xs,ys size of image in pixelsp[y][x].dd is pixel at (x,y) position as 32 bit integer typeclear(color) - clears entire imageresize(xs,ys) - resizes image to new resolutionbmp - VCL encapsulated GDI Bitmap with Canvas access
You can tweak the adjust randomness in Diamond&Square to change the terrain smoothness. Also the height limits and tresholds can be tampered with.
To achieve more brunching like rivers seed more start points in clusters so they should merge in time into single or more rivers.
