opengl es - Thick 3D textured lines with constant screen width -

i'm trying draw 3d textured lines constant screen width. (opengl es 2.0)

in reality wanted have simple "volumetric" lines minimum guaranteed (clamped) screen width. when far away don't become thin.

any input on how there appreciated but have feedback on problem i'm facing right now. misunderstanding of how perspective correct interpolation works...

so have triangle strip this:

struct tvtx {   vec3  mpos;   vec3  mdir;   vec2  mtexcoord; };  auto vtxs = std::vector< tvtx >{   { vec3{ -1,0,0 }, vec3{ 1,0,0 }, vec2{ -1,  1 } },   { vec3{ -1,0,0 }, vec3{ 1,0,0 }, vec2{ -1, -1 } },   { vec3{ -1,0,0 }, vec3{ 1,0,0 }, vec2{  0,  1 } },   { vec3{ -1,0,0 }, vec3{ 1,0,0 }, vec2{  0, -1 } },   { vec3{  1,0,0 }, vec3{ 1,0,0 }, vec2{  0,  1 } },   { vec3{  1,0,0 }, vec3{ 1,0,0 }, vec2{  0, -1 } },   { vec3{  1,0,0 }, vec3{ 1,0,0 }, vec2{  1,  1 } },   { vec3{  1,0,0 }, vec3{ 1,0,0 }, vec2{  1, -1 } } }; 

and transform in "world" space this:

pt1 = vec3{  -0.450794667,  0.454502404,-0.437951744 }; pt2 = vec3{   -0.248224616,0.682369053, -0.886776387 };  auto center = ( pt1 + pt2 ) * 0.5f; auto right = normalize( pt2 - pt1 ); auto = normalize( cross( vec3( 0,0,1 ), right ) ); auto forw = cross( right, ); auto mtx = mat4{ vec4( right, 0.0 ), vec4( up, 0.0 ), vec4( forw, 0.0   ), vec4( center, 1.0f ) }; 

my vertex shader like:

attribute vec3  apos; attribute vec3  adir; attribute vec2  atexcoord;  uniform mat4  umvpmtx; uniform vec2  uscreenextent;  // half size of screen uniform vec2  uinflate;       // amount of line "grow" in pixels  varying mediump vec2 vtexcoord;  void main() {   vec4  clippos1 = umvpmtx * vec4( apos, 1.0 );   // point on "direction" line, transform in screen , screen space direction vector   // don't know how transform direction vector world screen space, naive way not work ( mvp * vec4( adir, 0.0 ) )   vec4  clippos2 = umvpmtx * vec4( apos + adir * 1000.0, 1.0 );   vec2  ndc1 = vec2( clippos1 ) / clippos1.w;   vec2  ndc2 = vec2( clippos2 ) / clippos2.w;   vec2  sc1 = ndc1 * uscreenextent;   vec2  sc2 = ndc2 * uscreenextent;   vec2  dir = normalize( sc2 - sc1 ) * sign( clippos1.w * clippos2.w ); // reduce damage if points lie on opposite sides of origin   vec2  perp = vec2( -dir.y, dir.x );   vec2  sc = sc1 + dir * atexcoord.x * uinflate.x + perp * atexcoord.y * uinflate.y;    vtexcoord = atexcoord;    // bring clip space   gl_position = vec4( sc / uscreenextent * clippos1.w, clippos1.zw ); } 

fragment shader like:

varying mediump vec2 vtexcoord;  void main() {   mediump float dist = clamp( length( vtexcoord ), 0.0, 1.0 );    gl_fragcolor = vec4( mix( vec3( 1.0, 0.25, 0.25 ), vec3( 0.25, 1.0, 0.25 ), dist ), 1.0 ); } 

notice atexcoord go -1 1!

what like:

the above picture has uinflate.x = 0, not have round caps...

what quad centered red line, fading green. because way alter x , y coordinates in screen space without touching z , w components gives triangles not co-planar anymore? have hard time visualizing this...

update
triangles co-planar in eye space size quite different (in eye space). looks quad on screen not quad anymore in eye space. guess main problem (because of perspective correct interpolation ??). question remains: how draw 3d textured lines constant (or controlled) screen width. 3d mean lines depth tested other 3d content.