opengl - How to flip Y-axis in GLSL shader before gl_FragColor = texture2D(*,*); -

i need flip textures upside-down in shaders before applying perspective transformations. modified verttexcoord in vert.glsl, don't know use in swap.glsl. way

gl_fragcolor = texture2d(texture, verttexcoord ); 

does not work, because need texture modified in perspective.

vert.glsl:

#define processing_color_shader  uniform mat4 transform; uniform mat4 texmatrix;  attribute vec4 vertex; attribute vec4 color; attribute vec2 texcoord;  varying vec4 vertcolor; varying vec4 verttexcoord;  void main() {   gl_position = transform * vertex;    vertcolor = color;   verttexcoord = texmatrix * vec4(texcoord, 1.0, 1.0); } 

swap.glsl:

#ifdef gl_es precision highp float; #endif  // general parameters uniform sampler2d from; uniform sampler2d to; uniform float progress; uniform vec2 resolution;  uniform float reflection; uniform float perspective; uniform float depth;  varying vec4 vertcolor; varying vec4 verttexcoord;  const vec4 black = vec4(0.0, 0.0, 0.0, 1.0); const vec2 boundmin = vec2(0.0, 0.0); const vec2 boundmax = vec2(1.0, 1.0);  bool inbounds (vec2 p) {   return all(lessthan(boundmin, p)) && all(lessthan(p, boundmax)); }  vec2 project (vec2 p) {   return p * vec2(1.0, -1.2) + vec2(0.0, -0.02); }  vec4 bgcolor (vec2 p, vec2 pfr, vec2 pto) {   vec4 c = black;   pfr = project(pfr);   if (inbounds(pfr)) {     c += mix(black, texture2d(from, pfr), reflection * mix(1.0, 0.0, pfr.y));   }   pto = project(pto);   if (inbounds(pto)) {     c += mix(black, texture2d(to, pto), reflection * mix(1.0, 0.0, pto.y));   }   return c; }  void main() {   vec2 p = gl_fragcoord.xy / resolution.xy;     vec2 pfr, pto = vec2(-1.);    float size = mix(1.0, depth, progress);   float persp = perspective * progress;   pfr = (p + vec2(-0.0, -0.5)) * vec2(size/(1.0-perspective*progress), size/(1.0-size*persp*p.x)) + vec2(0.0, 0.5);    size = mix(1.0, depth, 1.-progress);   persp = perspective * (1.-progress);   pto = (p + vec2(-1.0, -0.5)) * vec2(size/(1.0-perspective*(1.0-progress)), size/(1.0-size*persp*(0.5-p.x))) + vec2(1.0, 0.5);    bool fromover = progress < 0.5;    if (fromover) {     if (inbounds(pfr)) {       gl_fragcolor = texture2d(from, pfr);     }     else if (inbounds(pto)) {       gl_fragcolor = texture2d(to, pto);     }     else {       gl_fragcolor = bgcolor(p, pfr, pto);     }   }   else {     if (inbounds(pto)) {       gl_fragcolor = texture2d(to, pto);     }     else if (inbounds(pfr)) {       gl_fragcolor = texture2d(from, pfr);     }     else {       gl_fragcolor = bgcolor(p, pfr, pto);     }   } } 

you sample texture at

(u,v) 

if want flip y-axis, sample at

(u, 1.0f -v) 

so updated main like:

void main() {   gl_position = transform * vertex;    vertcolor = color;   newtcoord = texcoord;   newtcoord.y = 1.0 - newtcoord.y;   verttexcoord = vec4(newtcoord, 1.0, 1.0); }