Customizing Page Curl Shader in OpenGL - opengl

Question

I am currently designing a fragment shader that shall display a page curl effect as a transition between two images. So I looked it up on the Internet and found this code that works as I want:
varying vec2 texCoord;
uniform sampler2D sourceTex;
uniform sampler2D targetTex;
uniform float time; // Ranges from 0.0 to 1.0
const float MIN_AMOUNT = -0.16;
const float MAX_AMOUNT = 1.3;
float amount = time * (MAX_AMOUNT - MIN_AMOUNT) + MIN_AMOUNT;
const float PI = 3.141592653589793;
const float scale = 512.0;
const float sharpness = 3.0;
float cylinderCenter = amount;
// 360 degrees * amount
float cylinderAngle = 2.0 * PI * amount;
const float cylinderRadius = 1.0 / PI / 2.0;
vec3 hitPoint(float hitAngle, float yc, vec3 point, mat3 rrotation) {
float hitPoint = hitAngle / (2.0 * PI);
point.y = hitPoint;
return rrotation * point;
}
vec4 antiAlias(vec4 color1, vec4 color2, float distance) {
distance *= scale;
if (distance < 0.0) return color2;
if (distance > 2.0) return color1;
float dd = pow(1.0 - distance / 2.0, sharpness);
return ((color2 - color1) * dd) + color1;
}
float distanceToEdge(vec3 point) {
float dx = abs(point.x > 0.5 ? 1.0 - point.x : point.x);
float dy = abs(point.y > 0.5 ? 1.0 - point.y : point.y);
if (point.x < 0.0) dx = -point.x;
if (point.x > 1.0) dx = point.x - 1.0;
if (point.y < 0.0) dy = -point.y;
if (point.y > 1.0) dy = point.y - 1.0;
if ((point.x < 0.0 || point.x > 1.0) && (point.y < 0.0 || point.y > 1.0)) return sqrt(dx * dx + dy * dy);
return min(dx, dy);
}
vec4 seeThrough(float yc, vec2 p, mat3 rotation, mat3 rrotation) {
float hitAngle = PI - (acos(yc / cylinderRadius) - cylinderAngle);
vec3 point = hitPoint(hitAngle, yc, rotation * vec3(p, 1.0), rrotation);
if (yc <= 0.0 && (point.x < 0.0 || point.y < 0.0 || point.x > 1.0 || point.y > 1.0)) {
return texture2D(targetTex, texCoord);
}
if (yc > 0.0) return texture2D(sourceTex, p);
vec4 color = texture2D(sourceTex, point.xy);
vec4 tcolor = vec4(0.0);
return antiAlias(color, tcolor, distanceToEdge(point));
}
vec4 seeThroughWithShadow(float yc, vec2 p, vec3 point, mat3 rotation, mat3 rrotation) {
float shadow = distanceToEdge(point) * 30.0;
shadow = (1.0 - shadow) / 3.0;
if (shadow < 0.0) shadow = 0.0;
else shadow *= amount;
vec4 shadowColor = seeThrough(yc, p, rotation, rrotation);
shadowColor.r -= shadow;
shadowColor.g -= shadow;
shadowColor.b -= shadow;
return shadowColor;
}
vec4 backside(float yc, vec3 point) {
vec4 color = texture2D(sourceTex, point.xy);
float gray = (color.r + color.b + color.g) / 15.0;
gray += (8.0 / 10.0) * (pow(1.0 - abs(yc / cylinderRadius), 2.0 / 10.0) / 2.0 + (5.0 / 10.0));
color.rgb = vec3(gray);
return color;
}
vec4 behindSurface(float yc, vec3 point, mat3 rrotation) {
float shado = (1.0 - ((-cylinderRadius - yc) / amount * 7.0)) / 6.0;
shado *= 1.0 - abs(point.x - 0.5);
yc = (-cylinderRadius - cylinderRadius - yc);
float hitAngle = (acos(yc / cylinderRadius) + cylinderAngle) - PI;
point = hitPoint(hitAngle, yc, point, rrotation);
if (yc < 0.0 && point.x >= 0.0 && point.y >= 0.0 && point.x <= 1.0 && point.y <= 1.0 && (hitAngle < PI || amount > 0.5)){
shado = 1.0 - (sqrt(pow(point.x - 0.5, 2.0) + pow(point.y - 0.5, 2.0)) / (71.0 / 100.0));
shado *= pow(-yc / cylinderRadius, 3.0);
shado *= 0.5;
} else
shado = 0.0;
return vec4(texture2D(targetTex, texCoord).rgb - shado, 1.0);
}
void main(void) {
const float angle = 30.0 * PI / 180.0;
float c = cos(-angle);
float s = sin(-angle);
mat3 rotation = mat3(
c, s, 0,
-s, c, 0,
0.12, 0.258, 1
);
c = cos(angle);
s = sin(angle);
mat3 rrotation = mat3(
c, s, 0,
-s, c, 0,
0.15, -0.5, 1
);
vec3 point = rotation * vec3(texCoord, 1.0);
float yc = point.y - cylinderCenter;
if (yc < -cylinderRadius) {
// Behind surface
gl_FragColor = behindSurface(yc, point, rrotation);
return;
}
if (yc > cylinderRadius) {
// Flat surface
gl_FragColor = texture2D(sourceTex, texCoord);
return;
}
float hitAngle = (acos(yc / cylinderRadius) + cylinderAngle) - PI;
float hitAngleMod = mod(hitAngle, 2.0 * PI);
if ((hitAngleMod > PI && amount < 0.5) || (hitAngleMod > PI/2.0 && amount < 0.0)) {
gl_FragColor = seeThrough(yc, texCoord, rotation, rrotation);
return;
}
point = hitPoint(hitAngle, yc, point, rrotation);
if (point.x < 0.0 || point.y < 0.0 || point.x > 1.0 || point.y > 1.0) {
gl_FragColor = seeThroughWithShadow(yc, texCoord, point, rotation, rrotation);
return;
}
vec4 color = backside(yc, point);
vec4 otherColor;
if (yc < 0.0) {
float shado = 1.0 - (sqrt(pow(point.x - 0.5, 2.0) + pow(point.y - 0.5, 2.0)) / 0.71);
shado *= pow(-yc / cylinderRadius, 3.0);
shado *= 0.5;
otherColor = vec4(0.0, 0.0, 0.0, shado);
} else {
otherColor = texture2D(sourceTex, texCoord);
}
color = antiAlias(color, otherColor, cylinderRadius - abs(yc));
vec4 cl = seeThroughWithShadow(yc, texCoord, point, rotation, rrotation);
float dist = distanceToEdge(point);
gl_FragColor = antiAlias(color, cl, dist);
}
I just want to modify the angle of the curl but it seems like there are too many dependencies that are not made dynamicly, does anybody know the math behind it well enough to help me or knows maybe even a better code sample to use?