Shader特效——“翻页”的实现 【GLSL】

precision mediump float;

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);
}