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OpenGL ES 动效滤镜实现

注:本文旨在记录笔者的学习过程,仅代表笔者个人的理解,如果有表述不准确的地方,欢迎各位指正!因为涉及到的概念来源自网络,所以如有侵权,也望告知!

前言

本文主要在GLSL显示普通图⽚的基础上实现动态滤镜效果。

正文

一、添加时间线

动效滤镜与上一篇文章提到静态的马赛克滤镜的不同之处,在于我们再实现着色器代码的时候添加了一个time参数,time参数是通过CADisplayLink定时器触发输入的。

- (void)startFilerAnimation {
    //1.CADisplayLink 定时器
    if (self.displayLink) {
        [self.displayLink invalidate];
        self.displayLink = nil;
    }
    //2.设置displayLink 的方法
    self.startTimeInterval = 0;
    self.displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(timeAction)];
    //3.将displayLink 添加到runloop 运行循环
    [self.displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes];
}

- (void)timeAction {
    //DisplayLink 的当前时间撮
    if (self.startTimeInterval == 0) {
        self.startTimeInterval = self.displayLink.timestamp;
    }
    //使用program
    glUseProgram(self.program);
    //绑定buffer
    glBindBuffer(GL_ARRAY_BUFFER, self.vertexBuffer);

    // 传入时间
    CGFloat currentTime = self.displayLink.timestamp - self.startTimeInterval;
    GLuint time = glGetUniformLocation(self.program, "Time");
    glUniform1f(time, currentTime);

    // 清除画布
    glClear(GL_COLOR_BUFFER_BIT);
    glClearColor(1, 1, 1, 1);

    // 重绘
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
    //渲染到屏幕上
    [self.context presentRenderbuffer:GL_RENDERBUFFER];
}
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二、实现动态滤镜

与静态的马赛克滤镜相同的,首先得能通过GLSL方式正常显示一张纹理图片。对应的顶点着色器和片元着色器的代码如下:

a.顶点着色器

attribute vec4 Position;
attribute vec2 TextureCoords;
varying vec2 TextureCoordsVarying;

void main (void) {
    gl_Position = Position;
    TextureCoordsVarying = TextureCoords;
}
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b.片元着色器

precision highp float;
uniform sampler2D Texture;
varying vec2 TextureCoordsVarying;

void main (void) {
    vec4 mask = texture2D(Texture, TextureCoordsVarying);
    gl_FragColor = vec4(mask.rgb, 1.0);
}
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1.缩放滤镜

实现原理通过修改顶点坐标和纹理坐标的对应关系来实现。

片元着色器实现:

precision highp float;

uniform sampler2D Texture;
varying vec2 TextureCoordsVarying;

uniform float Time;

const float PI = 3.1415926;

void main (void) {
    float duration = 0.6;
    float maxAmplitude = 0.3;

    float time = mod(Time, duration);
    float amplitude = 1.0 + maxAmplitude * abs(sin(time * (PI / duration)));

    float weakX = 0.5 + (TextureCoordsVarying.x - 0.5) / amplitude;
    float weakY = 0.5 + (TextureCoordsVarying.y - 0.5) / amplitude;
    vec2 weakTextureCoords = vec2(weakX, weakY);

    vec4 mask = texture2D(Texture, weakTextureCoords);
    gl_FragColor = vec4(mask.rgb, 1.0);
}
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2.灵魂出窍滤镜

实现原理:通过两个纹理图层的叠加,并且上⾯的图层随着时间的推移,会逐渐放⼤且不透明度逐渐降低来实现。

片元着色器实现:

precision highp float;

uniform sampler2D Texture;
varying vec2 TextureCoordsVarying;

uniform float Time;

void main (void) {
    float duration = 0.7;
    float maxAlpha = 0.4;
    float maxScale = 1.8;

    float progress = mod(Time, duration) / duration; // 0~1
    float alpha = maxAlpha * (1.0 - progress);
    float scale = 1.0 + (maxScale - 1.0) * progress;

    float weakX = 0.5 + (TextureCoordsVarying.x - 0.5) / scale;
    float weakY = 0.5 + (TextureCoordsVarying.y - 0.5) / scale;
    vec2 weakTextureCoords = vec2(weakX, weakY);

    vec4 weakMask = texture2D(Texture, weakTextureCoords);

    vec4 mask = texture2D(Texture, TextureCoordsVarying);

    gl_FragColor = mask * (1.0 - alpha) + weakMask * alpha;
}
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3.抖动滤镜

实现原理:通过对像素点颜⾊进行偏移,加上微弱的放⼤效果来实现

片元着色器实现:

precision highp float;

uniform sampler2D Texture;
varying vec2 TextureCoordsVarying;

uniform float Time;

void main (void) {
    float duration = 0.7;
    float maxScale = 1.1;
    float offset = 0.02;

    float progress = mod(Time, duration) / duration; // 0~1
    vec2 offsetCoords = vec2(offset, offset) * progress;
    float scale = 1.0 + (maxScale - 1.0) * progress;

    vec2 ScaleTextureCoords = vec2(0.5, 0.5) + (TextureCoordsVarying - vec2(0.5, 0.5)) / scale;

    vec4 maskR = texture2D(Texture, ScaleTextureCoords + offsetCoords);
    vec4 maskB = texture2D(Texture, ScaleTextureCoords - offsetCoords);
    vec4 mask = texture2D(Texture, ScaleTextureCoords);

    gl_FragColor = vec4(maskR.r, mask.g, maskB.b, mask.a);
}
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4.闪白滤镜

实现原理:通过添加⽩⾊图层 ,⽩⾊图层的透明度随着时间变化来实现。

片元着色器实现:

precision highp float;

uniform sampler2D Texture;
varying vec2 TextureCoordsVarying;

uniform float Time;

const float PI = 3.1415926;

void main (void) {
    float duration = 0.6;

    float time = mod(Time, duration);

    vec4 whiteMask = vec4(1.0, 1.0, 1.0, 1.0);
    float amplitude = abs(sin(time * (PI / duration)));

    vec4 mask = texture2D(Texture, TextureCoordsVarying);

    gl_FragColor = mask * (1.0 - amplitude) + whiteMask * amplitude;
}
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5.毛刺滤镜

实现原理:通过修改纹理坐标对纹理进行撕裂,加上微弱的像素点颜⾊偏移来实现。我们让每⼀⾏像素随机偏移 -1 ~ 1 的距离(这⾥的 -1 ~ 1 是对于纹理坐标来说的),但是如果整个画⾯都偏移⽐较⼤的值,那我们可能都看不出原来图像的样⼦。所以我们的逻辑是,设定⼀个阈值,⼩于这个阈值才进⾏偏移,超过这个阈值则乘上⼀个缩⼩系数。

片元着色器实现:

precision highp float;

uniform sampler2D Texture;
varying vec2 TextureCoordsVarying;

uniform float Time;

const float PI = 3.1415926;

float rand(float n) {
    return fract(sin(n) * 43758.5453123);
}

void main (void) {
    float maxJitter = 0.06;
    float duration = 0.3;
    float colorROffset = 0.01;
    float colorBOffset = -0.025;

    float time = mod(Time, duration * 2.0);
    float amplitude = max(sin(time * (PI / duration)), 0.0);

    float jitter = rand(TextureCoordsVarying.y) * 2.0 - 1.0; // -1~1
    bool needOffset = abs(jitter) < maxJitter * amplitude;

    float textureX = TextureCoordsVarying.x + (needOffset ? jitter : (jitter * amplitude * 0.006));
    vec2 textureCoords = vec2(textureX, TextureCoordsVarying.y);

    vec4 mask = texture2D(Texture, textureCoords);
    vec4 maskR = texture2D(Texture, textureCoords + vec2(colorROffset * amplitude, 0.0));
    vec4 maskB = texture2D(Texture, textureCoords + vec2(colorBOffset * amplitude, 0.0));

    gl_FragColor = vec4(maskR.r, mask.g, maskB.b, mask.a);
}
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6.幻觉滤镜

实现原理:通过残影和颜⾊偏移的叠加来实现。在移动的过程中,每经过⼀段时间间隔,根据当前的位置去创建⼀个新层,并且新层的不透明度随着时间逐渐减弱。于是在⼀个移动周期内,可以看到很多透明度不同的层叠加在⼀起,从⽽形成残影的效果。同时在移动的过程中,每间隔⼀段时间,遗失了⼀部分红⾊通道的值在原来的位置,并且这部分红⾊通道的值,随着时间偏移,会逐渐恢复。

片元着色器实现:

precision highp float;

uniform sampler2D Texture;
varying vec2 TextureCoordsVarying;

uniform float Time;

const float PI = 3.1415926;
const float duration = 2.0;

vec4 getMask(float time, vec2 textureCoords, float padding) {

    vec2 translation = vec2(sin(time * (PI * 2.0 / duration)),
                            cos(time * (PI * 2.0 / duration)));

    vec2 translationTextureCoords = textureCoords + padding * translation;
    vec4 mask = texture2D(Texture, translationTextureCoords);

    return mask;
}

float maskAlphaProgress(float currentTime, float hideTime, float startTime) {
    float time = mod(duration + currentTime - startTime, duration);
    return min(time, hideTime);
}

void main (void) {
    float time = mod(Time, duration);
    float scale = 1.2;
    float padding = 0.5 * (1.0 - 1.0 / scale);
    vec2 textureCoords = vec2(0.5, 0.5) + (TextureCoordsVarying - vec2(0.5, 0.5)) / scale;

    float hideTime = 0.9;
    float timeGap = 0.2;

    float maxAlphaR = 0.5; // max R
    float maxAlphaG = 0.05; // max G
    float maxAlphaB = 0.05; // max B

    vec4 mask = getMask(time, textureCoords, padding);
    float alphaR = 1.0; // R
    float alphaG = 1.0; // G
    float alphaB = 1.0; // B

    vec4 resultMask = vec4(0, 0, 0, 0);

    for (float f = 0.0; f < duration; f += timeGap) {
        float tmpTime = f;
        vec4 tmpMask = getMask(tmpTime, textureCoords, padding);

        //
        float tmpAlphaR = maxAlphaR - maxAlphaR * maskAlphaProgress(time, hideTime, tmpTime) / hideTime;
        float tmpAlphaG = maxAlphaG - maxAlphaG * maskAlphaProgress(time, hideTime, tmpTime) / hideTime;
        float tmpAlphaB = maxAlphaB - maxAlphaB * maskAlphaProgress(time, hideTime, tmpTime) / hideTime;

        resultMask += vec4(tmpMask.r * tmpAlphaR,
                           tmpMask.g * tmpAlphaG,
                           tmpMask.b * tmpAlphaB,
                           1.0);
        alphaR -= tmpAlphaR;
        alphaG -= tmpAlphaG;
        alphaB -= tmpAlphaB;
    }
    resultMask += vec4(mask.r * alphaR, mask.g * alphaG, mask.b * alphaB, 1.0);

    gl_FragColor = resultMask;
}
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