Mirror filter for GPUImage

GPUImageMirrorFilter.h

//
//  GPUImageMirrorFilter.h
//

#import <GPUImage/GPUImageFilter.h>

@interface GPUImageMirrorFilter : GPUImageFilter {
    GLint verticalMirrorUniform, horizontalMirrorUniform;
}

@property (nonatomic) CGFloat verticalMirror;
@property (nonatomic) CGFloat horizontalMirror;

@end

GPUImageMirrorFilter.m

//
//  GPUImageMirrorFilter.m
//

#import "GPUImageMirrorFilter.h"

NSString *const kGPUImageMirrorFragmentShaderString = SHADER_STRING
(
varying lowp vec2 textureCoordinate;
uniform sampler2D inputImageTexture;

uniform lowp float verticalMirror;
uniform lowp float horizontalMirror;

void main(void) {
    mediump vec2 p = textureCoordinate;

    if (verticalMirror < 0.0) {
        if (p.x > -verticalMirror) {
            p.x = -2.0 * verticalMirror - p.x;
        }
    }
    else if (verticalMirror > 0.0) {
        if (p.x < verticalMirror) {
            p.x = 2.0 * verticalMirror - p.x;
        }
    }

    if (horizontalMirror < 0.0) {
        if (p.y > horizontalMirror) {
            p.y = -2.0 * horizontalMirror - p.y;
        }
    }
    else if (horizontalMirror > 0.0) {
        if (p.y < horizontalMirror) {
            p.y = 2.0 * horizontalMirror - p.y;
        }
    }

    if (p.x > 1.0 || p.x < 0.0 || p.y > 1.0 || p.y < 0.0) {
        gl_FragColor = vec4(0.0);
    } else {
        lowp vec4 outputColor = texture2D(inputImageTexture, p);
        gl_FragColor = outputColor;
    }
}
);


@implementation GPUImageMirrorFilter

- (id)init {
    if (!(self = [self initWithFragmentShaderFromString:kGPUImageMirrorFragmentShaderString])) {
        return nil;
    }

    return self;
}

- (id)initWithFragmentShaderFromString:(NSString *)fragmentShaderString {
    if (!(self = [super initWithFragmentShaderFromString:fragmentShaderString])) {
        return nil;
    }

    verticalMirrorUniform = [filterProgram uniformIndex:@"verticalMirror"];
    self.verticalMirror = 0;

    horizontalMirrorUniform = [filterProgram uniformIndex:@"horizontalMirror"];
    self.horizontalMirror = 0;

    return self;
}

- (void)setVerticalMirror:(CGFloat)newValue {
    NSAssert(newValue < 1.f && newValue > -1.f, @"Mirror position must be between -1 and 1. Current value: %f", newValue);
    _verticalMirror = newValue;
    [self setFloat:_verticalMirror forUniform:verticalMirrorUniform program:filterProgram];
}

- (void)setHorizontalMirror:(CGFloat)newValue {
    NSAssert(newValue < 1.f && newValue > -1.f, @"Mirror position must be between -1 and 1. Current value: %f", newValue);
    _horizontalMirror = newValue;
    [self setFloat:_horizontalMirror forUniform:horizontalMirrorUniform program:filterProgram];
}

@end