`LittlePlanet` projection with GPUImage.framework

GPUImageLittlePlanetProjectionFilter.h

//
//  GPUImageLittlePlanetProjectionFilter.h
//
//  Created by Toshiyuki Suzumura on 2013/07/02.
//  Copyright (c) 2013 Toshiyuki Suzumura. All rights reserved.
//

#import "GPUImageFilter.h"

@interface GPUImageLittlePlanetProjectionFilter : GPUImageFilter

@property (nonatomic) CGSize renderSize;
@property (nonatomic, setter = setSkyRatio:) float skyRatio;

@end

GPUImageLittlePlanetProjectionFilter.m

//
//  GPUImageLittlePlanetProjectionFilter.m
//
//  Created by Toshiyuki Suzumura on 2013/07/02.
//  Copyright (c) 2013 Toshiyuki Suzumura. All rights reserved.
//

#import "GPUImageLittlePlanetProjectionFilter.h"

NSString* const kGPUImageLittlePlanetProjectionVertexShaderString = SHADER_STRING
(
 attribute vec4 position;
 attribute vec4 inputTextureCoordinate;
 varying vec2 textureCoordinate;
 uniform float skyRatio;
 
 void main()
 {
     gl_Position = position;
     textureCoordinate = (inputTextureCoordinate.xy - vec2(0.5, 0.5))*skyRatio;
 }
);


NSString* const kGPUImageLittlePlanetProjectionFragmentShaderString = SHADER_STRING
(
 varying highp vec2 textureCoordinate;
 uniform sampler2D inputImageTexture;
 
 const highp float M_2PI = M_PI*2.0;
 
 void main()
 {
     highp float x = textureCoordinate.x;
     highp float y = textureCoordinate.y;
     highp float R = sqrt(x*x + y*y);
     highp float S = atan(x, y);
     highp float r = 2.0*atan(1.0, R);  // R=0 => r=M_PI
     
     S = S/M_2PI;
     S = S - floor(S);
     gl_FragColor = texture2D(inputImageTexture, vec2(S, r/M_PI));
 }
);



@implementation GPUImageLittlePlanetProjectionFilter

- (id)init
{
    self = [super initWithVertexShaderFromString:kGPUImageLittlePlanetProjectionVertexShaderString
                        fragmentShaderFromString:kGPUImageLittlePlanetProjectionFragmentShaderString];
    if (self) {
        _renderSize = CGSizeMake(512, 512);
        [self setSkyRatio:10];
    }
    return self;
}

- (CGSize)sizeOfFBO
{
    return _renderSize;
}

- (void)setSkyRatio:(float)skyRatio
{
    _skyRatio = skyRatio;
    [self setFloat:_skyRatio forUniformName:@"skyRatio"];
}

@end