I am using opengl & using example of GLPaint. I have created app which apply twirl on image(texture) when user touch on image. I have issue that I need to save first touch and then apply next phase and so on. But on next touch the previous applied effect get removed.
I have tried to use depthbuffer & framebuffer both, but not able to get desired result. I am attaching all my code and shaders:
Code :
#import <QuartzCore/QuartzCore.h>
#import <OpenGLES/EAGLDrawable.h>
#import <GLKit/GLKit.h>
 
#import "PaintingView.h"
 
enum {
    ATTRIB_VERTEX,
    NUM_ATTRIBS
};
 
@interface PaintingView()
{
    // The pixel dimensions of the backbuffer
    GLint backingWidth;
    GLint backingHeight;
 
    EAGLContext *context;
 
    // OpenGL names for the renderbuffer and framebuffers used to render to this view
    GLuint viewRenderbuffer, viewFramebuffer, texture;
 
    // OpenGL name for the depth buffer that is attached to viewFramebuffer, if it exists (0 if it does not exist)
    GLuint depthRenderbuffer;
 
 
    Boolean needsErase;
 
    // Shader objects
//    GLuint vertexShader;
//    GLuint fragmentShader;
//    GLuint shaderProgram;
 
    // Buffer Objects
    GLuint vboId;
 
    BOOL initialized;
 
    GLint inputImageTexture2Uniform, filterPositionAttribute, filterTextureCoordinateAttribute;
 
    NSString *vertexShader, *fragmentShader;
 
    Boolean firstTouch;
 
    CGPoint twirlCenter;
}
 
// Program Handle
@property (assign, nonatomic, readonly) GLuint program;
 
// Attribute Handles
@property (assign, nonatomic, readonly) GLuint aPosition;
 
@property(nonatomic, readwrite) CGPoint location;
@property(nonatomic, readwrite) CGPoint previousLocation;
 
 
@end
 
@implementation PaintingView
 
// Implement this to override the default layer class (which is [CALayer class]).
// We do this so that our view will be backed by a layer that is capable of OpenGL ES rendering.
+ (Class)layerClass
{
    return [CAEAGLLayer class];
}
 
// The GL view is stored in the nib file. When it's unarchived it's sent -initWithCoder:
- (id)initWithCoder:(NSCoder*)coder {
    if ((self = [super initWithCoder:coder])) {
        CAEAGLLayer *eaglLayer = (CAEAGLLayer *)self.layer;
 
        eaglLayer.opaque = NO;
        // In this application, we want to retain the EAGLDrawable contents after a call to presentRenderbuffer.
        eaglLayer.drawableProperties = [NSDictionary dictionaryWithObjectsAndKeys:
                                        [NSNumber numberWithBool:YES], kEAGLDrawablePropertyRetainedBacking, kEAGLColorFormatRGBA8, kEAGLDrawablePropertyColorFormat, nil];
 
        context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
 
        //        context = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];
 
        if (!context || ![EAGLContext setCurrentContext:context]) {
            return nil;
        }
 
        // Set the view's scale factor as you wish
        self.contentScaleFactor = [[UIScreen mainScreen] scale];
 
        // Make sure to start with a cleared buffer
        needsErase = YES;
    }
    return self;
}
 
// If our view is resized, we'll be asked to layout subviews.
// This is the perfect opportunity to also update the framebuffer so that it is
// the same size as our display area.
-(void)layoutSubviews
{
    [EAGLContext setCurrentContext:context];
 
    if (!initialized) {
        initialized = [self initGL];
    }
    else {
        [self resizeFromLayer:(CAEAGLLayer*)self.layer];
    }
}
 
- (BOOL)initGL
{
    // Generate IDs for a framebuffer object and a color renderbuffer
    glGenFramebuffers(1, &viewFramebuffer);
    glGenRenderbuffers(1, &viewRenderbuffer);
 
    glBindFramebuffer(GL_FRAMEBUFFER, viewFramebuffer);
    glBindRenderbuffer(GL_RENDERBUFFER, viewRenderbuffer);
    // This call associates the storage for the current render buffer with the EAGLDrawable (our CAEAGLLayer)
    // allowing us to draw into a buffer that will later be rendered to screen wherever the layer is (which corresponds with our view).
    [context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(id<EAGLDrawable>)self.layer];
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, viewRenderbuffer);
 
    glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &backingWidth);
    glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &backingHeight);
 
    // For this sample, we do not need a depth buffer. If you do, this is how you can create one and attach it to the framebuffer:
    //    glGenRenderbuffers(1, &depthRenderbuffer);
    //    glBindRenderbuffer(GL_RENDERBUFFER, depthRenderbuffer);
    //    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, backingWidth, backingHeight);
    //    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthRenderbuffer);
 
    if(glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
    {
        NSLog(@"failed to make complete framebuffer object %x", glCheckFramebufferStatus(GL_FRAMEBUFFER));
        return NO;
    }
 
    // Setup the view port in Pixels
    glViewport(0, 0, backingWidth, backingHeight);
 
    // Create a Vertex Buffer Object to hold our data
    glGenBuffers(1, &vboId);
 
    // Load the brush texture
    //    brushTexture = [self textureFromName:@"transprnt.png"];
 
    // Load shaders
    [self setupShaders];
 
    inputImageTexture2Uniform = [self uniformIndex:@"inputImageTexture"];
    UIImage *uyet = [UIImage imageNamed:@"kerala.jpg"];
    [self setImage:uyet];
 
 
 
 
    // Enable blending and set a blending function appropriate for premultiplied alpha pixel data
    glEnable(GL_BLEND);
    glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
 
 
    return YES;
}
 
- (void)setupShaders
{
    vertexShader = @"RWTBase";
    fragmentShader = @"TwirlShader";
    // Program
    _program = [self programWithVertexShader:vertexShader fragmentShader:fragmentShader];
 
    // Attributes
    filterPositionAttribute = glGetAttribLocation(_program, "aPosition");
    filterTextureCoordinateAttribute = glGetAttribLocation(_program, "inputTextureCoordinate");
 
    glEnableVertexAttribArray(filterPositionAttribute);
    glEnableVertexAttribArray(filterTextureCoordinateAttribute);
 
    glBindAttribLocation(_program,
                         filterPositionAttribute,
                         [@"aPosition" UTF8String]);
    glBindAttribLocation(_program,
                         filterTextureCoordinateAttribute,
                         [@"inputTextureCoordinate" UTF8String]);
 
 
 
    // Program
    glUseProgram(_program);
 
    static const GLfloat textureCoordinates[] = {
        0.0f, 0.0f,
        1.0f, 0.0f,
        0.0f, 1.0f,
        1.0f, 1.0f,
    };
    static const GLfloat vertices[] = {
        -1.0f, -1.0f,
        1.0f, -1.0f,
        -1.0f,  1.0f,
        1.0f,  1.0f,
    };
 
    glVertexAttribPointer(filterPositionAttribute, 2, GL_FLOAT, 0, 0, vertices);
    glVertexAttribPointer(filterTextureCoordinateAttribute, 2, GL_FLOAT, 0, 0, textureCoordinates);
 
}
 
- (BOOL)resizeFromLayer:(CAEAGLLayer *)layer
{
    // Allocate color buffer backing based on the current layer size
    glBindRenderbuffer(GL_RENDERBUFFER, viewRenderbuffer);
    [context renderbufferStorage:GL_RENDERBUFFER fromDrawable:layer];
    glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &backingWidth);
    glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &backingHeight);
 
    // For this sample, we do not need a depth buffer. If you do, this is how you can allocate depth buffer backing:
    //    glBindRenderbuffer(GL_RENDERBUFFER, depthRenderbuffer);
    //    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, backingWidth, backingHeight);
    //    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthRenderbuffer);
 
    if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
    {
        NSLog(@"Failed to make complete framebuffer objectz %x", glCheckFramebufferStatus(GL_FRAMEBUFFER));
        return NO;
    }
 
 
    // Update viewport
    glViewport(0, 0, backingWidth, backingHeight);
 
    return YES;
}
 
// Releases resources when they are not longer needed.
- (void)dealloc
{
    // Destroy framebuffers and renderbuffers
    if (viewFramebuffer) {
        glDeleteFramebuffers(1, &viewFramebuffer);
        viewFramebuffer = 0;
    }
    if (viewRenderbuffer) {
        glDeleteRenderbuffers(1, &viewRenderbuffer);
        viewRenderbuffer = 0;
    }
    if (depthRenderbuffer)
    {
        glDeleteRenderbuffers(1, &depthRenderbuffer);
        depthRenderbuffer = 0;
    }
    // vbo
    if (vboId) {
        glDeleteBuffers(1, &vboId);
        vboId = 0;
    }
 
    // tear down context
    if ([EAGLContext currentContext] == context)
        [EAGLContext setCurrentContext:nil];
}
 
// Custom Methods....
/*
 ***********
 *         *
 *   ATG   *
 *         *
 ***********
 */
#pragma mark - Compile & Link
- (GLuint)programWithVertexShader:(NSString*)vsh fragmentShader:(NSString*)fsh {
    // Build shaders
    GLuint vertexShader1 = [self shaderWithName:vsh type:GL_VERTEX_SHADER];
    GLuint fragmentShader1 = [self shaderWithName:fsh type:GL_FRAGMENT_SHADER];
 
    // Create program
    GLuint programHandle = glCreateProgram();
 
    // Attach shaders
    glAttachShader(programHandle, vertexShader1);
    glAttachShader(programHandle, fragmentShader1);
 
    // Link program
    glLinkProgram(programHandle);
 
    // Check for errors
    GLint linkSuccess;
    glGetProgramiv(programHandle, GL_LINK_STATUS, &linkSuccess);
    if (linkSuccess == GL_FALSE) {
        GLchar messages[1024];
        glGetProgramInfoLog(programHandle, sizeof(messages), 0, &messages[0]);
        NSLog(@"%@:- GLSL Program Error: %s", [self class], messages);
    }
 
    // Delete shaders
    glDeleteShader(vertexShader1);
    glDeleteShader(fragmentShader1);
 
    return programHandle;
}
 
- (GLuint)shaderWithName:(NSString*)name type:(GLenum)type {
    // Load the shader file
    NSString* file;
    if (type == GL_VERTEX_SHADER) {
        file = [[NSBundle mainBundle] pathForResource:name ofType:@"vsh"];
    } else if (type == GL_FRAGMENT_SHADER) {
        file = [[NSBundle mainBundle] pathForResource:name ofType:@"fsh"];
    }
 
    // Create the shader source
    const GLchar* source = (GLchar*)[[NSString stringWithContentsOfFile:file encoding:NSUTF8StringEncoding error:nil] UTF8String];
 
    // Create the shader object
    GLuint shaderHandle = glCreateShader(type);
 
    // Load the shader source
    glShaderSource(shaderHandle, 1, &source, 0);
 
    // Compile the shader
    glCompileShader(shaderHandle);
 
    // Check for errors
    GLint compileSuccess;
    glGetShaderiv(shaderHandle, GL_COMPILE_STATUS, &compileSuccess);
    if (compileSuccess == GL_FALSE) {
        GLchar messages[1024];
        glGetShaderInfoLog(shaderHandle, sizeof(messages), 0, &messages[0]);
        NSLog(@"%@:- GLSL Shader Error: %s", [self class], messages);
    }
 
    return shaderHandle;
}
 
// Touch Methiods..
#pragma mark - Touches
 
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
 
//    CGRect                bounds = [self.view bounds];
//    UITouch*            touch = [[event touchesForView:self.view] anyObject];
//    //    NSLog(@"Hellossss");
//    firstTouch = YES;
//    _location = [touch locationInView:self.view];
//    _location.y = bounds.size.height - _location.y;
}
 
// Handles the continuation of a touch.
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
 
    CGRect              bounds = [self bounds];
    UITouch*            touch = [[event touchesForView:self] anyObject];
 
 
    UITouch* touchEvent = [touches anyObject];
    CGPoint locationInView = [touchEvent locationInView:self];
    twirlCenter = getNormalizedPoint(self, locationInView);
 
 
//    [EAGLContext setCurrentContext:context];
//    glBindFramebuffer(GL_FRAMEBUFFER, viewFramebuffer);
 
    // Render the stroke
    [self applyEffect];
 
//    [self drawEffects];
    if (firstTouch) {
        firstTouch = NO;
        _previousLocation = [touch previousLocationInView:self];
        _previousLocation.y = bounds.size.height - _previousLocation.y;
    } else {
        _location = [touch locationInView:self];
        _location.y = bounds.size.height - _location.y;
        _previousLocation = [touch previousLocationInView:self];
        _previousLocation.y = bounds.size.height - _previousLocation.y;
    }
 
    // Render the stroke
    [self renderLineFromPoint:_previousLocation toPoint:_location];
}
 
-(void)drawEffects {
//    [EAGLContext setCurrentContext:context];
//    glBindFramebuffer(GL_FRAMEBUFFER, viewFramebuffer);
 
    // Draw
    glBindBuffer(GL_ARRAY_BUFFER, vboId);
 
    glUseProgram(_program);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
 
 
    // Display the buffer
    glBindRenderbuffer(GL_RENDERBUFFER, viewRenderbuffer);
    [context presentRenderbuffer:GL_RENDERBUFFER];
}
 
-(void)applyEffect {
 
    //        [EAGLContext setCurrentContext:context];
    //        glBindFramebuffer(GL_FRAMEBUFFER, viewFramebuffer);
 
    GLint radiusIndex = [self uniformIndex:@"radius"];
    glUniform1f(radiusIndex, 0.1);
 
    GLint angleIndex = [self uniformIndex:@"angle"];
    glUniform1f(angleIndex, -0.5);
 
    //    twirlCenter = CGPointMake(1.0, 0.0);
    GLint centerIndex = [self uniformIndex:@"center"];
 
    GLfloat positionArray[2];
    positionArray[0] = twirlCenter.x;
    positionArray[1] = twirlCenter.y;
    glUniform2fv(centerIndex, 1, positionArray);
 
}
 
// Drawings a line onscreen based on where the user touches
- (void)renderLineFromPoint:(CGPoint)start toPoint:(CGPoint)end
{
    static GLfloat*     vertexBuffer = NULL;
    static NSUInteger   vertexMax = 64;
    NSUInteger          vertexCount = 0,
    count,
    i;
 
    [EAGLContext setCurrentContext:context];
    glBindFramebuffer(GL_FRAMEBUFFER, viewFramebuffer);
 
    // Convert locations from Points to Pixels
    CGFloat scale = self.contentScaleFactor;
    start.x *= scale;
    start.y *= scale;
    end.x *= scale;
    end.y *= scale;
 
    // Allocate vertex array buffer
    if(vertexBuffer == NULL)
        vertexBuffer = malloc(vertexMax * 2 * sizeof(GLfloat));
 
    // Add points to the buffer so there are drawing points every X pixels
    count = MAX(ceilf(sqrtf((end.x - start.x) * (end.x - start.x) + (end.y - start.y) * (end.y - start.y)) / 3), 1);
    for(i = 0; i < count; ++i) {
        if(vertexCount == vertexMax) {
            vertexMax = 2 * vertexMax;
            vertexBuffer = realloc(vertexBuffer, vertexMax * 2 * sizeof(GLfloat));
        }
 
        vertexBuffer[2 * vertexCount + 0] = start.x + (end.x - start.x) * ((GLfloat)i / (GLfloat)count);
        vertexBuffer[2 * vertexCount + 1] = start.y + (end.y - start.y) * ((GLfloat)i / (GLfloat)count);
        vertexCount += 1;
    }
 
    // Load data to the Vertex Buffer Object
    glBindBuffer(GL_ARRAY_BUFFER, vboId);
    glBufferData(GL_ARRAY_BUFFER, vertexCount*2*sizeof(GLfloat), vertexBuffer, GL_DYNAMIC_DRAW);
 
    glEnableVertexAttribArray(ATTRIB_VERTEX);
    glVertexAttribPointer(ATTRIB_VERTEX, 2, GL_FLOAT, GL_FALSE, 0, 0);
 
    // Draw
    glBindTexture(GL_TEXTURE_2D, texture);
 
    glUseProgram(_program);
 
 
    glDrawArrays(GL_POINTS, 0, (int)vertexCount);
 
    // Display the buffer
    glBindRenderbuffer(GL_RENDERBUFFER, viewRenderbuffer);
    [context presentRenderbuffer:GL_RENDERBUFFER];
}
 
///fsdffdf
 
 
static CGPoint getNormalizedPoint(UIView* view, CGPoint locationInView)
{
    const float normalizedX = (locationInView.x / view.bounds.size.width) * (2.f - 1.f);
    const float normalizedY = ((view.bounds.size.height - locationInView.y) / view.bounds.size.height) * (2.f - 1.f);
 
    return CGPointMake(normalizedX, normalizedY);
}
 
// set Img...
 
- (void)setImage:(UIImage *)image
{
    // Create an RGBA bitmap context
    CGImageRef CGImage = image.CGImage;
    GLint width = (GLint)CGImageGetWidth(CGImage);
    GLint height = (GLint)CGImageGetHeight(CGImage);
    size_t bitsPerComponent = 8;
    size_t bytesPerRow = width * 4;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context1 = CGBitmapContextCreate(NULL, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedFirst|kCGBitmapByteOrder32Little);
    // Invert vertically for OpenGL
    CGContextTranslateCTM(context1, 0, height);
    CGContextScaleCTM(context1, 1, -1);
    CGContextDrawImage(context1, CGRectMake(0, 0, width, height), CGImage);
    GLubyte *textureData = (GLubyte *)CGBitmapContextGetData(context1);
 
    //    [self setContentSize:CGSizeMake(width, height)];
    //    [self _setTextureData:textureData width:width height:height];
    [self generateDefaultTextureWithWidth:width height:height data:textureData];
 
    CGContextRelease(context1);
    CGColorSpaceRelease(colorSpace);
}
 
- (GLuint)generateDefaultTextureWithWidth:(GLint)width height:(GLint)height data:(GLvoid *)data
{
    //    texture = 0;
    glActiveTexture(GL_TEXTURE0);
    glGenTextures(1, &texture);
    glBindTexture(GL_TEXTURE_2D, texture);
    glUniform1i(inputImageTexture2Uniform, 0);
 
 
    //    glGenTextures(1, &texture);
    //    glBindTexture(GL_TEXTURE_2D, texture);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_BGRA, GL_UNSIGNED_BYTE, data);
    //    glBindTexture(GL_TEXTURE_2D, 0);
    return texture;
}
 
- (GLuint)uniformIndex:(NSString *)uniformName
{
    return glGetUniformLocation(_program, [uniformName UTF8String]);
}
 
- (void)setPaintLine12
{
    [self performSelector:@selector(drawEffects) withObject:nil afterDelay:0.5];
}
 
// Erases the screen
- (void)erase
{
    NSLog(@"erase");
}
 
- (BOOL)canBecomeFirstResponder {
    return YES;
}
 
@end

Also attaching shader as follows:
Code :
precision highp float;
 
varying vec2 textureCoordinate;
 
uniform sampler2D inputImageTexture;
 
uniform vec2 center;
uniform float radius;
uniform float angle;
 
void main()
{
    vec2 textureCoordinateToUse = textureCoordinate;
    float dist = distance(center, textureCoordinate);
    if (dist < radius)
    {
        textureCoordinateToUse -= center;
        float percent = (radius - dist) / radius;
        float theta = percent * percent * angle * 8.0;
        float s = sin(theta);
        float c = cos(theta);
        textureCoordinateToUse = vec2(dot(textureCoordinateToUse, vec2(c, -s)), dot(textureCoordinateToUse, vec2(s, c)));
        textureCoordinateToUse += center;
    }
 
    gl_FragColor = texture2D(inputImageTexture, textureCoordinateToUse );
}

Please help me to debug what is wrong and how to resolve this.
You can also run complete project from here Twirl on Touch-github