iPhone Software Development: How to skip provision

In some OpenGL 1.x books you might notice that the gl commands (like belows) are within the code block of glBegin() and glEnd() pair. These gl* commands must be converted to Vertices Array in order to be useful for OpenGL ES for iPhone.


glBegin(GL_LINE_STRIP);
z = -50.0f;
for(angle = 0.0f; angle <= (2.0f*GL_PI)*3.0f; angle += 0.1f)
{
  x = 50.0f*sin(angle);
  y = 50.0f*cos(angle);
  // Specify the point and move the z value up a little
  glVertex3f(x, y, z);
  z += 0.5f;
}
// Done drawing points
glEnd();


Typically, you remove the glBegin() and glEnd() commands and create the vertices array and implement the vertices position calculation (if any) inside the setupView and then remove other unsupported gl* commands before putting them to the iPhone OpneGL ES project code.

Here are some of the typical drawings in 3D. This one is for a rotating Spiral

To use the source codes here, you just need to create a new project from OpenGL ES Application template of XCode and copy the source codes of EAGLView.m from below and paste them for Build & Go in XCode.

EAGLView.m (for Spiral) Select all

//
// EAGLView.m
// Spiral
//

#import <QuartzCore/QuartzCore.h>
#import <OpenGLES/EAGLDrawable.h>

#import "EAGLView.h"

#define USE_DEPTH_BUFFER 0

// A class extension to declare private methods
@interface EAGLView ()

@property (nonatomic, retain) EAGLContext *context;
@property (nonatomic, assign) NSTimer *animationTimer;

- (BOOL) createFramebuffer;
- (void) destroyFramebuffer;

@end

@implementation EAGLView

@synthesize context;
@synthesize animationTimer;
@synthesize animationInterval;

// You must implement this method
+ (Class)layerClass {
return [CAEAGLLayer class];
}

#define kAnimationFrequency 60.0

//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])) {
// Get the layer
CAEAGLLayer *eaglLayer = (CAEAGLLayer *)self.layer;

eaglLayer.opaque = YES;
eaglLayer.drawableProperties = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:NO], kEAGLDrawablePropertyRetainedBacking, kEAGLColorFormatRGBA8, kEAGLDrawablePropertyColorFormat, nil];

context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES1];

if (!context || ![EAGLContext setCurrentContext:context]) {
[self release];
return nil;
}

animationInterval = 1.0 / kAnimationFrequency;
[self setupView];
}
return self;
}

#define GL_PI 3.1415f
GLfloat linesVertices[186];

- (void)setupView {

// setup the projection matrix
glMatrixMode(GL_PROJECTION);
glLoadIdentity();

//glOrthof(-1.0f, 1.0f, -1.5f, 1.5f, -1.0f, 1.0f);
GLfloat nRange = 100.0f;
GLfloat w = 320.0f, h = 480.0f;
glOrthof (-nRange*w/h, nRange*w/h, -nRange, nRange, -nRange, nRange);

glMatrixMode(GL_MODELVIEW);

GLfloat x,y,z,angle; // Storage for coordinates and angles
int c = 0;
z = -50.0f;

// Loop around in a circle three times
for(angle = 0.0f; angle <= (2.0f*GL_PI)*3.0f; angle += 0.1f)
{
// Calculate x and y values on the circle
x = 50.0f*sin(angle);
y = 50.0f*cos(angle);
// glVertex3f(x, y, z);
linesVertices[c++] = x;
linesVertices[c++] = y;
linesVertices[c++] = z;

// Bump up the z value
z += 0.5f;
}
}

- (void)drawView {

static GLfloat xRot = 0.0f;
static GLfloat yRot = 0.0f;
static GLfloat zRot = 1.0f;

const GLubyte linesColors[] = {
0.0f, 0.0f, 0.0f, 1.0f,
};

[EAGLContext setCurrentContext:context];
glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
glViewport(0, 0, backingWidth, backingHeight);

// Clear background color
glClearColor(0.5f, 0.5f, 0.5f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);

glRotatef(xRot, 0.0f, 0.0f, 0.0f);
glRotatef(yRot, 1.0f, 1.0f, 0.0f);
glRotatef(zRot, 0.0f, 0.0f, 1.0f);

// Set Line Width
glLineWidth(3.0f);

glVertexPointer(3, GL_FLOAT, 0, linesVertices);

// Set drawing color to green
glColor4f(0.0f, 1.0f, 0.0f, 0.0f);
glColorPointer(4, GL_UNSIGNED_BYTE, 0, linesColors);
glEnableClientState(GL_VERTEX_ARRAY);

glDrawArrays(GL_LINES, 0, 189);

glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
[context presentRenderbuffer:GL_RENDERBUFFER_OES];

static NSTimeInterval lastDrawTime;
if (lastDrawTime)
{
NSTimeInterval timeSinceLastDraw = [NSDate timeIntervalSinceReferenceDate] - lastDrawTime;
xRot += 0.1 * timeSinceLastDraw;
}
lastDrawTime = [NSDate timeIntervalSinceReferenceDate];

}

- (void)layoutSubviews {
[EAGLContext setCurrentContext:context];
[self destroyFramebuffer];
[self createFramebuffer];
[self drawView];
}

- (BOOL)createFramebuffer {

glGenFramebuffersOES(1, &viewFramebuffer);
glGenRenderbuffersOES(1, &viewRenderbuffer);

glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
[context renderbufferStorage:GL_RENDERBUFFER_OES fromDrawable:(CAEAGLLayer*)self.layer];
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, viewRenderbuffer);

glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);

if (USE_DEPTH_BUFFER) {
glGenRenderbuffersOES(1, &depthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, depthRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_DEPTH_COMPONENT16_OES, backingWidth, backingHeight);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, depthRenderbuffer);
}

if(glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES) != GL_FRAMEBUFFER_COMPLETE_OES) {
NSLog(@"failed to make complete framebuffer object %x", glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES));
return NO;
}

return YES;
}

- (void)destroyFramebuffer {

glDeleteFramebuffersOES(1, &viewFramebuffer);
viewFramebuffer = 0;
glDeleteRenderbuffersOES(1, &viewRenderbuffer);
viewRenderbuffer = 0;

if(depthRenderbuffer) {
glDeleteRenderbuffersOES(1, &depthRenderbuffer);
depthRenderbuffer = 0;
}
}

- (void)startAnimation {
self.animationTimer = [NSTimer scheduledTimerWithTimeInterval:animationInterval target:self selector:@selector(drawView) userInfo:nil repeats:YES];
}

- (void)stopAnimation {
self.animationTimer = nil;
}

- (void)setAnimationTimer:(NSTimer *)newTimer {
[animationTimer invalidate];
animationTimer = newTimer;
}

- (void)setAnimationInterval:(NSTimeInterval)interval {

animationInterval = interval;
if (animationTimer) {
[self stopAnimation];
[self startAnimation];
}
}

- (void)dealloc {

[self stopAnimation];

if ([EAGLContext currentContext] == context) {
[EAGLContext setCurrentContext:nil];
}

[context release];
[super dealloc];
}

@end

And this one is for a rotating Fanned Circle.

EAGLView.m (Fanned Circle) Select all

//
// EAGLView.m
// Fanned Circle
//

#import <QuartzCore/QuartzCore.h>
#import <OpenGLES/EAGLDrawable.h>

#import "EAGLView.h"

#define USE_DEPTH_BUFFER 0

// A class extension to declare private methods
@interface EAGLView ()

@property (nonatomic, retain) EAGLContext *context;
@property (nonatomic, assign) NSTimer *animationTimer;

- (BOOL) createFramebuffer;
- (void) destroyFramebuffer;

@end

@implementation EAGLView

@synthesize context;
@synthesize animationTimer;
@synthesize animationInterval;

// You must implement this method
+ (Class)layerClass {
return [CAEAGLLayer class];
}

#define kAnimationFrequency 60.0

//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])) {
// Get the layer
CAEAGLLayer *eaglLayer = (CAEAGLLayer *)self.layer;

eaglLayer.opaque = YES;
eaglLayer.drawableProperties = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:NO], kEAGLDrawablePropertyRetainedBacking, kEAGLColorFormatRGBA8, kEAGLDrawablePropertyColorFormat, nil];

context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES1];

if (!context || ![EAGLContext setCurrentContext:context]) {
[self release];
return nil;
}

animationInterval = 1.0 / kAnimationFrequency;
[self setupView];
}
return self;
}

#define GL_PI 3.1415f
GLfloat linesVertices[186];

- (void)setupView {

// setup the projection matrix
glMatrixMode(GL_PROJECTION);
glLoadIdentity();

//glOrthof(-1.0f, 1.0f, -1.5f, 1.5f, -1.0f, 1.0f);
GLfloat nRange = 100.0f;
GLfloat w = 320.0f, h = 480.0f;
glOrthof (-nRange*w/h, nRange*w/h, -nRange, nRange, -nRange, nRange);

glMatrixMode(GL_MODELVIEW);

GLfloat x,y,z,angle; // Storage for coordinates and angles
int c;
z = 0.0f;
c = 0;
for(angle = 0.0f; angle <= GL_PI; angle += (GL_PI / 20.0f))
{
// Top half of the circle
x = 50.0f*sin(angle);
y = 50.0f*cos(angle);
// glVertex3f(x, y, z);
linesVertices[c++] = x;
linesVertices[c++] = y;
linesVertices[c++] = z;

// Bottom half of the circle
x = 50.0f*sin(angle+GL_PI);
y = 50.0f*cos(angle+GL_PI);
// glVertex3f(x, y, z);
linesVertices[c++] = x;
linesVertices[c++] = y;
linesVertices[c++] = z;
}
}

- (void)drawView {

static GLfloat xRot = 0.0f;
static GLfloat yRot = 0.0f;
static GLfloat zRot = 1.0f;

const GLubyte linesColors[] = {
0.0f, 1.0f, 0.0f, 1.0f,
};

[EAGLContext setCurrentContext:context];
glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
glViewport(0, 0, backingWidth, backingHeight);

// Clear background color
glClearColor(0.5f, 0.5f, 0.5f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);

glRotatef(xRot, 1.0f, 0.0f, 0.0f);
glRotatef(yRot, 0.0f, 1.0f, 0.0f);
glRotatef(zRot, 0.0f, 0.0f, 1.0f);

// Setup and render the points
glEnable(GL_POINT_SMOOTH);
glPointSize(1.0);
glVertexPointer(3, GL_FLOAT, 0, linesVertices);
// Set drawing color to green
glColor4f(0.0f, 1.0f, 0.0f, 0.0f);
glColorPointer(4, GL_UNSIGNED_BYTE, 0, linesColors);
glEnableClientState(GL_VERTEX_ARRAY);

glDrawArrays(GL_LINES, 0, 189);

glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
[context presentRenderbuffer:GL_RENDERBUFFER_OES];

static NSTimeInterval lastDrawTime;
if (lastDrawTime)
{
NSTimeInterval timeSinceLastDraw = [NSDate timeIntervalSinceReferenceDate] - lastDrawTime;
zRot+=1.2 * timeSinceLastDraw;
}
lastDrawTime = [NSDate timeIntervalSinceReferenceDate];

}

- (void)layoutSubviews {
[EAGLContext setCurrentContext:context];
[self destroyFramebuffer];
[self createFramebuffer];
[self drawView];
}

- (BOOL)createFramebuffer {

glGenFramebuffersOES(1, &viewFramebuffer);
glGenRenderbuffersOES(1, &viewRenderbuffer);

glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
[context renderbufferStorage:GL_RENDERBUFFER_OES fromDrawable:(CAEAGLLayer*)self.layer];
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, viewRenderbuffer);

glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);

if (USE_DEPTH_BUFFER) {
glGenRenderbuffersOES(1, &depthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, depthRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_DEPTH_COMPONENT16_OES, backingWidth, backingHeight);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, depthRenderbuffer);
}

if(glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES) != GL_FRAMEBUFFER_COMPLETE_OES) {
NSLog(@"failed to make complete framebuffer object %x", glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES));
return NO;
}

return YES;
}

- (void)destroyFramebuffer {

glDeleteFramebuffersOES(1, &viewFramebuffer);
viewFramebuffer = 0;
glDeleteRenderbuffersOES(1, &viewRenderbuffer);
viewRenderbuffer = 0;

if(depthRenderbuffer) {
glDeleteRenderbuffersOES(1, &depthRenderbuffer);
depthRenderbuffer = 0;
}
}

- (void)startAnimation {
self.animationTimer = [NSTimer scheduledTimerWithTimeInterval:animationInterval target:self selector:@selector(drawView) userInfo:nil repeats:YES];
}

- (void)stopAnimation {
self.animationTimer = nil;
}

- (void)setAnimationTimer:(NSTimer *)newTimer {
[animationTimer invalidate];
animationTimer = newTimer;
}

- (void)setAnimationInterval:(NSTimeInterval)interval {

animationInterval = interval;
if (animationTimer) {
[self stopAnimation];
[self startAnimation];
}
}

- (void)dealloc {

[self stopAnimation];

if ([EAGLContext currentContext] == context) {
[EAGLContext setCurrentContext:nil];
}

[context release];
[super dealloc];
}

@end

And also I have found a very nice method saveCurrentScreenToPhotoAlbum to capture the OpenGL view screen here. And below is an implementation on how to capture the screen in iPhone Simulator. You just touch/click the info button at the lower left bottom of iPhone Screen to trigger the screen capture to the Photo Album (Simulator or actual device).

To use the source codes here, you just need to create a new project from OpenGL ES Application template of XCode and copy the source codes of EAGLView.m from below and paste them for Build & Go in XCode. For this screenshot functionality, you need to add the CoreGraphics Framework to the Xcode Project before build & go.

EAGLView.m (Fanned Circle with Screen Capture) Select all

//
// EAGLView.m
// Fanned Circle with Screen Capture
//
#import <QuartzCore/QuartzCore.h>
#import <OpenGLES/EAGLDrawable.h>

#import "EAGLView.h"

#define USE_DEPTH_BUFFER 0

// A class extension to declare private methods
@interface EAGLView ()

@property (nonatomic, retain) EAGLContext *context;
@property (nonatomic, assign) NSTimer *animationTimer;

- (BOOL) createFramebuffer;
- (void) destroyFramebuffer;

@end

@implementation EAGLView

@synthesize context;
@synthesize animationTimer;
@synthesize animationInterval;

// You must implement this method
+ (Class)layerClass {
return [CAEAGLLayer class];
}

#define kAnimationFrequency 60.0

//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])) {
// Get the layer
CAEAGLLayer *eaglLayer = (CAEAGLLayer *)self.layer;

eaglLayer.opaque = YES;
eaglLayer.drawableProperties = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:NO], kEAGLDrawablePropertyRetainedBacking, kEAGLColorFormatRGBA8, kEAGLDrawablePropertyColorFormat, nil];

context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES1];

if (!context || ![EAGLContext setCurrentContext:context]) {
[self release];
return nil;
}

animationInterval = 1.0 / kAnimationFrequency;
[self setupView];
}
return self;
}

#define GL_PI 3.1415f
GLfloat linesVertices[186];

- (void)setupView {

UIButton *infoButton = [UIButton buttonWithType:UIButtonTypeInfoLight];
[infoButton addTarget:self action:@selector(saveCurrentScreenToPhotoAlbum) forControlEvents:UIControlEventTouchUpInside];
infoButton.alpha = 0.5f;
infoButton.frame = CGRectMake(17, self.bounds.size.height-33, 33, 33);
[self addSubview:infoButton];
[self bringSubviewToFront:infoButton];

// setup the projection matrix
glMatrixMode(GL_PROJECTION);
glLoadIdentity();

//glOrthof(-1.0f, 1.0f, -1.5f, 1.5f, -1.0f, 1.0f);
GLfloat nRange = 100.0f;
GLfloat w = 320.0f, h = 480.0f;
glOrthof (-nRange*w/h, nRange*w/h, -nRange, nRange, -nRange, nRange);

glMatrixMode(GL_MODELVIEW);

GLfloat x,y,z,angle; // Storage for coordinates and angles
int c;
z = 0.0f;
c = 0;
for(angle = 0.0f; angle <= GL_PI; angle += (GL_PI / 20.0f))
{
// Top half of the circle
x = 50.0f*sin(angle);
y = 50.0f*cos(angle);
// glVertex3f(x, y, z);
linesVertices[c++] = x;
linesVertices[c++] = y;
linesVertices[c++] = z;

// Bottom half of the circle
x = 50.0f*sin(angle+GL_PI);
y = 50.0f*cos(angle+GL_PI);
// glVertex3f(x, y, z);
linesVertices[c++] = x;
linesVertices[c++] = y;
linesVertices[c++] = z;
}

}

- (void)drawView {

static GLfloat xRot = 0.0f;
static GLfloat yRot = 0.0f;
static GLfloat zRot = 1.0f;

const GLubyte linesColors[] = {
0.0f, 1.0f, 0.0f, 1.0f,
};

[EAGLContext setCurrentContext:context];
glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
glViewport(0, 0, backingWidth, backingHeight);

// Clear background color
glClearColor(0.5f, 0.5f, 0.5f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);

glRotatef(xRot, 1.0f, 0.0f, 0.0f);
glRotatef(yRot, 0.0f, 1.0f, 0.0f);
glRotatef(zRot, 0.0f, 0.0f, 1.0f);

// Setup and render the points
glEnable(GL_POINT_SMOOTH);
glPointSize(1.0);
glVertexPointer(3, GL_FLOAT, 0, linesVertices);
// Set drawing color to green
glColor4f(0.0f, 1.0f, 0.0f, 0.0f);
glColorPointer(4, GL_UNSIGNED_BYTE, 0, linesColors);
glEnableClientState(GL_VERTEX_ARRAY);

glDrawArrays(GL_LINES, 0, 189);

glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
[context presentRenderbuffer:GL_RENDERBUFFER_OES];

static NSTimeInterval lastDrawTime;
if (lastDrawTime)
{
NSTimeInterval timeSinceLastDraw = [NSDate timeIntervalSinceReferenceDate] - lastDrawTime;
zRot+=0.1 * timeSinceLastDraw;
}
lastDrawTime = [NSDate timeIntervalSinceReferenceDate];

}

// callback for CGDataProviderCreateWithData
void releaseScreenshotData(void *info, const void *data, size_t size) {
free((void *)data);
};

// callback for UIImageWriteToSavedPhotosAlbum
- (void)image:(UIImage *)image didFinishSavingWithError:(NSError *)error contextInfo:(void *)contextInfo {
NSLog(@"ScreenSave finished\n");
[image release]; // release image
}

- (void)saveCurrentScreenToPhotoAlbum {
NSInteger myDataLength = backingWidth * backingHeight * 4;
// allocate array and read pixels into it.
GLuint *buffer = (GLuint *) malloc(myDataLength);
glReadPixels(0, 0, backingWidth, backingHeight, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
// gl renders “upside down” so swap top to bottom into new array.
for(int y = 0; y < backingHeight / 2; y++) {
for(int x = 0; x < backingWidth; x++) {
//Swap top and bottom bytes
GLuint top = buffer[y * backingWidth + x];
GLuint bottom = buffer[(backingHeight - 1 - y) * backingWidth + x];
buffer[(backingHeight - 1 - y) * backingWidth + x] = top;
buffer[y * backingWidth + x] = bottom;
}
}
// make data provider with data.
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer, myDataLength, releaseScreenshotData);
// prep the ingredients
const int bitsPerComponent = 8;
const int bitsPerPixel = 4 * bitsPerComponent;
const int bytesPerRow = 4 * backingWidth;
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
// make the cgimage
CGImageRef imageRef = CGImageCreate(320, 480, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);
CGColorSpaceRelease(colorSpaceRef);
CGDataProviderRelease(provider);
// then make the UIImage from that
UIImage *myImage = [[UIImage alloc] initWithCGImage:imageRef]; // change this to manual alloc/init instead of autorelease
CGImageRelease(imageRef); // YOU CAN RELEASE THIS NOW
UIImageWriteToSavedPhotosAlbum(myImage, self, @selector(image:didFinishSavingWithError:contextInfo:), nil); // add callback for finish saving
}

- (void)layoutSubviews {
[EAGLContext setCurrentContext:context];
[self destroyFramebuffer];
[self createFramebuffer];
[self drawView];
}

- (BOOL)createFramebuffer {

glGenFramebuffersOES(1, &viewFramebuffer);
glGenRenderbuffersOES(1, &viewRenderbuffer);

glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
[context renderbufferStorage:GL_RENDERBUFFER_OES fromDrawable:(CAEAGLLayer*)self.layer];
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, viewRenderbuffer);

glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);

if (USE_DEPTH_BUFFER) {
glGenRenderbuffersOES(1, &depthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, depthRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_DEPTH_COMPONENT16_OES, backingWidth, backingHeight);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, depthRenderbuffer);
}

if(glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES) != GL_FRAMEBUFFER_COMPLETE_OES) {
NSLog(@"failed to make complete framebuffer object %x", glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES));
return NO;
}

return YES;
}

- (void)destroyFramebuffer {

glDeleteFramebuffersOES(1, &viewFramebuffer);
viewFramebuffer = 0;
glDeleteRenderbuffersOES(1, &viewRenderbuffer);
viewRenderbuffer = 0;

if(depthRenderbuffer) {
glDeleteRenderbuffersOES(1, &depthRenderbuffer);
depthRenderbuffer = 0;
}
}

- (void)startAnimation {
self.animationTimer = [NSTimer scheduledTimerWithTimeInterval:animationInterval target:self selector:@selector(drawView) userInfo:nil repeats:YES];
}

- (void)stopAnimation {
self.animationTimer = nil;
}

- (void)setAnimationTimer:(NSTimer *)newTimer {
[animationTimer invalidate];
animationTimer = newTimer;
}

- (void)setAnimationInterval:(NSTimeInterval)interval {

animationInterval = interval;
if (animationTimer) {
[self stopAnimation];
[self startAnimation];
}
}

- (void)dealloc {

[self stopAnimation];

if ([EAGLContext currentContext] == context) {
[EAGLContext setCurrentContext:nil];
}

[context release];
[super dealloc];
}

@end

In case you might also want to know how to do screenshot for a non-OpenGL ES content programatically. This is the code for viewcontroller. If you put the code in a UIView object change self.view to self

- (void) snapUIView
{
    UIGraphicsBeginImageContext(self.view.bounds.size);
    [self.view.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *myImage = UIGraphicsGetImageFromCurrentImageContext();     UIGraphicsEndImageContext();
    UIImageWriteToSavedPhotosAlbum(myImage, nil, nil, nil);
}

 
 
 

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