Image editing using iphone.

Question

I'm creating an image editing application for iphone. i would like to enable the user to pick an image from the photolibrary, edit it (grayscale, sepia,etc) and if possible, save back to the filesystem. I've done it for picking image (the simplest thing, as you know using imagepicker) and also for creating the grayscale image. But, i got stuck with sepia. I don't know how to implement that. Is it possible to get the values of each pixel of the image so that we can vary it to get the desired effects. Or any other possible methods are there??? pls help...

Answer 1

The Apple image picker code will most likely be holding just the file names and some lower-res renderings of the images in RAM til the last moment when a user selects an image.

When you ask for the full frame buffer of the image, the CPU suddenly has to do a lot more work decoding the image at full resolution, but it might be even as simple as this to trigger it off:

CFDataRef CopyImagePixels(CGImageRef inImage)
{
    return CGDataProviderCopyData(CGImageGetDataProvider(inImage));
}

/* IN MAIN APPLICATION FLOW - but see EDIT 2 below */
const char* pixels = [[((NSData*)CopyImagePixels([myImage CGImage]))
  autorelease] bytes]; /* N.B. returned pixel buffer would be read-only */

This is just a guess as to how it works, really, but based on some experience with image processing in other contexts. To work out whether what I suggest makes sense and is good from a memory usage point of view, run Instruments.

The Apple docs say (related, may apply to you):

You should avoid creating UIImage objects that are greater than 1024 x 1024 in size. Besides the large amount of memory such an image would consume, you may run into problems when using the image as a texture in OpenGL ES or when drawing the image to a view or layer. This size restriction does not apply if you are performing code-based manipulations, such as resizing an image larger than 1024 x 1024 pixels by drawing it to a bitmap-backed graphics context. In fact, you may need to resize an image in this manner (or break it into several smaller images) in order to draw it to one of your views. [ http://developer.apple.com/iphone/library/documentation/UIKit/Reference/UIImage%5FClass/Reference/Reference.html ]

AND

Note: Prior to iPhone OS 3.0, UIView instances may have a maximum height and width of 1024 x 1024. In iPhone OS 3.0 and later, views are no longer restricted to this maximum size but are still limited by the amount of memory they consume. Therefore, it is in your best interests to keep view sizes as small as possible. Regardless of which version of iPhone OS is running, you should consider using a CATiledLayer object if you need to create views larger than 1024 x 1024 in size. [ http://developer.apple.com/iPhone/library/documentation/UIKit/Reference/UIView%5FClass/UIView/UIView.html ]

Also worth noting:-

(a) Official how-to

http://developer.apple.com/iphone/library/qa/qa2007/qa1509.html

(b) From http://cameras.about.com/od/cameraphonespdas/fr/apple-iphone.htm

"The image size uploaded to your computer is at 1600x1200, but if you email the photo directly from the iPhone, the size will be reduced to 640x480."

(c) Encoding large images with JPEG image compression requires large amounts of RAM, depending on the size, possibly larger amounts than are available to the application.

(d) It may be possible to use an alternate compression algorithm with (if necessary) its malloc rewired to use temporary memory mapped files. But consider the data privacy/security issues.

(e) From http://stackoverflow.com/questions/504843/iphone-sdk-after-a-certain-number-of-characters-entered-the-animation-just-won

"I thought it might be a layer size issue, as the iPhone has a 1024 x 1024 texture size limit (after which you need to use a CATiledLayer to back your UIView), but I was able to lay out text wider than 1024 pixels and still have this work."

Sometimes the 1024 pixel limit may appear to be a bit soft, but I would always suggest you program defensively and stay within the 1024 pixel limit if you can.

EDIT 1

Added extra line break in code.

EDIT 2

Oops! The code gets a read-only copy of the data (there is a diference between CFMutableDataRef and CFDataRef). Because of limitations on available RAM, you then have to make a lower-res copy of it by smooth-scaling it down yourself, or to copy it into a modifiable buffer, if the image is large, you may need to write it in bands to a temporary file, release the unmodifiable data block and load the data back from file. And only do this of course if having the data in a temporary file like this is acceptable. Painful.

EDIT 3

Here's perhaps a better idea maybe try using a destination bitmap context that uses a CFData block that is a memory-mapped CFData. Does that work? Again only do this if you're happy with the data going via a temporary file.

EDIT 4

Oh no, it appears that memory mapped read-write CFData not available. Maybe try mmap BSD APIs.

EDIT 5

Added "const char*" and "pixels read-only" comment to code.