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Get binary data from UIImage
Closed 5 years ago.
I have a confusion regarding getting number of bytes in UIImage
To get number of bytes we use NSdata byts,
I have have an image of size 128X160 size, and NSData bytes tell it have 400669 bytes
But as per calculation in 3 channel image of 8 bit color i.e 24bit per pixel the number of bytes should be 3X128X160 = 61440 byts
Please help and let me know why NSData tell different in number of bytes?
Thanks
The underlying data of a UIImage can vary, so for the same "image" one can have varying sizes of data. One thing you can do is use UIImagePNGRepresentation or UIImageJPEGRepresentation to get the equivalent NSData constructs for either, then check the size of that.
Related
We have an UIImage of size 1280*854 and we are trying to save it in png format.
NSData *pngData = UIImagePNGRepresentation(img);
The problem is that the size of pngData is 9551944 which is inappropriately large for the input image size. Even considering 24 bit PNG, at the max it should be 1280*854*3 (3 for 24 bit png).
BTW, this is only happening with images scaled with UIGraphicsGetImageFromCurrentImageContext. We also noticed that image._scale is set to 2.0 in image returned by UIGraphicsGetImageFromCurrentImageContext.
Any idea what's wrong.
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convert UIImage to NSData
(7 answers)
Closed 7 years ago.
I'm taking a "snapshot" of the image context in UIGraphicsBeginImageContextWithOptions(UIScreen.mainScreen().bounds.size, true, 0) and eventually creating a UIImage using
var renderedImage = UIGraphicsGetImageFromCurrentImageContext()
However I need to get the NSData representation of this UIImage without using UIImageJPEGRepresentation or UIImagePNGRepresentation (because these produce files that are way larger than the original UIImage). How can I do this?
these produce files that are way larger than the original UIImage). How can I do this?
Image files contain compressed data, while NSData is raw i.e. not compressed. Therefore NSData will in about all cases be larger, when written into a file.
More info at another question: convert UIImage to NSData
I'm not sure what you mean by "way larger" than the original UIImage. The data backing the UIImage object is at least as big as the data you would get by converting it into a JPG, and roughly equivalent to the data you would get by converting it to a PNG.
The rendered image will be twice the screen size in points, because you have rendered a retina screen into an image.
You can avoid this and render the image as non-retina by making your image context have a scale of 1:
UIGraphicsBeginImageContextWithOptions(UIScreen.mainScreen().bounds.size, true, 1)
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How to easily resize/optimize an image size with iOS?
(18 answers)
Closed 9 years ago.
I have image with only red and white color. In image processing, we can reduce image from 24 bit to 8 bit or something like that.
Is it possible to reduce image size? In my iPad application, I can save image as png or jpeg. But I want to reduce the size more. How should I write code?
Have you looked into the method UIImageJPEGRepresentation? Once you have your UIImage you just need to do something like:
NSData* imgData = UIImageJPEGRepresentation(myImage, 0.4); //0.4 is the compression rate.
[imgData writeToURL:myFileURL atomically:YES];
I would like to be able to calculate the total number of bytes a UIImage uses in memory.
I can make a rough estimate by multiplying the width by the height and then by a multiplier number of bytes, but I'd like to calculate the size exactly if possible.
In general, objects don't have a single meaningful "size", since they can allocate and release any number of other objects privately as needed. sizeof(*myObj) only gives you the size of the top level structure, not a very useful number. If you need the complete memory impact of allocating and using an object, run under Instruments and watch allocations.
For a UIImage, its practical size is the size of whatever is backing it, typically either an NSData containing a PNG, or a CGimageRef, plus the object overhead. (There's also the pixel buffer when it gets rendered to the screen or other context; but that buffer belongs to the view or context in question, not the UIImage. If a UIView is doing the rendering then that buffer is likely in GL texture memory anyway.)
[UIImage imageWithData:[NSData dataWithContentsOfFile:#"foo.png"]] gives you a UIImage that is the same size as the foo.png file, plus some inconsequential overhead. [UIImage imageNamed:#"foo.png"] does the same thing, except that the class maintains a cache table of one object per filename, and will cause that object to dump its memory copy of the png in low-memory situations, reducing its "size" to just the overhead.
imageWithCGImage: and variants give you an UIImage that uses a CGImage reference as its backing store, and CGImages can be any number of things depending on their source. If you've been painting in one, it's probably an uncompressed pixel buffer. Calculate its size exactly as you propose above. If you need what its size "would be" if it were from a file, inspect the result of the UIImagePNGRepresentation or UIImageJPEGRepresentation functions.
Width * height * 4 will get you close. I'm not sure there's a way to get the exact size, since width is rounded out to an arbitrary, undocumented boundary (at least 4 pixels or 16 bytes, I gather), and there are several extra internal pieces of the object that you'd need to count. Plus likely there are internal attributes that are hung on the object or not, based on its use.
I had to solve this for a twitter app I was writing. Twitter rejects images larger than 3MB, so I needed to compress the image just enough to get below the 3MB limit. Here is the code snippet I used:
float compression = 1.0f;
NSData* data = UIImageJPEGRepresentation(photo, compression);
while(data.length > 3145728) //3MB
{
compression -= .1f;
NSLog(#"Compressing Image to: %lf", compression);
data = UIImageJPEGRepresentation(photo, compression);
NSLog(#"Image Bytes: %i", data.length);
}
The compression algorithm I used is non-optimized.
So, What is it doing?
Good question! The UIImageJPEGRepresentation method returns a byte array. To get the size, simply check the length of the array!
There is also a UIImagePNGRepresentation method. Keep in mind, these method are having to build byte arrays, and if needed convert the binary representation of the data. This can take a bit of time. Luckily in my case, most images taken by the iPhone are already less than 3MB, and will only need the compression if there is a wide range of colors; but calling the UIImageJPEGRepresentation method repeatedly (which could happen in my posted code) can take some time.
I'm using some images as background, and for retina display they are 640x960 pixels and saved as PNG.
Becasue they eat-up so much memory, I've saved them as RGB-565 (2 bytes per pixel, 5 pixels for red and blue, 6 pixels for green, no transparency), but the memory allocated is the same.
Is the UIImage class aware of those specific formats ?
For the memory usage, I think UIImage think is a RGBA8888 (4 bytes per pixel, 8 bits per channel).
thanks,
r.
UIImages contain some nifty optimization strategies. One of them is probably converting an image to the native format (ARGB32 or something, I forget...) for faster rendering at run-time. At other times, it might choose to keep the image in its original PNG compressed form.
Ah see: http://developer.apple.com/library/ios/#documentation/uikit/reference/UIImage_Class/Reference/Reference.html says "Windows Bitmap Format (BMP) files that are formatted as RGB-565 are converted to ARGB-1555 when they are loaded.". The same will probably hold for your images.