I am writing an iOS app which need to load a lot of full-screen size images into OpenGL. Image storage space is a concern, so I am using png or jpg. Problem is: Loading the image data takes way too long (100s of ms on an iPad2).
I was wondering if someone knows the fastest way to load good-quality compressed image data on an iPad or similar? This could include switching to different image compression formats (?)
Here's what I've found:
Loading jpg via the iOS implementation of libturbo-jpeg is faster than any way I found for png.
If you split jpg images in two, you can parallel-decompress via GCD which almost cuts the load time in half. Still not enough!
The PowerVR compressed PVRTC format can be loaded very quickly, but both compression ratio and image quality are way outside what I need...
No idea how jpeg-2000 would perform, but it seems to be optimized for compression ratio rather than decoding speed.
Any ideas?? This must be a common problem for games or similar..
Are there any newer image formats (jpg is 1986!!) which have portable implementation, maybe slightly less compression than jpg, but decode much faster?
This library might help you out:
https://github.com/path/FastImageCache
Related
I realize that PNG is normally the web standard, but I might have a bit of a unique situation. I'm building an app that downloads lots of images from a server. The download needs to be as quick as possible, especially over 3G connections. I've been looking into the best compression for the images on the server. Ideally, every image I use can be quickly resized for the app using a drag and drop converter of some sort. When I convert one of the images to a PNG, the file size is around 130KB. When converted to an acceptable JPG quality however, the file size comes in at well under 50KB. Should I use JPGs in the application?
Thanks in advance,
It actually depends on the type of images.
The basic rule is to use JPEG for photos and PNG for everything else.
Why is it so?
JPEG compression is designed for photos - photos can handle lossy compression. On the other hand, PNG is compression designed for images which have similar pixels close to each other (straight lines, areas filled with one color, texts, etc.). If you try to use JPEG on an image which is not a photo, you'll be surprised - it won't look very good.
PNG is a standard - but for icons, backgrounds, patterns etc; not for photos.
Yes you can use jpg. You will not encounter any problem. It is a tradeoff between quality and size. Jpeg is usually smaller because it is compressed so as to reduce size. Png is lossless compressed.
I have made applications that use both png and jpeg and they both work like a charm.
EDIT: You can also check out this link PNG vs JPEG
PNG images are huge in size, use JPG as long as it's possible (when you don't need transparency)
Xcode optimizes png images so they are loaded faster, but is this also recommended for other image resources that are not for buttons and UI (Photos for example)?
Or is it a standard to have UI images as png and "photos" as JPEG?
What is the best practice here?
The best practice is to use PNG files as often as possible when using images within your app. iOS and Xcode load and display PNGs quickest as opposed to any other type of format. If given a choice, go with PNG.
When you use any other file type (or if you load a non-optimized PNG files), your iPhone has to do the byte-swapping and alpha premultiplication at load-time (and possibly re-do the alpha multiplication at display time). Your application basically has to do the same processing that Xcode does, but it's doing it at run-time instead of at build-time. This is going to cost you both in terms of processor cycles and memory overhead. One of the reasons why Mobile Safari is the biggest memory hog of the built-in iPhone applications is because the images it has to load in order to display web-pages are all non-optimized images, mostly JPEGs. Since JPEG is a compressed format, it has the added extra step of having to decompress the image into memory before it can do the premultiplication and byte-swapping.
For what it's worth, Apple recommends using the PNG format for images. There's a lot more information on this topic in this Apple documentation.
I've read many of the png vs. jpg threads here and elsewhere. I didn't find this aspect covered for small images that are downloaded from a server.
A short recap:
Xcode optimizes png images that are delivered with the app bundle in a way that they are optimized for the iOS hardware ("png magic")
png images support transparency (which I don't need)
png is the better choice from graphics, jpg for pictures (we have pictures)
I'm building an app that periodically downloads feeds that contain links to thumbnail images (size 80x80). These images are presented side by side the text content in a tableview. I can influence which format is used (jpg, png) on the server side.
If I use an uncompressed png format, it will have about 17k size for one image. This is quite large. And since this png doesn't use the "png magic" of Xcode, the iPhone still might need quite some cpu to get them into the table view compared to an "Xcode prepared" png.
The same image as a compressed jpg is only 3k which is great.
Question: are there lab comparisons that show the real world performance of these 2 formats?
Another one: has anyone used jpgs of a similar size (80x80, 3k) successfully in a table-view?
Many thanks in advance
What do you mean "lab comparisons"? PNG is going to do better with flat colors -- it uses variations on run-length and dictionary encoding as I understand. JPEG will be better with images containing subtle gradients, and loses data mostly in jumps in luminosity which are hard for the human eye to see. "Better" here refers only to file size. It sounds like you would want JPEGs here.
I have an app that will display a bunch of images in a slideshow. Those images will be part of the bundle, thus distributed with the app.
All the images are photographs or photographic, etc.
I've read that it's preferred to use PNG as the image format, but seeing that the JPG version will be much smaller, I'd rather be using that.
Are there any guidelines which format to use and in which case?
PNG's are pixel perfect (non-lossy), and require very little extra CPU energy to display. However, large PNGs may take longer to read from storage than more compressed image formats, and thus be slower to display.
JPG's are smaller to store, but lossy (amount depends on compression level), and to display them requires a much more complicated decoding algorithm. But the typical compression and image quality is usually quite sufficient for photos.
Use JPG's for photos and for anything large, and PNG's for anything small and/or designed to be displayed "pixel perfect" (e.g. small icons) or as a part of a composited transparent overlay, etc.
Apple optimizes PNG images that are included in your iPhone app bundle. In fact, the iPhone uses a special encoding in which the color bytes are optimized for the hardware. XCode handles this special encoding for you when you build your project. So, you do see additional benefits to using PNG's on an iPhone other than their size consideration. For this reason it is definitely recommended to use PNG's for any images that appear as part of the interface (in a table view, labels, etc).
As for displaying a full screen image such as a photograph you may still reap benefits with PNG's since they are non-lossy and the visual quality should be better than a JPG not to mention resource usage with decoding the image. You may need to decrease the quality of your JPG's in order to see a real benefit in file size but then you are displaying non-optimal images.
File size is certainly a factor but there are other considerations at play as well when choosing an image format.
There is one important thing to think about with PNGs. If a PNG is included in your Xcode build it will be optimized for iOS. This is called PNG crush. If your PNG is downloaded at run time it will not be crushed. Crushed PNGs run about the same as 100% JPGs. Lower quality JPGs run better than higher quality JPGs. So from a performance standpoint from fastest to slowest it would go low quality JPG, high quality JPG, PNG Crushed, PNG.
If you need to download PNGs you should consider crushing the PNGs on the server before the download.
http://www.cocoanetics.com/2011/10/avoiding-image-decompression-sickness/
The Cocoanetics blog published a nice iOS performance benchmark of JPGs at various quality levels, and PNGs, with and without crushing.
From his conclusion:
If you absolutely need an alpha channel or have to go with PNGs then
it is advisable to install the pngcrush tool on your web server and
have it process all your PNGs. In almost all other cases high quality
JPEGs combine smaller file sizes (i.e. faster transmission) with
faster compression and rendering.
It turns out that PNGs are great for small images that you would use
for UI elements, but they are not reasonable to use for any full
screen applications like catalogues or magazines. There you would want
to choose a compression quality between 60 and 80% depending on your
source material.
In terms of getting it all to display you will want to hang onto
UIImage instances from which you have drawn once because those have a
cached uncompressed version of the file in them. And where you don’t
the visual pause for a large image to appear on screen you will have
to force decompression for a couple of images in advance. But bear in
mind that these will take large amounts of RAM and if you are
overdoing it that might cause your app to be terminated. NSCache is a
great place to place frequently used images because this automatically
takes care of evicting the images when RAM becomes scarce.
It is unfortunate that we don’t have any way to know whether or not an
image still needs decompressing or not. Also an image might have
evicted the uncompressed version without informing us as to this
effect. That might be a good Radar to raise at Apple’s bug reporting
site. But fortunately accessing the image as shown above takes no time
if the image is already decompressed. So you could just do that not
only “just in time” but also “just in case”.
Just thought I'd share a bit of decompression performance data...
I'm doing some prototyping of a 360 degree viewer - a carousel where the user can spin through a series of photos taken from different angles, to give the impression of being able to smoothly rotate an object.
I have loaded the image data into an array of NSData's to take file i/o out of the equation, but create NSImage's on the fly. Testing at near max frame rate (~25 fps) and watching in Instruments I see the app is clearly CPU-bound and there's an approximately 10% increase in CPU load showing ~275 kb png's vs. ~75 kb jpg's.
I can't say for sure but my guess is the CPU limit is just from general program execution and moving all the data around in memory, but that image decompression is done on the GPU. Either way and the JPG vs. PNG performance argument looks to favour JPG, especially when the smaller file sizes (and therefore smaller sizes of objects in memory at least in some parts of the chain) is taken into consideration.
Of course every situation is different, there's no substitute for testing...
I have found massive differences in animation performance when using jpegs vs png. For example placing three screen-sized jpegs side by side in a UIScrollView and scrolling horizontally on an iPhone4 results in lag and a thoroughly unpleasant jerky animation. With non-transparent pngs of the same dimensions the scrolling is smooth. I never use jpegs, even if the image is large.
I think if you want to use transparent, you have no choice except PNG. But, if your background is opaque already, then you may use JPG. That is the only difference I can see
'Use JPEG for photos' as mentioned in Human Interface Guidelines under section Produce artwork in the appropriate format.
I am grabbing an image from the camera roll and then using it for a while as well as save it to disk as a PNG on the iPhone. I am getting the odd crash, presumably due to out of memory.
Does it make a difference if I save it as PNG or JPG (assuming I choose note to degrade the quality in the JPG case)? Specifically:
is more memory then used by the UIImage after I reload it off of disk if I saved it as a PNG?
is it possible the act of saving as PNG uses up more memory transiently during the saving process?
I had been assuming the UIImage was a format neutral representation and it shouldn't matter, but I thought I should verify.
I am getting the odd crash, presumably due to out of memory
Then STOP WHAT YOU ARE DOING RIGHT NOW and first figure out if that's actually the cause of the crash. Otherwise there's a very good chance that you're chasing the wrong problem here, fixing a memory problem that doesn't exist while ignoring the real cause of the crash. If you want to fix a crash, start by figuring out what caused the crash. Following up on what's "presumably" the problem is a recipe for wasted time and effort.
I have an application on the store that needs to save intermediate versions of an image as it's being edited. In the original version, I used PNG format for saving, to avoid quality loss from loading and saving JPEG multiple times.
Sometime around the 2.2 software release, Apple introduced a change into the PNG writing code, such that it takes many times longer to save PNG data from some images. I ended up having to change to saving in JPEG format, because my application was timing out when trying to save images on exit.
Also, you'll run into issues because saving in PNG format doesn't preserve the "orientation" information in the UIImage, so a picture taken in Portrait orientation with the built-in camera will appear rotated after you save and reload it.
It depends on what type of images you're dealing with. If you're dealing with photographic images, JPEGs will almost always be smaller than PNGs, with no discernable loss of detail as can be seen by the human eye.
Conversely, if you're dealing with highly non-photographic images such as GUI elements or images with large blocks of solid colors, then PNGs and JPEGs will be comparable in size, but the PNG will save losslessly whereas the JPEG will be lossy and have very visible artifacts. If you have a really simple image (very large blocks of constant colors, e.g.), then a PNG will very likely be much smaller than a JPEG, and again will not have any compression artifacts.
The act of saving an image as a PNG or JPEG should not take up very much transient memory. When an image is in memory, it is typically stored uncompressed in memory so that it can be drawn to the screen very quickly, as opposed to having to decompress it every time you want to render it. Compared to the size of the uncompressed image, the amount of extra temporary storage you need to compress it is very small. If you can fit the uncompressed image in memory, you don't have to worry about the memory used while compressing it.
And of course, once you write the image to the file system in non-volatile storage and free the in-memory image, it really doesn't matter how big the compressed image is, because it doesn't take up main memory any more. The size of the compressed image only affects how much flash storage it's using, which can be an issue, but it does not affect high likely your app is to run out of memory.
Your crashes may be from a known memory leak in the UIImagePickerController.
This should help you fix that.
I don't have any hard data, but I'd assume that PNGs are preferable because Apple seems to use PNGs virtually everywhere in iPhone OS.
However, if you've already got the code set up for writing PNGs, it shouldn't be too hard to change it to write JPEGs, should it? Just try both methods and see which works better.
Use PNG wherever possible. As part of the compilation XCode runs all PNG files through a utility (pngcrush) to compress and optimize them.
is more memory then used by the UIImage after I reload it off of
disk if I saved it as a PNG?
=> No, it's the same memory size if you import from 2 images that have same resolution and same number of channels. (such as RGBA)
is it possible the act of saving as PNG uses up more memory transiently during the saving process?
=> No, it only effect your disk memory.