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.
Related
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
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 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 have an iPhone application that downloads images from the internet and saves them for display later.
The user can select the images to view from a UITableView, the table view has custom cells which display thumbnails of the original images in varying sizes.
When the large image is first downloaded it is scaled to thumbnail size and the thumbnail is saved using UIImagePNGRepresentation.
What I would like to do is save the thumbnail in the optimized iPhone PNG format. How can I do that? does it happen magically just by loading the original large image into memory and saving it? Do I have to do any further processing on the thumbnail before saving?
Chances are the UIImagePNGRepresentation does not create the images, as they are non-comformant PNGs, and can't be read by anything else. If they API generated PNGs that could not be read I think it would document that, and anyone who uploads PNGs from the phone would notice that they did not work.
The optimization is useful, but most of the optimized PNGs are part of the UI where the optimization is done as part of the build process. Chances are the cost of performing the optimization will offset any gains you get from it.
So it turns out that the RGB565 colorspace used in the optimized format is simply not available in a CGGraphicsContext, which is the rendering class used by all the UIwhatever components.
So if I DID write some code to change the color space of the image before saving it, I couldn't get the texture back into a UI class. The only way to use it is to load it directly into the OpenGL innards and use OpenGL in my app.
Don't worry about the "optimized PNG" format, as it isn't making any significant difference.
It does not affect rendering speed at all, and loading speed is dictated by file size more than file format.
So simply save it in a format that will give you smallest files. If you're not using transparency, then it might be JPEG.
If you need transparency, and can spend more CPU time when saving images, then include pngquant in your program (it's under BSD-like license) and shrink those PNGs to 8-bit palette.