When should a photo be cached - android

Is there a general guideline or rule of thumb on when I should cache a photo?
For example I have a grid view of smaller icon images. This page isn't at the beginning of the app so there's no guarantee a user sees it, these images could change server-side but for the most part they won't. Should I cache them because there are so many or not worry about them because they are small?
Another example would be if I click on one of the images mentioned above it will lead me to a view with one larger image, should I cache this image since it's larger or should I let the user make a network request each time?
I'm not looking for an answer for these specific questions, but rather a guideline to follow.
To clarify by caching I mean saving the file to storage. Also, is there a better method than this?
Thanks for the help!

Caching is an important feature in user experience, so even if there's no guarantee the user sees that particular 'page', it a nice thing to have.
Saving images in a cache depends directly on many variables:
- How much time does it take to regenerate images if not cached
- How many images are we talking about
- How often do they change
- What size is each image
For example, having 100 thumbnails of images that change every month makes a lot of sense. Caching 200 images with 1024x768 resolution that change very often doesn't.... or does it? The answer is very variable.
Also cache should always have storage boundaries. Even in the previous example with the large images, you could limit the cache to a a certain storage capacity.
Normally the cache saves a local copy of the most recently used items. But you can implement a more sophisticated algorithm, for example, saving the most visited images instead of caching the most recently used.

Related

Best practices for photo storage

I want to make an Android application that has the same look as Instagram, which means every profile has a gallery of photos.
My question is about storing and retrieve those photos. I want to know what is the best practice for photo storage in a database (MySQL), more precisely:
Should I store photos with original size (2mb for example) or resize them with smaller size?
Should I store them with two version (smaller size photo, and normal size photo)?
Saving images in the database looks like overkill, it's not needed. Saving on disk should be fine.
Although you should focus on your business logic & leave this to image libraries which does this stuff & do best for memory & performance.
Some famous libs are:
Picasso
Glide
Universal image loader
Fresco
& you will find many more
Almost each library does lots of stuff for you, such as:
In memory cache(LRU Cache) - so that UI thread is not blocked from doing I/O
operation from disks
Disk Cache - To keep images saved for further
app sessions, also when memory cache reaches its limit its have to be
evicted from it., you can specify disk cache size.
Various image scaling strategies
Image transformations
Clearing up bitmaps for freeing up memory
Network operations on background threads
Background threads management & so on..
My suggestion is to go with one of the image library
I dont think the best practice is to store the photos in a database.
In the DB you store the link/location of the photos example a url to a photo.
About smaller size photo it depends on what you are trying to accomplish because now you need more space to pay for and maybe less bandwidth because you can put thumbnails and let it only load the original size when a user clicks on it. So it all depends what and how you want to archieve
Best practice for storing images is to simply cache them as files and not save them in a database. You could save them as blobs in an sql db but they should be less than 32px by 32px to make things smooth. Everything else should be saved as files. Let me give you an example of WhatsApp.
WhatApp caches most of it's images as files and only saves the pixelated thumbnail in a database.

In Glide 3.x, Is it possible to use different cache directories for different images?

My overall object is to delete certain type of images (which are temporary) after a certain period like after one month, from disk cache of the my app.
I'm using Glide, so I know that using Glide, I can clear entire disk Cache but my goal is to clear only a certain type of images from disk cache. Like for example, live banner images from my app (which keeps changing after certain period). Problem is, they keeps adding space even though one gets inactive. So ideally I should delete all old banner images from cache.
What I'm thinking is have a separate Glide Module just for these banner images and setup a different internal cache directory for these images, and after a month, ask Glide to delete/clear this directory.
Another thing is to use Glide's signature concept which invalidates the images.
For both of above ideas, my problem and question is that I want to use only for certain type of images but more I read about them, it feels like once I use them, they apply to all images being download using Glide
Even if I use multiple Glide module, they all gets used for all the images (that's why there is a concept of conflict management for glide modules)
I might be wrong, and there might be indeed a way to get a solution to my problem. All I need is your help if you know a way :)
I'm aware that if nothing works out for my solution, then to keep my app's size impact low, I'll have to decreased overall internal disk cache size which is 250MB by default.
Thanks.

Thumbnail creation. On the fly or create small image?

The question regarding thumbnail creation seems to get asked a great deal, and each time there seems to be a great many different solutions proposed. From my experience none of the supplied answers are actually sufficient and all seem to have their own flaws.
a) Memory issues when re-sizing
b) Performance issues / slow to render
I therefore wanted to post a question regarding a very common functional requirement that I am hoping someone can assist with.
'User generates a report using activity and takes associated photo using internal camera. The report along with the image path is saved to SQLLite database. User wants to view reports in a scrollable list view at some point in the future. Number of reports could be 1..100'
So given these requirements the questions I have are as follows
1) Given that to data I have not found a reliable / performent / memory optimised way of resizing full size images at run time to a given thumbnail, should I create a smaller image at image capture time?
2) If you need to display many items in a ListView should you use paging or should it be possible to display a 100 items in one ListView?
Thanks
Regarding number 2: What you could do is load a subset first (say the first 30, depending on your listview item size), and load the rest asynchronously and append them to your adapter. This will decrease the loading time of your screen.
1/ Why do you want to reinvent the wheel :) There are libraries that already do for you and do it well. See picasso or AQuery for instance (there are other libraries too).
Basically, they have a thumbnail cache somewhere on the SDcard/internal memory to keep last N requested thumbnails.
I don't know if you are keeping the image data within the database, in which case you may have to adapt one of those libraries to load the image data from the DB and not from a file as it is usually the case.
2/ Why do you want to reinvent the wheel :) There are libraries that already do for you and do it well. See https://github.com/commonsguy/cwac-endless for instance (and google for more)

Android Photo / Thumbnail Strategy using DiskLRUCache

I'm using Jake Wharton's DiskLruCache lib.
I'm curious about app performance, caching strategy, using caching both in a view and across a whole application. Most of the time, the image won't change.
Let's say for example I have a 320x320 photo on my server. I open the stream, save the image.
In my list views, I show bitmaps and in the detail, I show a larger image. Should I save a thumbnail bitmap too? Is that more efficient?
What is your experience with sharing the cache "object" across the entire app (let's say I have multiple views that might leverage the same data. What are the issues with this?
For the sake of performance and currency, what if the image changes on the server. What's the best strategy to know that it's changed? I don't have access to modified date. Only size and yet, I don't really want to query size every time either. Set a flag in the app on the server and then query the flag?
In a traditional application (if there is such a thing), what's the best practice for clearing the cache from time to time? (indent weirded out.)
(I was inspired to write this after seeing all of the performance improvements by Facebook in iOS. I don't have billions to do caching but I would like to at least be smart about it! LOL)
A lot of these answers depend on the type of app you're writing, how important image updates are (and how likely images will change, etc), and total images produced. Aside from disk caching, you should be using memory caching as well, especially in ListViews and other areas where the same images will be scrolled through repeatedly. Look at LruCache and read the Caching Bitmaps entry from Google.
320x320 is probably too big for a listview, you will likely want to create thumbnails (depending on device, and how you're implementing your listviews).
1) You should be using Disk caching fairly aggressively (how you define that is up to the app you're writing). Use the external storage directory, and if they have a few GB left over, it's not an issue if you take 100 mb for your app for instance. It can all be cleared away if it's ever needed anyway.
2) There shouldn't be an issue. Disk IO (even to a flash medium) should never be handled on the main thread. Use AsyncTasks to load the images. There can only be one main foreground activity at once anyway, and while an activity is sleeping, it shouldn't be trying to read from the disk anyway.
3) Again this depends on how you're implementing your app. You should be able to get additional information when retrieving the file (even Apache can tell your app the last modified date).
3.1) You can have a sqllite db that keeps track of how often certain images are used, and most recent read. If the most recent read is a few days old, let that image expire.
Edit: There's a library from Jake Wharton now called Picasso that I would recommend using, which handles the Network/local IO as well as memory and disk caching. Check it ou here: http://square.github.io/picasso/ . A lot of what you will need to do can be accomplished with one line: Picasso.with(this).load(imageFileURL).into(imageView);

removing image data from memory

I have to believe there's a way to clear image data from memory once it's no longer required, but despite exhaustive searching I can't find a solution. Both stack and the google android dev list are full of questions regarding OOM errors, specifically "bitmap size exceeds VM budget", but I still don't see a clear answer.
I understand there are hard memory limits on devices, and I understand it's not realistic to load up and display or cache large amounts of image data, but there should be away to discard data that's no longer required.
For example, imagine this very basic hypothetical app, that emulates a lot of the behavior of the native gallery app:
An image gallery that allows the user to peruse images from a remote server.
There might be any number of images on that server.
The app displays 1 image at a time, and allows a user to go back or forward 1 image at a time through button presses or swiping.
There'd be a maximum of 3 images rendered at any one time (so the user can see the one immediately to the left or right of the current image when swiping). All other image data should be discarded.
Images are loaded using URL.openStream and Drawable.createFromStream or BitmapFactory.decodeStream. Streams are closed appropriately.
Images are sized appropriately on the server before being fetched.
Loading happens in AsyncTasks. Tasks that are no longer needed (due to moving away from an image with an incomplete task) are cancelled. Any references in the AyncTask are WeaklyReferenced.
When any image is no longer required, it's "cleared" via:
getBackground().setCallback(null)
Listeners are set to null
setImageDrawable/Bitmap(null)
removeView
This simple construct, that takes into account all the suggest practices I'm aware of, will inevitably crash with an OOM error at some point. Using BitmapFactory.Options inSampleSize and inPreferredConfig will delay the inevitable, but not forever, and at the cost of image quality. In this example, I've used remote images, but the issue exists with images stored in /assets/ or in internal memory, etc.
My feeling is that if we can display X amount of image data at one point, and we take all steps to remove that image data from memory, we should be able to display that same amount of data later, without having to compensate for what has happened before.
With the sheer quantity of questions about this very issue, I'd hope to have a standard solution documented, but if there is one, I can't find it. I've seen answers posted by Romain Guy, who otherwise seems very generous with his knowledge and active in the community, that say something like "Simple. Don't use so much memory". OK. Tell me how.
I should also mention that System.gc does nothing to help this. I'm also aware of bitmap.recycle, but unless I'm mistaken this can't be used in this fashion.
Am I missing something fundamental? Is there a way to discard image data once it's no longer being used? What is missing from the above to create a simple photo gallery? Assuming the built-in gallery app uses the framework and not the NDK, I imagine there has to be a way...
TYIA.
/this question has also been posted on the android developer google group list.
Through my work with Prime I found a few tips, one of which you have not mentioned. When you decode your Bitmaps make sure to use the inPurgeable and inInputShareable flags in your BitmapFactory.Options. That will help a little bit but I would recommend you look at my implementation of image loading in Prime. I use it in all of my products without any memory issues. I have found that 95% of memory problems are from the incorrect usage of the Bitmap class.
There is a very detailed article about the use of bitmaps on the android developer website.
Did you look at it ?
It explains how to load, cache and display bitmaps efficently and how to get rid of this famous OutofMemoryError.
There is also a sample application from an image gallery.
I think that's what you're looking for.

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