I have an application that allows the user to load and display images. In the QML front end I am using a fileDialog to get the file URLs which are then loaded using an image loader in C++.
The issue I have at the moment is that if you want to load multiple images you have to load them all at once because once the fileURLs changes, the old URLs are lost.
What is the best way to go about storing the URLs so that I can keep the images displayed?
solution 1:
C++ side, Use a QStringList or a QVector to store paths and load Images from it. Qml side, each time you get new URL push it back to your QStringList.
solution 2:
Declare a qml array (property var array:[]) and store in it the new urls, load images from this list.
Related
I would like an easy approach to preload/cache all of my static image assets so that they can be rendered/served without a delay.
I've seen that there is a precacheImage() call that can be used to pre-load/cache the AssetImage. This needs a context and it is recommended to call this in the didChangeDependencies() override.
Shouldn't there be a way to make this easier and more general? My app uses a total of 1.5 MB of image data (and I've included 2.0x and 3.0x upscaled versions in that number). PNG images that are 50 KB (and no upscaled versions) takes a noticeable amount of time to display, maybe 300-600ms on both emulator and fast devices. These are local assets, not fetched over the network. I find that irritating and I'm frustrated that there isn't a better way to handle this?
I've also seen the tip to use FadeInImage but again - it's not really what I'm looking for.
I'm displaying the image in a stateless widget (a custom button). It's not possible to use precacheImage in a stateless widget afaik. So I'd need to build the Image.asset() in my parent widget, call precacheImage and then pass the image widget to my stateless widget and render it in build - this is cumbersome.
Furthermore, the images will be displayed in different places (different parent widgets). Sometimes the image widgets differ in size between widgets and since size is parameters to Image.asset() I guess I would need to precache each unique size and pass these precached image widgets around. Isn't it possible to tell Flutter to "cache" the data of the PNG so that when the Image.asset is requested it reads the PNG from cache - without having to pass around precached image widgets?
I would like a "precacheAllImageAssets()call or callprecacheImage()` with a string so that each Image.asset() that references that same asset would be cached.
I guess that Flutter internally caches the image widget (including it's size and other properties) as some internal render object that is cached. Thus pre-caching two different sizes of the same image would require two different caches. With that being said - I'd still want a precacheAllImageAssets() call that could at least read the PNG data into memory and just serve it quicker even if would need to do some processing to get the PNG data to an actual widget with a size before it could be rendered. With such a cache I could maybe get a render delay of < 50 ms instead of the current 300-600 ms.
Any idea if this is possible? If not possible - am I missing something obvious or could this be a (likely) future improvement of the Flutter framework?
Here is my similar precacheAllImageAssets(), but you need to list all image path by yourself.
final List _allAsset = [
///tabbar
'images/tabbar/tabar_personal.png',
'images/tabbar/tabar_personal_slt.png',
'images/tabbar/tabar_home.png',
'images/tabbar/tabar_home_slt.png',
'images/...'
'images/...'
}
void main() {
final binding = WidgetsFlutterBinding.ensureInitialized();
binding.addPostFrameCallback((_) async {
BuildContext context = binding.renderViewElement;
if(context != null)
{
for(var asset in _allAsset)
{
precacheImage(AssetImage(asset), context);
}
}
});
}
UPDATE: after some research I've figured out that previous version of my answer was not correct. Here is relevant one (you can see old one in edit history).
You do not need to use same ImageProvider for images to precache. So you can run precacheImage() at init time and then create another image with same path and it is gonna be obtained from cache (if it was not cleared one way on another).
Internally precacheImage() uses ImageProvider.obtainKey() which is pair of (imagePath, scale) as a key to store image in in-memory cache. So as long as you are using same key it does not matter which instance of ImageProvider/Image you are using.
For futher insights you can inspect ImageCache documentation. Specifically, take a look at putIfAbsent() method as it's main caching endpoint. To understand how images generate their key (at which they are stored in ImageCache) try to start with ImageProvider class and then look into it's implementations.
I have a set of images (url addresses). Some of them are of acceptable size, but some are too big. When I load it, the app crashes because of big images.
Is there a way to get the image size (megabytes) before loading it? This way I can remove large images from the list, and avoid loading them.
If the server where the images are located supports it, you can issue a HEAD request. This can be done in Qt using e.g. QNetworkAccessManager::head.
It should also be possible in QML/Javascript, see HTTP HEAD Request in Javascript/Ajax?.
My site have 2 pages and the 2 pages contain a similar picture. If a user comes to the first page, he downloads the picture and then come to the second page, if I make the website so that the picture is shared between the 2 pages then the user no need to download the picture again?
If I want to put the same picture but different in size on the webpage, is it better to make 2 pictures by using image software editor or using CSS to change the width and the height of the picture?
you have answered your question.
if the image is from the same source, and if you have configured the caching on webserver correctly (and if the client has enabled cache), then there are no re-requests sent to view the same resource.
you dont need to create multiple images for different sizes, use html image attributes to show it in the grid dimensions you wish to.
Exception: if the original image is quite large, and you are not sure if the user will want to view the image, then create smaller image for faster loading. Thumbnails on a photo album is a good example for that. There is a program called re-sizer which accepts a folder and create a new set of images with the required dimensions
Resources below
Image Resizer
HTTP Compression
I have a web application running on local host. The requirement is to load multiple rectangular jpg images (96 images, average 7k in size each) and show on home page when it runs. Images are showed in a grid of 8x12 rows/columns. I am loading image by setting the 'src' attribute of the 'img' in javascript. The url assigned to the 'src' attribute is same for all images but the image id is different. Images are loading but the main issue is that they are not loading very quickly and they are some what loading in a sequence means 1,2,3,4... and so on but some images are not loaded in sequence. I want to improve the performance of this page. I have tried the figure out the timings at different points like:
When call is originated form client (image src attribute is set)
When server is receiving call. (the page on server which serve individual image)
When server is about to return the image.
When on client side image is received/showed (image loaded event called in javascript)
It turned out after looking at the collected data that main time is lost between 1 and 2 above that is between the client side call is originated and server is receiving call for a particular image.
I have also tried setting parameters like maxWorkerThreads, minWorkerThreads, requestQueueLimit and maxconnection in machine.config but no significant improvement yet.
Can someone please help me in this situation as i am stuck here since many days and i am really short of time now. Desperately needs to improve the performance of these images loads.
Thanks in advance.
Since you stress the lack of time, I would advise you to try a jQuery plugin that loads images on demand. The gallery in which you load the pictures, does the user scroll in it? Or is it 1 big field?
If it's a gallery the user can scroll in, I'd strongly suggest you take a look at lazyload. What this plugin does, is it fetches the image only when it is needed. As long as it's not on the screen, it will not be loaded. For a brief example, have a look at this demo.
I hope this can help you out.
I want to load some images into a ListBox. These images are inside XAP.
Someone tell me that I can use ResourceManager, but I don't know how to get a list for all images inside a folder like "/Assets/Images/".
These images are added as Content.
Any advice?
If images are set to be content, then you can access them by their relative URLs. So, for example, in code-behind you could do something like this:
BitmapImage image = new BitmapImage(new Uri("/Assets/Images/image1.png",UriKind.Relative));
ImageControl.Source = image;
Same way you can reference them in XAML. However, on Windows Phone you cannot directly list them. You could have an XML file, for example, that keeps the names of existing images, and then read it and read images recursively.
You can't list the content resources - see List content files in a windows phone 7 app?. The workaround is to hardcode the list in your program.
To load the image see the examples under Application.GetResourceStream