iOS - View high resolution image

Question

I want to display a high resolution image in imageview. Here is what i do

Prepare UIImage in the background
When the file is loaded, switch to main thread
Display the image [ by using UIImageView's setImage: ]

The problem is there is a lag in the 3rd step. It takes few seconds and loads the file. And if its a large image, I get the memory warning and a crash. So is there a way by which i can draw images from top to bottom ( like how the browser does) ? Also I need to preserve the quality of the image.

Is there a way by which i can achieve my requirement. Thanks in advance

Scaling causes a degradation in the quality. I need to support zooming too — Dhilip, Apr 25 '13 at 07:15

score 1 · Answer 1 · edited May 23 '17 at 11:43

UIImage will not decode its underlying image data until it is actually requested, which will happen on the main thread when you assign it to an image view. So although you are trying to load the image in a background thread, the work is being delayed and actually occurs on the main thread.

The workarounds to this issue are pretty hacky and usually involve writing the image to a new graphics context to force the decoding, and then creating a CGImageRef or UIImage from that. This all takes place on the background thread. By the time you ship the new image to the main thread, all the decoding has already taken place and you shouldn't see a delay. This question has some answers which demonstrate this technique.

Drawing images from top-to-bottom is not possible with the standard APIs provided by Apple (as far as I know). You would have to write your own streaming image decoder which would be a significant task.

Thanks Mike.. i am looking into it – Dhilip Apr 25 '13 at 07:17 — Dhilip, Apr 25 '13 at 07:17

score 1 · Answer 2 · answered Apr 23 '13 at 06:58

1

You can use SDWebImage framework which helps you for :

1.An asynchronous image downloader

2.Automatic image caching .

3.Avoid duplications .

You can download this from:https://github.com/rs/SDWebImage

About the memory warning it is not only due to your application its due to effective memory use from all the application running on the background so try to stop the background running applications.

answered Apr 23 '13 at 06:58

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Thanks for the suggestion.SDWebImage does some compression and decompression where i lose the quality. It would be rude to request the user to kill background apps – Dhilip Apr 25 '13 at 07:16

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

And if its a large image, I get the memory warning and a crash.

it seems you are hitting some "hard" (memory related) limit of your device trying to load and display the image into memory at once.

So is there a way by which i can draw images from top to bottom ( like how the browser does)?

You should try with CATiledLayer, which provides a way to draw very large images without incurring a memory hit.

Here you can find a tutorial about it.

You could also give a look at the PhotoScrollerNetwork project:

blazingly fast tile rendering - visually much much faster than Apple's code (which uses png files in the file system)

you supply a single jpeg file or URL and this code does all the tiling for you, quickly and painlessly

Thanks sergio.. i am looking into it – Dhilip Apr 25 '13 at 07:14 — Dhilip, Apr 25 '13 at 07:14

iOS - View high resolution image

3 Answers3