Why SD 5.5 use Thumbnail Decoding for large image to replace Downsizing in SD 4.0

[KittenYang]

I noticed that SD 5.5 moved the Downsizing codes to [SDImageCoderHelper decodedAndScaledDownImageWithImage:image limitBytes:bytes]; and not used.
Instead，SD 5.5.0 use the Thumbnail Decoding https://github.com/SDWebImage/SDWebImage/wiki/Advanced-Usage#thumbnail-decoding-550

So I write a test, both decode a large image, the limited size is 60M which means the output image maximum size is small than (3960, 3960). Here is the result.

SD 4.x downsizing:

SD 5.5 Thumbnail Decoding

Notice the peak memory, the Thumbnail Decoding workaround is way more higher than Downsizing workaround.

So why use Thumbnail Decoding instead of Downsizing.Do I miss some context?

[dreampiggy]

@KittenYang Hi. can you provide your image url, or any demo ?

The image bitmap buffer memory size, should be only related to image resolution.

For 3960 x 3960, it MUST BE 3960*3960*4 bytes, which is 59.820556640625 MB.

If not, it's a bug, but not designed behavior. (The thumbnail decoding does not take effect at all ?)

[KittenYang]

@dreampiggy The Thumbnail Decoding does take effect. But the Peak Memory is still High. Look at the picture 2, the peak memory is 279.15M but picture 1 is only 52.69M. I uploaded a demo project you can test.

https://github.com/KittenYang/SDThumbnailDecodingPeakMemoryTest

Notice: please use Instruments - Allocations to monitor memory.

[KittenYang]

@dreampiggy Hi, have u tested it?