SDWebImageScaleDownLargeImages may turn a big picture into a very small picture which is not what we expect

[ericdream]

New Issue Checklist

• [x] I have read and understood the CONTRIBUTING guide
• [x] I have read the Documentation
• [x] I have searched for a similar issue in the project and found none

Issue Info

Info | Value |
-------------------------|-------------------------------------|
Platform Name | ios
Platform Version | all
SDWebImage Version | The latest version
Integration Method | cocoapods
Xcode Version | Xcode 11
Repro rate | all the time
Repro with our demo prj | e.g. does it happen with our demo project?
Demo project link | e.g. link to a demo project that highlights the issue

Issue Description and Steps

When I use SDWebImageScaleDownLargeImages， this property turn a big picture into a very small picture which is not what we expect，
so I found the code:
thumbnailOptions[(__bridge NSString *)kCGImageSourceCreateThumbnailFromImageIfAbsent] = @(YES);
KCGImageSourceCreateThumbnailFromImageIfAbsent I think is the key problem，
Specifies whether a thumbnail should be automatically created for an image if a thumbnail isn't present in the image source file.

So I think we should replace KCGImageSourceCreateThumbnailFromImageIfAbsent with kCGImageSourceCreateThumbnailFromImageAlways。thanks!

[dreampiggy]

See wiki: https://github.com/SDWebImage/SDWebImage/wiki/Advanced-Usage#thumbnail-decoding-550

SDWebImageScaleDownLargeImages from v5.5.0, will translate to .imageThumbnailPixelSize. Which will use thumbnail decoding to create a smaller size.

By default, it should produce a image with 3966 x 3966 pixels, unless you modify the SDImageCoderHelper.defaultScaleDownLimitBytes

[dreampiggy]

Or, maybe your image URL is a HEIC which contains a embed thumbnail image ? (Note only HEIF image format have this feature).

This may cause that Image/IO return back the embed thumbnail image, not what your set pixel size.

[dreampiggy]

So I think we should replace KCGImageSourceCreateThumbnailFromImageIfAbsent with kCGImageSourceCreateThumbnailFromImageAlways。thanks!

Maybe you didn't read the source code carefully. The kCGImageSourceCreateThumbnailWithTransform already get applied. See: https://github.com/SDWebImage/SDWebImage/blob/5.8.0/SDWebImage/Core/SDImageIOAnimatedCoder.m#L204

[ericdream]

Or, maybe your image URL is a HEIC which contains a embed thumbnail image ? (Note only HEIF image format have this feature).

This may cause that Image/IO return back the embed thumbnail image, not what your set pixel size.

Yes, that's the case,ti`s a embed thumbnail image,
So how do I solve this problem, I want to use SDWebImageScaleDownLargeImages to solve the big picture problem.

[dreampiggy]

Can you show the image URL, give a example so that I can debug with.

Maybe we can just add a backup logic: If the returned back CGImage from ImageIO, the size is much more smaller than the kCGImageSourceThumbnailMaxPixelSize we provided, then we use the full image for decoding

Actually, I think this is a bug of Apple. Because following the documentation:

https://developer.apple.com/documentation/imageio/kcgimagesourcecreatethumbnailfromimagealways?language=objc

Read it again, it should Produce the image from full image when the embed thumbnail image is smaller than the size we passed to the CGImageSourceCreateThumbnailAtIndex

[ericdream]

Can you show the image URL, give a example so that I can debug with.

Maybe we can just add a backup logic: If the returned back CGImage from ImageIO, the size is much more smaller than the kCGImageSourceThumbnailMaxPixelSize we provided, then we use the full image for decoding

Actually, I think this is a bug of Apple. Because following the documentation:

https://developer.apple.com/documentation/imageio/kcgimagesourcecreatethumbnailfromimagealways?language=objc

Read it again, it should Produce the image from full image when the embed thumbnail image is smaller than the size we passed to the CGImageSourceCreateThumbnailAtIndex

http://resource-fat.7kid.com/7kid/moment/img/20206/55e26e85-a962-4d98-a51f-107b55911b24_6.jpg

example:

```NSString *url = @"http://resource-fat.7kid.com/7kid/moment/img/20206/55e26e85-a962-4d98-a51f-107b55911b24_6.jpg";
UIImageView *imageView = [[UIImageView alloc] initWithFrame:self.view.bounds];
imageView.contentMode = UIViewContentModeScaleAspectFit;
[self.view addSubview:imageView];
[imageView sd_setImageWithURL:[NSURL URLWithString:url] placeholderImage:nil options:SDWebImageScaleDownLargeImages];

[dreampiggy]

@ericdream Released 5.8.1