Quality issues for animated images

[sergeiromanchuk]

Description

My code includes the setting of this limitation:

/// Limits scale down to 24MB.
SDImageCoderHelper.defaultScaleDownLimitBytes = 24 * 1_024 * 1_024

When I get an animated image data that weighs 1.8 MB, the quality is severely degraded, and if I saw correctly my final image was 0.5 MB. I tried to play around with setting the value and in the end I got an image of 7200 bytes with the 1 MB limit set.

This was strange to me, so I tried exploring the codebase and seemed to find the problem in this line: https://github.com/SDWebImage/SDWebImageWebPCoder/pull/75/files#diff-e9c046b25a4d1b5fd2e490c2ebc802f1a63ef0459e0ad74dd3647f72d9c68323R94

Am I misunderstanding the expected behavior? Because it looks like a bug to me. Since it is strange that by setting the limit to 24 MB I get image quality degradation and image size reduction when its image data size is only 1.77 MB.

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Also maybe you can tell me why when I try to download a heavy image and several small images, if the heavy image was added earlier, I don't get a response for the small images until the large one is downloaded? (It is important to note that I am considering the case of downloading an image from disk or over the network, only in these cases this happens).

[dreampiggy]

Image Data Size (JPEG/WebP/PNG)
!=
Image RAM (buffer) size (CGImage/UIImage/CIImage)

Image Data are "Compressed Format" of a vector, vector element is pixel (normally just Pixel8888, a 3/4 component uint8_t, or just think as a uint24_t/uint32_t)

The calculation is correct actually. Image Buffer Size on RAM is just

BytesPerPixel * PixelCont
= BytesPerPixel * PixelPerFrame * FrameCount
= BytesPerPixel * ImageWidth * ImageHeight * FrameCount
= BytesPerComponent * ComponentCount * ImageWidth * ImageHeight * FrameCount, Component == 4 if contains alpha, else 3 for RGB
= sizeof(PixelType) * ComponentCount * ImageWidth * ImageHeight * FrameCount, if you use RGBA8888, the BytesPerComponent is 8 (sizeof uint8_t)

[dreampiggy]

This scaleDownLimitBytes can be set for Per-Image-Request, not that global control. So you can limit a larger bytes for animated image

Pass SDWebImageContextImageScaleDownLimitBytes on loadImageWithURL or setImageWithURL API from the top-level

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Another addition of behavior is that we separate the limitBytes for static image and animated image, they can have different scale down limit bytes.

[dreampiggy]

⬆️ If you're interested in the feature above, leave comments here

When finished, I can submit a feature request to https://github.com/SDWebImage/SDWebImage and implement it