TecRah

Perhaps the programmers in India know that it’s almost Chinese New Year, and they’ve added a festive bug to the iOS 18 2 system update.

The thing is, some users whose iPhones have been updated to iOS 18 2, viewed the picture in the photo album, took a screenshot, then viewed the screenshot and took another screenshot, and then took a few screenshots, and then the color of the picture would become more and more red.

I would call it a Chinese New Year limited for fruit fans. Flirting aside, Tony is actually quite curious as to what this bug is actually due to.

There’s a theory on the web that the iPhone uses the sRGB color gamut when viewing pictures in albums, but when taking a screenshot on the system, the picture is saved in P3 color gamut mode. The problem is that P3 has a larger color description range than sRGB, so the red that sRGB understands is not the same as the red that P3 understands.

For example, pure red in the sRGB gamut is labeled 255, 0, 0, and the R, G, and B values in parentheses are the coordinates of the color’s position in color space. If mapped correctly into the P3 color space, pure red would be labeled 234, 51, 35.

So far, the iPhone has made some kind of mistake in mapping the colors, but it just doesn’t work. Repeatedly taking screenshots is the same as jumping between two different color gamut ranges and adding this bug on top of each other, which leads to a bigger and bigger color deviation, and the photo ends up being red.

Of course, this is just a theory of the netizens, but Tony also briefly tested it.

I first found an image with the original color gamut of sRGB, and then found that if I took a screenshot with a full screen display, it would not turn red. But with a little system UI, such as the status bar and background interface, it will be screenshot into the P3 color gamut, and then screenshot will become more and more red, the probability is that the system interface of the P3 color gamut mapping problem.

Screenshot in full screen, still sRGB color gamut Screenshot with system UI exposed, saved as P3 color gamut.

It’s hard not to be reminded of the new feature Apple announced a while back, when it suddenly has this bug after not having any problems in the past.

Here’s the thing: Apple introduced a new HDR standard, Adaptive HDR, at the WWDC24 global developer conference. With this new standard, not only albums, info, quick views and previews will support HDR, but even screenshots and recorded files will be rendered and displayed in HDR.

So Apple’s screenshots use the P3 color gamut, which has a wider color gamut and more colors, but so far it seems the code isn’t tuned right. As expected, Apple wanted to show off its high standards, but in reality, it’s created a new bug.

However, the specific reason in the end is not such a thing, may also have to wait for Apple’s official conclusion, but brush to this iOS 18 out of the bug after the news, Tony head immediately thought of the year Android a thing.

Slightly older friends may still have the impression that in the era of the prevalence of the bar forum fighting, many emoticons every more save uploaded once, the screen will become a little bit muddy, and in the end, and even the color will be strangely green, electronic pulp is how it came about.

Why does the picture become blurry? On the surface, we all know the reason is that the user did not choose the original picture when uploading the photo, coupled with the platform in order to save storage space and network bandwidth, the picture is compressed.

But in reality, even if it is compressed, at most, the clarity is worse, and it should not turn green. So essentially, it’s still the same year that Android poked a big hole in the compression algorithm when making the conversion.

Specifically, most display devices, such as the cell phone display in our hands, are based on RGB color mode. However, in the process of image and video storage and transmission, there is a more abbreviated and less computationally intensive color mode called YUV, for those who are interested in this part, you can search for it by yourself, Tony here will not be long-winded.

Anyway, under normal circumstances, the algorithm for converting from RGB to YUV color mode is very mature. Even if there is a bit of image quality loss during the data conversion process, it’s not that bad.

But the interface for compressing images on the Android side, a library called Skia to be precise, has come up with a lazy solution to optimize the speed of operation

Standard JPEG images use at least 16 bits of precision when converting the color space, that is, 2 16 different brightness or chrominance levels, but Skia reduces it to 8 bits.

Just to show the contrast of the bit depths

Yes, it’s just a lazy way to reduce the precision of the algorithm. This leads to larger and larger color deviations.

Plus Android has some algorithmic problems in order to speed up the process. Generally speaking, most of the RGB values are converted to YUV values by complicated formulas with decimals, but Android chooses to round up the part after the decimal point directly. Note the rounding, which makes Tony, who is used to rounding, spit out a mouthful of blood.

The result is that the brightness value Y keeps getting smaller, the image keeps getting darker, the chroma value UV also gets smaller, and the color keeps shifting to green.

Then, after the continuous saving and uploading of images during the spread of Bitmap, the color space transformation algorithm repeatedly compressed the image, and the image became greener and greener. This bug was not fixed until Android 7 in mid-April 2016, and the bug has been passed down to this day.

I believe that through this example of coding error conversion in Android, you can already see that the color display on the computer is a very complex process, especially now there are several other commonly used different color gamuts.

For example, the standard sRGB, there are manufacturers to promote a wider color gamut of P3, or when you watch HDR video, to switch to the BT2020 color gamut and so on.

The subject of display is becoming more and more complex, and it’s not just Apple and Android that have had trouble with color space – in fact, Microsoft still has problems with it.

For example, in Windows and Chrome, which we use every day, there is a bug that has existed for two years and has not been fixed. When using HDR monitors, the screen often maps out a bleak white color display effect (left), which causes Tony to turn off the HDR option for daily use, even though he has purchased an HDR-enabled monitor.

I’ve mastered reproducing the bug by taking a screenshot of it.

Speaking of this, some of you may have questions, not always praised Apple’s color management solution to do well? Then these problems Apple also have?

In fact, you can test the screen display performance of the test site wide gamut, through the classic wide color gamut test page to verify the site link is tucked in the original article in the reading, can be a key to direct. If you’re using an Android phone or other device, you may only be able to see the darker W’s if you save the image to your system album, but not in your browser. This is not a problem with the iPhone.

The image on the website looks like this, but it’s not the compressed image below!

While Android phones and other devices have wide color gamut displays, they don’t have full third-party connectivity, so they’re often just awkwardly left to their own devices, beautifully displayed in their system albums. But in the Apple camp, there is no need to distinguish between the device interface and APP, the user can get a basically consistent display effect.

And until today, the vast majority of Android phones and Windows computers, in the view of the color space other than sRGB content, either need to study the complex configuration file, or need to install specialized software. It can be said that there is basically no color management to speak of.

Finally, back to the issue of iPhone screenshots turning red, based on the information obtained so far, Tony is basically convinced that it is the introduction of screenshot HDR that is causing the problem. Hopefully Apple will fix this bug soon, after all, Apple is already the furthest along in color management HDR of all the systems out there.

By Levi

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Editorial Sunshine

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