>>1513231Not quite. They do some extra stuff like convert from baseline to progressive JPEG.
Also, with PNG, the pixels where alpha=0 are altered to reduce file size.
However, every transformation done is effectively lossless with respect to the underlying raw pixel data.
For PNG, altering the fully transparent pixels doesn't matter because they're invisible anyway.
Some people think it's a lossy recompression because of changes in hash and file size.
Another thing is that that stripping the metadata also removes any embedded ICC profiles, often resulting in less saturated colors.
And yet, the pixels are still very much the same.
They're just being wrongly interpreted as sRGB instead of e.g. Display P3 or Adobe RGB.
For example, a fully satured red in sRGB is #FF0000.
When an image is saved with Display P3, that red is stored as #EA3323.
Because that ICC profile has a wider gamut, it doesn't need as much saturation to represent the same color.
When that image is displayed by a program that is aware of ICC profiles, it gets reconverted to #FF0000 for a display that expect sRGB (as most do).
When the profile gets stripped, that conversion can't happen, so it's assumed to be sRGB, and the red shows up as #EA3323.
That might seem like the pixel changed, but really, it's the same raw value that was stored to begin with.
