[ios, macos] Fix snapshot scale

[frederoni]

Fixes #10089
Fixes the scale of the snapshot on iOS by letting CGImage resize the output.
Snapshotting on macOS already works as intended.

@1ec5 👀
@fabian-guerra feel free to cherry-pick this into #10020

[fabian-guerra]

@frederoni I realized that the following code made the image on macOS be resized 4x

[compositedImage lockFocus];
                    [logoImage drawInRect:CGRectMake(MGLLogoImagePosition.x, MGLLogoImagePosition.y, logoImage.size.width,logoImage.size.height)];
                    [compositedImage unlockFocus];

I'm changing that.

@1ec5 @frederoni thinking about #10089 makes me realize that if we don't keep the scale and we make 456×480 be 1:1 pixel ratio that would be misleading developers since that would be a scale back for retina screens and images won't look good. Consider the scenario where someone takes a screenshot on an iPhone 7 (375×667) then they present the screenshot in the next controller (full screen) someone would expect a 750×1334 size image that will look good, instead we will return the 375×667 version and the OS will have to scale up resulting on a quality loss.

[frederoni]

I realized that the following code made the image on macOS be resized 4x

[compositedImage lockFocus];
[logoImage drawInRect:CGRectMake(MGLLogoImagePosition.x, MGLLogoImagePosition.y, logoImage.size.width,logoImage.size.height)];
[compositedImage unlockFocus];

I don't get how that could result in 4x but I assume

_scale = [NSScreen mainScreen].backingScaleFactor;

mainScreen could be an issue if you have multiple screens.

Consider the scenario where someone takes a screenshot on an iPhone 7 (375×667) then they present the screenshot in the next controller (full screen) someone would expect a 750×1334 size image that will look good, instead we will return the 375×667 version and the OS will have to scale up resulting on a quality loss.

The UIImage should remain at 375×667 but exporting the image should result in size×scale. Which happens correctly in this branch. As you can see below, there's no rendering/resizing artifact in either of the versions.

Snapshotter	Logical resolution	Resolution	img
MGLMapSnapshotter (this branch)	375 x 667	750 x 1334
OS Snapshot	375 x 667	750 x 1334

[fabian-guerra]

@frederoni thank you for clarifying this. That's what I meant the resulting image should be 750 x 1334. @1ec5 is this what you mean?

This is what is get in my current branch could you please test if still happens in your branch?:

--->[compositedImage lockFocus];
(lldb) po compositedImage
<NSImage 0x60800047f580 Size={2052, 1434} Reps=(
    "<NSCGImageSnapshotRep:0x608000272b00 cgImage=<CGImage 0x6080001c3660>\n\t<<CGColorSpace 0x618000025e60> (kCGColorSpaceDeviceRGB)>\n\t\twidth = 2052, height = 1434, bpc = 8, bpp = 32, row bytes = 8208 \n\t\tkCGImageAlphaPremultipliedLast | 0 (default byte order) \n\t\tis mask? No, has mask? No, has matte? No, should interpolate? No>"
)>

--->[compositedImage unlockFocus];
(lldb) po compositedImage
<NSImage 0x60800047f580 Size={2052, 1434} Reps=(
    "<NSCGImageSnapshotRep:0x60800006d140 cgImage=<CGImage 0x6080001c2ee0>\n\t<<CGColorSpace 0x618000025ea0> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; Color LCD)>\n\t\twidth = 4104, height = 2868, bpc = 8, bpp = 32, row bytes = 16448 \n\t\tkCGImageAlphaPremultipliedFirst | kCGImageByteOrder32Little \n\t\tis mask? No, has mask? No, has matte? No, should interpolate? Yes>"
)>

[fabian-guerra]

@frederoni I can confirm that the 4x scaling on macOS happens with this branch too.

When I comment this code:

[compositedImage lockFocus];
[logoImage drawInRect:CGRectMake(MGLLogoImagePosition.x, MGLLogoImagePosition.y, logoImage.size.width,logoImage.size.height)];
[compositedImage unlockFocus];

I get an image sized 2052 × 1434

Instead of 4104 × 2868

(Download and compare please)

[frederoni]

Yep, this change only fixes the iOS version.

[fabian-guerra]

@frederoni I fixed the issue https://github.com/mapbox/mapbox-gl-native/pull/10020/files#diff-56bc5c1e41273957efd873aa83e6c522R159 I had to do something similar to what you did for iOS.

[1ec5]

65936d7 fixes the macOS implementation to produce a correctly sized image.

[1ec5]

The map isn’t necessarily opaque. As in #7859, if the background style layer has a non-opaque fill color, an alpha channel may be needed.

[fabian-guerra]

I created this ticket to follow up the tail work.