How to detect scale factor program way? CGDisplayCopyAllDisplayModes doesn't return all resolutions.

[freedbrt]

I created a new display, and I see more resolutions at MacOS Display settings tab than api call CGDisplayCopyAllDisplayModes returned to me. I understand it's related to scaled resolutions, but how to get scale factor of virtual display programmatically?

I use this code for testing

#import <Cocoa/Cocoa.h>
#import <CoreGraphics/CoreGraphics.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // Get an array of all connected screens
        NSArray<NSScreen *> *screens = [NSScreen screens];
        
        // Iterate through each screen
        for (NSScreen *screen in screens) {
            // Get the current screen's display ID
            uint32_t displayID = (uint32_t)[[screen deviceDescription][@"NSScreenNumber"] unsignedIntValue];
            
            // Get the current display mode
            CGDisplayModeRef currentMode = CGDisplayCopyDisplayMode(displayID);
            
            // Get the backing scale factor for this screen
            CGFloat backingScaleFactor = [screen backingScaleFactor];
            
            NSLog(@"Screen with display ID: %u (backingScaleFactor: %.2f)", displayID, backingScaleFactor);
            NSLog(@"Supported resolutions:");
            
            // Get all supported display modes
            CFArrayRef allModes = CGDisplayCopyAllDisplayModes(displayID, NULL);
            CFIndex count = CFArrayGetCount(allModes);
            
            // Iterate through all supported modes
            for (CFIndex i = 0; i < count; i++) {
                CGDisplayModeRef mode = (CGDisplayModeRef)CFArrayGetValueAtIndex(allModes, i);
                size_t width = CGDisplayModeGetWidth(mode);
                size_t height = CGDisplayModeGetHeight(mode);
                size_t pixelWidth = CGDisplayModeGetPixelWidth(mode);
                size_t pixelHeight = CGDisplayModeGetPixelHeight(mode);
                
                // Calculate the scale factor
                CGFloat scaleFactorWidth = (CGFloat)pixelWidth / (CGFloat)width;
                CGFloat scaleFactorHeight = (CGFloat)pixelHeight / (CGFloat)height;
                
                // Check if this mode is the current mode
                if (mode == currentMode) {
                    NSLog(@"* %zux%zu (pixel: %zux%zu) (scale: %.2fx%.2f) (current)", width, height, pixelWidth, pixelHeight, scaleFactorWidth, scaleFactorHeight);
                } else {
                    NSLog(@"  %zux%zu (pixel: %zux%zu) (scale: %.2fx%.2f)", width, height, pixelWidth, pixelHeight, scaleFactorWidth, scaleFactorHeight);
                }
            }
            
            // Release the display mode references
            CGDisplayModeRelease(currentMode);
            CFRelease(allModes);
            
            NSLog(@"\n");
        }
    }
    return 0;
}

clang -framework Cocoa -framework CoreGraphics -o ScreenResolutions ScreenResolutions.m
./ScreenResolutions

And I received:

Supported resolutions:
  1280x720 (pixel: 1280x720) (scale: 1.00x1.00)
  1312x738 (pixel: 1312x738) (scale: 1.00x1.00)
  1344x756 (pixel: 1344x756) (scale: 1.00x1.00)
  1376x774 (pixel: 1376x774) (scale: 1.00x1.00)
  1408x792 (pixel: 1408x792) (scale: 1.00x1.00)
  1440x810 (pixel: 1440x810) (scale: 1.00x1.00)
  1472x828 (pixel: 1472x828) (scale: 1.00x1.00)
  1504x846 (pixel: 1504x846) (scale: 1.00x1.00)
  1536x864 (pixel: 1536x864) (scale: 1.00x1.00)
  1568x882 (pixel: 1568x882) (scale: 1.00x1.00)
  1600x900 (pixel: 1600x900) (scale: 1.00x1.00)
  1632x918 (pixel: 1632x918) (scale: 1.00x1.00)
  1664x936 (pixel: 1664x936) (scale: 1.00x1.00)
  1696x954 (pixel: 1696x954) (scale: 1.00x1.00)
  1728x972 (pixel: 1728x972) (scale: 1.00x1.00)
  1760x990 (pixel: 1760x990) (scale: 1.00x1.00)
  1792x1008 (pixel: 1792x1008) (scale: 1.00x1.00)
  1824x1026 (pixel: 1824x1026) (scale: 1.00x1.00)
  1856x1044 (pixel: 1856x1044) (scale: 1.00x1.00)
  1888x1062 (pixel: 1888x1062) (scale: 1.00x1.00)
  1920x1080 (pixel: 1920x1080) (scale: 1.00x1.00)
  1952x1098 (pixel: 1952x1098) (scale: 1.00x1.00)
  1984x1116 (pixel: 1984x1116) (scale: 1.00x1.00)
  2016x1134 (pixel: 2016x1134) (scale: 1.00x1.00)
  2048x1152 (pixel: 2048x1152) (scale: 1.00x1.00)
  2080x1170 (pixel: 2080x1170) (scale: 1.00x1.00)
  2112x1188 (pixel: 2112x1188) (scale: 1.00x1.00)
  2144x1206 (pixel: 2144x1206) (scale: 1.00x1.00)
  2176x1224 (pixel: 2176x1224) (scale: 1.00x1.00)
  2208x1242 (pixel: 2208x1242) (scale: 1.00x1.00)
  2240x1260 (pixel: 2240x1260) (scale: 1.00x1.00)
  2272x1278 (pixel: 2272x1278) (scale: 1.00x1.00)
  2304x1296 (pixel: 2304x1296) (scale: 1.00x1.00)
  2336x1314 (pixel: 2336x1314) (scale: 1.00x1.00)
  2368x1332 (pixel: 2368x1332) (scale: 1.00x1.00)
  2400x1350 (pixel: 2400x1350) (scale: 1.00x1.00)
  2432x1368 (pixel: 2432x1368) (scale: 1.00x1.00)
  2464x1386 (pixel: 2464x1386) (scale: 1.00x1.00)
  2496x1404 (pixel: 2496x1404) (scale: 1.00x1.00)
  2528x1422 (pixel: 2528x1422) (scale: 1.00x1.00)
  2560x1440 (pixel: 2560x1440) (scale: 1.00x1.00)

But in the MacOS settings I see more

In the BetterDisplay I see Resolution step multiplier 2.0, but how to get it? Can you give me any suggestion how to solve it?

[waydabber]

If you mean the backingScaleFactor, that is always fixed at 2.0 for HiDPI resolutions and 1.0 for LoDPI resolutions in current macOS versions. For virtual screen framebuffer to real pixel scaling factor is meaningless as there is no real display so there is no additional scaling, just the framebuffer.