Difference between EDID vs CGDisplayScreenSize

edid_physical_size.mm

#import <Cocoa/Cocoa.h>
#import <IOKit/graphics/IOGraphicsLib.h>

char* getEDID(CGDirectDisplayID displayId) {
    char* result = NULL;
    
    io_registry_entry_t displayport = CGDisplayIOServicePort(displayId);
    NSDictionary* dict = (NSDictionary*) IODisplayCreateInfoDictionary(displayport, kIODisplayOnlyPreferredName);
    
    NSData* edid = [dict objectForKey: [NSString stringWithUTF8String: kIODisplayEDIDKey]];
    if (edid) {
        result = (char*)[edid bytes];
    }
    
    return result;
}


int main() {
    for(NSScreen* screen in [NSScreen screens]) {
        NSDictionary *description = [screen deviceDescription];
        NSSize displayPixelSize = [[description objectForKey:NSDeviceSize] sizeValue];
        CGDirectDisplayID displayId = (CGDirectDisplayID) [[description objectForKey:@"NSScreenNumber"] unsignedIntValue];
        CGSize displayPhysicalSize = CGDisplayScreenSize(displayId);
        char* edid = getEDID(displayId);
        
        // See https://en.wikipedia.org/wiki/Extended_Display_Identification_Data
        // The image sizes are on 12bits so we have to recompose them
        int horizontalImgSize = (((edid[54 + 14] >> 4) & 0x0F) << 8) | ((unsigned char) edid[54 + 12]);
        int verticalImgSize = ((edid[54 + 14] & 0x0F) << 8) | ((unsigned char) edid[54 + 13]);
        
        NSLog(@"PPI using CGDisplayScreenSize : is %0.2fx%0.2f", (displayPixelSize.width / displayPhysicalSize.width) * 25.4f, (displayPixelSize.height / displayPhysicalSize.height) * 25.4f);
        NSLog(@"PPI using EDID : is %0.2fx%0.2f", (displayPixelSize.width / horizontalImgSize) * 25.4f, (displayPixelSize.height / verticalImgSize) * 25.4f);
    }
    return 0;
}