fix: get screen density of android devices

[hiking93]

Image compression is using screenSize.scale as the resize factor:

mobile-mcp/src/server.ts

Line 439 in 4c026a6

screenshot = image.resize(Math.floor(pngSize.width / screenSize.scale))

But scale is currently hard-coded to 1 for Android, so while the file size is reduced, the resolution of the image won't be scaled, causing #140.

[krismuhi]

thanks. we are gonna check this soon

[gmegidish]

@hiking93 This looks neat and is a very good patch. Only change I'll ask is that instead of throwing an exception (which is very unlikely to happen in our scenario, the device is working properly), let's assume the dpi is 160 so there's no scaling. It'll make the code simpler and remove an exception that needs to be tested. What do you think, should we make this change?

[gmegidish]

@hiking93 I just realized something. my emulated device has value of 560, which means 560/160=3.5

I just need to test this thoroughly to make sure taps on screenshot coordinates dont get broken (they're not stable regardless because of the way LLMs work). Need to make sure imagemagick supports scaling down by 3.5 factor.

[hiking93]

@hiking93 This looks neat and is a very good patch. Only change I'll ask is that instead of throwing an exception (which is very unlikely to happen in our scenario, the device is working properly), let's assume the dpi is 160 so there's no scaling. It'll make the code simpler and remove an exception that needs to be tested. What do you think, should we make this change?

Sure, I've updated the implementation: 8e4267c

[hiking93]

Need to make sure imagemagick supports scaling down by 3.5 factor.

I tried a Pixel 9 Pro Fold emulator:

• wm size: 2076x2152
• wm density: 390 (scale = 2.4375)

mobile_take_screenshot created a screenshot image with size 851 × 882, seems to be working well.

[hiking93]

I just need to test this thoroughly to make sure taps on screenshot coordinates don't get broken (they're not stable regardless because of the way LLMs work).

To my knowledge, mobile_get_screen_size, mobile_list_elements_on_screen, and mobile_click_on_screen_at_coordinates all work with raw pixels on the Android side (instead of “points” on iOS), so I think density doesn't affect them. May double-check on that, though.