expose getScreenImage

[brigand]

I'm not sure if this is correct, but it seems to work. I'm very new to C/C++ stuff.

On a 300ms interval at 2560x1440 the memory usage goes from 20mb to 250mb and back down again.

It might be significantly faster if I can reuse buffers. Also, it's returning a SlowBuffer in 0.10, and the docs advise against using SlowBuffer for large data.

# 150ms interval, measure `robot.getScreenImage(0, 0, 2560, 1440);` with process.hrtime
Execution time (hr): 0s 97.8387ms
Execution time (hr): 0s 70.431239ms
Execution time (hr): 0s 70.394325ms
Execution time (hr): 0s 79.024596ms
Execution time (hr): 0s 96.242392ms
Execution time (hr): 0s 78.415336ms
Execution time (hr): 0s 73.583015ms
Execution time (hr): 0s 73.15636ms
Execution time (hr): 0s 77.463437ms
Execution time (hr): 0s 75.860151ms
Execution time (hr): 0s 129.808632ms
Execution time (hr): 0s 125.236072ms
Execution time (hr): 0s 127.089746ms
Execution time (hr): 0s 97.068304ms
Execution time (hr): 0s 87.929368ms
Execution time (hr): 0s 130.232699ms
Execution time (hr): 0s 135.494717ms
Execution time (hr): 0s 101.097455ms
Execution time (hr): 0s 85.203419ms
Execution time (hr): 0s 86.463413ms
Execution time (hr): 0s 116.454051ms
Execution time (hr): 0s 72.302956ms
Execution time (hr): 0s 64.864825ms
Execution time (hr): 0s 66.089723ms
Execution time (hr): 0s 77.848504ms
Execution time (hr): 0s 70.759078ms
Execution time (hr): 0s 75.107431ms
Execution time (hr): 0s 68.109502ms
Execution time (hr): 0s 70.178252ms
Execution time (hr): 0s 65.27376ms
Execution time (hr): 0s 100.983102ms
Average execution time (hr): 2s 88.58079225806452ms

[octalmage]

Looks good but I've been waiting to add screen related code until we got the Windows build working. These functions add a level of complexity on Windows and I feel that the Windows builds are a priority.

I've got similar code here:

https://github.com/octalmage/robotjs/tree/bitmap

#13 related.

[brigand]

Cool, I should have checked the branches :-)

[octalmage]

No worries! I actually came here to approve this pull request but someone JUST made progress on the windows build like an hour ago.

Really appreciate the work!