x11_screenshotter.cc

#include <nan.h>
#include <node.h>
#include <X11/Xlib.h>
#include <X11/extensions/Xrandr.h>

using namespace v8;

void GetScreenshot(const Nan::FunctionCallbackInfo<Value>& args) {
    // Get both expected arguments as integers.
    Isolate* isolate = args.GetIsolate();
    if (args.Length() != 2) {
        isolate->ThrowException(Exception::Error(String::NewFromUtf8(isolate, "Wrong number of arguments").ToLocalChecked()));
        return;
    }
    Local<Value> firstArg = args[0];
    if (!firstArg->IsNumber()) {
        isolate->ThrowException(Exception::Error(String::NewFromUtf8(isolate, "Argument is not a number").ToLocalChecked()));
        return;
    }
    int boundsX = firstArg.As<Number>()->Value();
    Local<Value> secondArg = args[1];
    if (!firstArg->IsNumber()) {
        isolate->ThrowException(Exception::Error(String::NewFromUtf8(isolate, "Argument is not a number").ToLocalChecked()));
        return;
    }
    int boundsY = secondArg.As<Number>()->Value();

    // Get the display and its default screen. Note screens are not monitors!
    Display* display = XOpenDisplay(NULL);
    Screen* screen = DefaultScreenOfDisplay(display);
    if (!screen) {
        isolate->ThrowException(Exception::Error(String::NewFromUtf8(isolate, "Could not get default screen").ToLocalChecked()));
        return;
    }

    // Get the screens root window. This will be what we want to capture.
    Window win = XRootWindowOfScreen(screen);

    // Find a monitor that matches the top-left corner we were given.
    bool found = false;
    int monitorCount;
    XRRMonitorInfo monitor;
    XRRMonitorInfo* monitors = XRRGetMonitors(display, win, true, &monitorCount);
    for (int i = 0; i < monitorCount; i++) {
        // Get the monitor.
        monitor = monitors[i];

        // Handle checking the top left.
        if (monitor.x == boundsX && monitor.y == boundsY) {
            printf("matched monitor at x: %d, y: %d\n", monitor.x, monitor.y);
            found = true;
        } else {
            printf("scanned monitor at x: %d, y: %d - no match!\n", monitor.x, monitor.y);
        }

        // At the end of everything, break.
        if (found) {
            break;
        }
    }
    XFree(monitors);

    // If said monitor was not found, throw an error.
    if (!found) {
        isolate->ThrowException(Exception::Error(String::NewFromUtf8(isolate, "Could not find monitor").ToLocalChecked()));
        return;
    }

    // Get the image from the window.
    XImage* image = XGetImage(display, win, monitor.x, monitor.y, monitor.width, monitor.height, AllPlanes, ZPixmap);
    XCloseDisplay(display);
    if (image == NULL) {
        isolate->ThrowException(Exception::Error(String::NewFromUtf8(isolate, "Could not get image").ToLocalChecked()));
        return;
    }

    // Create a new allocation and turn it into RGBA.
    size_t size = image->width * image->height * 4;
    uint8_t* bytes = new uint8_t[size];
    for (int blockStart = 0; blockStart < size; blockStart += 4) {
        bytes[blockStart] = image->data[blockStart + 2];
        bytes[blockStart + 1] = image->data[blockStart + 1];
        bytes[blockStart + 2] = image->data[blockStart];
        bytes[blockStart + 3] = image->data[blockStart + 3];
    }

    // Create an array buffer from this new allocation and free the old image.
    Nan::MaybeLocal<Object> buf = Nan::NewBuffer((char*)bytes, size);
    XFree(image);

    // Set the buffer as the return value.
    args.GetReturnValue().Set(buf.ToLocalChecked());
}

void Initialize(Local<Object> exports) {
    Local<Context> context = exports->CreationContext();
    exports->Set(context,
        Nan::New("getScreenshotForBounds").ToLocalChecked(),
        Nan::New<FunctionTemplate>(GetScreenshot)
            ->GetFunction(context)
            .ToLocalChecked());
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Initialize)