Fix JPEG thumbnail problems

Due to the fact that we run JS behaviors before we capture the
screenshot, the browser could be scrolled down in the page. When we
don't capture the full page, we may get a screenshot of the bottom part of
the page and not the top. To fix that we run `window.scroll(0, 0)`
before capturing the screenshot.

We change method `BrozzlerWorker.full_and_thumb_jpegs` to
`BrozzlerWorker.thumb_jpeg`. That's because we already get a JPEG now
from the browser after our changes at `Browser.screenshot`.

`thumb_jpeg` only returns a thumbnail now. There is no need to read PNG
and convert to JPEG. This means that screenshots will be a bit faster
now :)

brozzler/browser.py

@@ -514,6 +514,11 @@ class Browser:
             self.websock_thread.on_response = None
 
     def _try_screenshot(self, on_screenshot, full_page=False):
+        """The browser instance must be scrolled to the top of the page before
+        trying to get a screenshot.
+        """
+        self.send_to_chrome(method='Runtime.evaluate', suppress_logging=True,
+                            params={'expression': 'window.scroll(0,0)'})
         for i in range(3):
             try:
                 jpeg_bytes = self.screenshot(full_page)
@@ -616,9 +621,9 @@ class Browser:
                             deviceScaleFactor=deviceScaleFactor,
                             screenOrientation=screenOrientation)
                 )
-            capture_params = {'format': 'jpeg', quality: 95, 'clip': clip}
+            capture_params = {'format': 'jpeg', 'quality': 95, 'clip': clip}
         else:
-            capture_params = {'format': 'jpeg', quality: 95}
+            capture_params = {'format': 'jpeg', 'quality': 95}
         self.websock_thread.expect_result(self._command_id.peek())
         msg_id = self.send_to_chrome(method='Page.captureScreenshot',
                                      params=capture_params)

brozzler/worker.py

@@ -165,22 +165,16 @@ class BrozzlerWorker:
             raise brozzler.ProxyError(
                     'proxy error on WARCPROX_WRITE_RECORD %s' % url) from e
 
-    def full_and_thumb_jpegs(self, large_png):
+    def thumb_jpeg(self, full_jpeg):
-        # these screenshots never have any alpha (right?)
+        """Create JPEG thumbnail.
-        img = PIL.Image.open(io.BytesIO(large_png)).convert('RGB')
+        """
-
+        img = PIL.Image.open(io.BytesIO(full_jpeg))
-        out = io.BytesIO()
-        img.save(out, "jpeg", quality=95)
-        full_jpeg = out.getbuffer()
-
         thumb_width = 300
         thumb_height = (thumb_width / img.size[0]) * img.size[1]
         img.thumbnail((thumb_width, thumb_height))
         out = io.BytesIO()
         img.save(out, "jpeg", quality=95)
-        thumb_jpeg = out.getbuffer()
+        return out.getbuffer()
-
-        return full_jpeg, thumb_jpeg
 
     def brozzle_page(self, browser, site, page, on_screenshot=None,
                      on_request=None, enable_youtube_dl=True):
@@ -226,15 +220,14 @@ class BrozzlerWorker:
         return outlinks
 
     def _browse_page(self, browser, site, page, on_screenshot=None, on_request=None):
-        def _on_screenshot(screenshot_png):
+        def _on_screenshot(screenshot_jpeg):
             if on_screenshot:
-                on_screenshot(screenshot_png)
+                on_screenshot(screenshot_jpeg)
             if self._using_warcprox(site):
                 self.logger.info(
                         "sending WARCPROX_WRITE_RECORD request to %s with "
                         "screenshot for %s", self._proxy_for(site), page)
-                screenshot_jpeg, thumbnail_jpeg = self.full_and_thumb_jpegs(
+                thumbnail_jpeg = self.thumb_jpeg(screenshot_jpeg)
-                        screenshot_png)
                 self._warcprox_write_record(
                         warcprox_address=self._proxy_for(site),
                         url="screenshot:%s" % str(urlcanon.semantic(page.url)),