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ImageGrab.grab(bbox=...) expects four pixel coordinates in the order (left, top, right, bottom). It does not expect (x, y, width, height). Even a correctly ordered tuple can capture the wrong area when its values came from logical GUI units or DPI-scaled cursor coordinates rather than the pixel space used by the captured image.
The reliable fix is to check both the tuple’s meaning and its units. Then account for platform-specific scaling and monitor layout before adjusting coordinates.
What ImageGrab expects in bbox
Pillow’s ImageGrab.grab takes a screen snapshot. Its bbox argument is a four-value box, (left, upper, right, lower), expressed in pixel coordinates. The first pair identifies one corner; the second pair identifies the opposite corner. In particular, right and lower are absolute positions, not a width and height.
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left, top, width, height = 120, 80, 300, 200
bbox = (left, top, left + width, top + height)
image = ImageGrab.grab(bbox=bbox)For the example, the resulting box is (120, 80, 420, 280). A tuple can contain integers and still describe the wrong region if its values are in the wrong order or use the wrong coordinate units.
Check the box before capturing
Print the values at the call site and verify they are integer pixel positions in left/top/right/bottom order. The box should have positive width and height: right > left and bottom > top. An empty or reversed extent can result from treating width and height as endpoints.
print("bbox:", bbox)
left, top, right, bottom = bbox
assert right > left and bottom > top
image = ImageGrab.grab(bbox=bbox)
print("captured size:", image.size, "mode:", image.mode)Pillow documents that pixels inside the bounding box are returned as RGBA on macOS and RGB otherwise. That mode difference is normal; it does not by itself indicate a bad crop.
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Why coordinate variables can be numerically valid but wrong
Width and height were passed as endpoints
This is the most direct variable-related mistake. Many interfaces describe a rectangle as x, y, width and height, while Pillow’s box describes two corners. Convert explicitly at the boundary between those representations:
def xywh_to_bbox(x, y, width, height):
return (int(x), int(y), int(x + width), int(y + height))
bbox = xywh_to_bbox(x, y, width, height)
snapshot = ImageGrab.grab(bbox=bbox)This conversion only fixes tuple semantics. It does not convert between logical points and physical screenshot pixels.
The coordinate source uses logical units, not screenshot pixels
GUI toolkits, accessibility APIs, cursor APIs, and screen-selection overlays may report coordinates in a coordinate system that differs from the captured image. Display scaling can make logical coordinates and physical pixels differ. Feeding one system’s values directly to another can shift, shrink, enlarge, or displace the crop.
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Compare the coordinates with ImageGrab.grab().size to understand the image dimensions you are working with. This is a diagnostic, not a universal conversion formula: a desktop image size alone does not establish the unit convention of every coordinate API or every monitor.
full = ImageGrab.grab()
print("full capture size:", full.size)
print("coordinates:", x, y, width, height)
print("candidate bbox:", xywh_to_bbox(x, y, width, height))macOS Retina scaling
Pillow’s documentation says macOS Retina captures are 2× by default. If your variables are logical points while the target region is addressed in physical pixels, the values may need consistent scaling. Pillow issue reports also describe screencapture producing 144-DPI physical pixels while the caller used 72-DPI virtual coordinates, so the reported fix was to scale all four box values. That report is a useful illustration, not proof that every macOS coordinate source should always be multiplied by two.
Determine how the variables were obtained and what units they represent before changing them. A screen-capture selection interface can also provide coordinates that are not pixel coordinates; a report for issue #3293 describes correcting the capture by using the pixel coordinate system. Treat that as a warning to verify the source rather than assume all overlay coordinates behave alike.
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Windows DPI virtualization
A Pillow issue report describes win32api.GetCursorPos() returning values that were wrong under display scaling. The reported remedy was to make the process per-monitor DPI aware before obtaining cursor coordinates. If a cursor position is your input, check the process’s DPI-awareness setting before collecting it, not just before calling ImageGrab.grab. Otherwise the values may already have been reported in a virtualized coordinate space.
Secondary monitors and negative coordinates
On Windows, Pillow’s current documentation notes that the top-left position can be negative when all_screens=True. A monitor positioned left of or above the primary display naturally has negative desktop x or y coordinates. Do not clamp these values to zero merely because they are negative: doing so changes the target location.
For a target outside the primary monitor, use all_screens=True and preserve signed coordinates. Pillow’s implementation obtains a desktop image plus an origin offset and crops relative to that offset. A capture or calculation that assumes the primary display begins at the desktop’s universal origin can therefore misplace or omit a secondary display; older issue reports also describe black output in multi-monitor scenarios.
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from PIL import ImageGrab
# Use desktop coordinates, including negative values where applicable.
bbox = (left, top, right, bottom)
image = ImageGrab.grab(bbox=bbox, all_screens=True)Use this option when the target lies outside the primary monitor. It is not a substitute for verifying the coordinate origin and units.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A practical debugging sequence
- Record the context. Note the Pillow version, operating system, monitor arrangement, display scale, and the source of the variables: cursor API, GUI toolkit, accessibility interface, or selection overlay.
- Print and label the values. Confirm the tuple is
(left, top, right, bottom), not(x, y, width, height). Check that all four values are integers and that right exceeds left and bottom exceeds top. - Convert dimensions if needed. For
(x, y, width, height), pass(x, y, x + width, y + height). - Check units against the capture. Compare the box with
ImageGrab.grab().size, and establish whether the source values are physical pixels or logical coordinates. Do not apply a scale factor until that is known. - Check platform-specific behavior. On Retina macOS, verify point-versus-pixel scaling. On Windows, check per-monitor DPI awareness if using cursor coordinates, and keep negative desktop coordinates for monitors left of or above the primary screen.
- Test the relevant capture scope. For a Windows target on another monitor, try
all_screens=True. Pillow uses different platform paths: current source describes macOS region capture throughscreencapture -Rand Windows capture followed by an offset-aware crop.
Common symptoms and fixes
| Symptom | Likely cause | What to check |
|---|---|---|
| The crop is too small, too large, or starts at the wrong edge | width and height were passed as absolute right and bottom endpoints |
Convert (x, y, width, height) to (x, y, x + width, y + height). |
| The box is valid but consistently scaled or offset | Coordinate units differ from the screenshot’s pixel coordinates | Identify whether the values came from logical units, a cursor API, a toolkit, or an overlay; verify platform scaling. |
| A macOS Retina region does not line up | Logical points and physical pixels may be mixed | Verify the coordinate source’s units and apply a consistent conversion only when supported by that source. |
| Windows cursor-based coordinates are wrong at non-default scaling | Process DPI virtualization may affect reported positions | Make the process per-monitor DPI aware before obtaining the cursor coordinates. |
| A secondary-monitor crop is black, missing, or misplaced | Monitor origin or desktop extent may be handled as if the primary screen were the whole desktop | On Windows, use all_screens=True when appropriate and preserve negative coordinates. |
| The returned image mode differs across machines | Platform-specific output mode | macOS returns the bounded pixels as RGBA; other platforms return RGB according to Pillow’s documentation. Handle mode explicitly if downstream code requires a particular one. |
Choose a fix based on how the coordinates were collected
| Coordinate source or setup | First thing to verify | Relevant adjustment |
|---|---|---|
| Rectangle supplied as x, y, width, height | Tuple semantics | Compute right and bottom from the origin plus dimensions. |
| macOS Retina display | Logical points versus screenshot pixels | Scale consistently only after identifying the units; do not assume every source uses the same scale. |
| Windows cursor position under display scaling | Process DPI awareness before reading coordinates | Use per-monitor DPI awareness as described in the reported issue. |
| Windows multi-monitor desktop | Whether the target lies beyond the primary monitor and whether its origin is negative | Use all_screens=True for the relevant target and retain signed coordinates. |
| Screen-selection overlay or GUI toolkit | Whether its coordinates are physical pixels | Establish the overlay’s coordinate system before passing values to Pillow. |
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FAQ
Does a negative bbox coordinate always mean the tuple is invalid?
No. On Windows with all_screens=True, a negative desktop position can represent a monitor placed left of or above the primary display.
Should I always multiply macOS coordinates by two?
No. First identify whether the specific coordinate source reports logical points or physical pixels; the correct conversion depends on that source.
Is RGB output on macOS an error?
Not necessarily. Pillow documents RGBA for the bounded pixels on macOS and RGB otherwise, so output mode varies by platform.
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