Bit block transfer, usually written BitBlt or bitblt and pronounced “bit blit,” is a graphics operation that copies or combines pixel data in a rectangular region from one image surface to another. A simple blit copies pixels; a raster operation (ROP) can instead combine source and destination values using rules such as AND, OR, or XOR. Windows GDI’s BitBlt function is one specific implementation, not the definition of the operation everywhere.
How a bit block transfer works
A bitmap stores an image as pixel values. A bit block transfer selects a rectangular block of pixels on a source surface and transfers it to a destination. With a direct copy, the source pixels replace the corresponding destination pixels. With a raster operation, the destination result is calculated by applying a selected rule to source and destination data.
That makes “blit” broader than simply copying bytes: the operation concerns pixel data and can apply a Boolean combination as it transfers. The available rules and details depend on the graphics API and device.
What BitBlt means in Windows GDI
In Win32 GDI, Microsoft’s BitBlt function transfers color data for a rectangle of pixels from a source device context (DC) to a destination DC. The call specifies the destination location and dimensions, the source DC and coordinates, and a raster-operation code. Coordinates are logical coordinates for the relevant DCs.
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The ROP determines how source and destination pixels contribute to the result. Examples in the Windows API include:
SRCCOPY: copy the source pixels to the destination.SRCAND: combine source and destination with a bitwise AND operation.SRCINVERT: combine them with XOR.SRCPAINT: combine them with OR.
These names and behaviors describe Win32 GDI’s ROP codes; other graphics systems may expose different operations or interfaces.
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Win32 BitBlt limitations and behavior
Microsoft’s documentation describes several constraints specific to the GDI function. They matter when a transfer behaves differently from a straightforward same-device copy:
- Clipping:
BitBltclips on the destination DC, not the source DC. - Color formats: If source and destination formats differ, the function may convert the source color format to match the destination. It does not perform color management.
- Different devices: The function fails if the source and destination DCs represent different devices. Microsoft recommends converting a memory bitmap to a device-independent bitmap (DIB) and using DIB APIs to display it in that situation.
- Device support: Not all devices support
BitBlt; support depends on the device and driver.
These are not universal rules for every bit-block transfer implementation. Windows also has a driver-level DrvBitBlt interface for transfers involving device-managed and GDI-managed surfaces, illustrating that the work may be handled at different implementation layers.
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How BitBlt differs from related operations
Related graphics operations are distinguished by what they do to the destination region, not just by their names. Microsoft lists MaskBlt, PlgBlt, and StretchBlt among related GDI functions.
| Operation | What changes |
|---|---|
BitBlt |
Transfers a rectangular pixel region; the ROP controls how source and destination data combine. |
PlgBlt |
Maps a source rectangle to a destination parallelogram and can optionally use a monochrome bitmap as a mask, according to Microsoft’s documentation. |
StretchBlt and MaskBlt |
Listed as related functions in the Win32 BitBlt reference; consult the relevant API documentation for their exact behavior and device support. |
Where the term came from
The historical account in Leptonica’s raster-operation documentation, dated November 23, 2006, distinguishes the PDP-10’s earlier BLT instruction—which transferred a contiguous block of memory—from a later two-dimensional raster operation for display buffers. It places an early version of that raster operation at Xerox PARC in the early 1970s, associated with Dan Ingalls and the Alto, and says Xerox called it “Bitblt,” short for “Bit Block Transfer.” This is a technical secondary account of the history rather than a definitive first-use record.
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