A survey of the various ways of creating GDI bitmaps with predefined data

Raymond

Suppose you have a buffer full of pixels and you want to create a GDI bitmap that contains those pixels.

We’ll start with the Create­Bitmap function. This creates a device-dependent bitmap with the pixels you provide. The weird thing about this function is that it’s a device-dependent bitmap, but there’s no parameter that specifies the device! As a result, the bitmap has the format you specify, but you can select it only into device contexts that are compatible with the bitmap format. For example, if you create a 4bpp bitmap, then you can select it only into 4bpp device contexts.¹

Next up is the misleadingly-named Create­DI­Bitmap function. Even though the DI stands for device independent, this function does not create a device-independent bitmap. It creates a device-dependent bitmap that is compatible with the provided device context. The reason it’s called DI is that you can provide pixels in a device-independent format, and those pixels will be used to initialize the bitmap. As noted in the documentation, the behavior is functionally equivalent to Create­Compatible­Bitmap followed by Set­DI­Bits.

If it’s a device-independent bitmap you want, then the function to use is Create­DIB­Section. The simplest use of this function creates a device-independent bitmap and gives you a pointer to the in-memory pixel buffer. You can then manipulate the pixel buffer directly, say, by memcpying the bytes from your original buffer.

The fancier use of this function creates a device-independent bitmap around existing memory. The memory needs to be in a file mapping object, either a file mapping object created from a file or (more often) a file mapping object created from the page file (in other words, a shared memory block). You can then specify the byte offset within the file mapping at which the pixel buffer starts. In this case, the memory is not copied; the memory in the file mapping object is the backing memory for the bitmap. If you modify the bitmap, then the contents of the file mapping object change; if you modify the contents of the file mapping object, you modify the bitmap.

Here’s the table:

Function Type of bitmap Resulting format Source pixels Must format match?
Create­Bitmap Device-dependent As specified Copied Yes
Create­DI­Bitmap Device-dependent Device-compatible Copied No
Create­DIB­Section without hSection Device-independent As specified Uninitialized (copy them yourself) Yes
Create­DIB­Section with hSection Device-independent As specified Shared Yes

In the above table, the Resulting format column describes the pixel format of the returned bitmap. The Source pixels column describes what happens to the pixels you pass as the source pixels: Are they copied into the bitmap, or does the bitmap share the memory with the source pixels? The Must format match? column specifies whether the format of the source pixels must match the pixel format of the returned bitmap. If Must format match? is No, then the system will perform a format conversion.

¹ Monochrome bitmaps are compatible with any device context and have special behavior when selected into color device contexts.

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