Understanding Bitmap memory layout

[Thingfish]

Hi

Trying to get an understanding of how bitmap data is arranged in
memory, but aside from bitmap.h and the plasma example there doesn't
seem to be much documentation. Have a few questions I hope you can
help answer :)

1) Taking a look at the AndroidBitmapInfo struct from bitmap.h:

typedef struct {
uint32_t width;
uint32_t height;
uint32_t stride;
int32_t format;
uint32_t flags; // 0 for now
} AndroidBitmapInfo;

I assume width and height, are bitmap dimensions in pixels. From what
I can google stride seems to be the width of a bitmap line in bytes,
possible including padding? Is this correct, and if so, is padding
relevant for all configs/formats?


2) Comparing enum AndroidBitmapFormat from bitmap.h and Bitmap.Config
from the Android SDK I find "matching" definitions such as
Config.ARGB_8888 from the SDK and ANDROID_BITMAP_FORMAT_RGBA_8888 from
the NDK. Doesn't the ordering of the letters usually signal the
bitmasks used to extract the individual components? So why are they
different, and which one is correct? :)

[Thingfish]

After a bunch of segfaults :) I can partially answer this myself.

1) Seems to be no extra padding for Config.ARGB_8888, not that
surprising since it automatically fits on 4-byte boundaries.

2) libswscale gives the correct result when the data is treated as
RGBA, so I guess the definitions from bitmap.h are more correct.

[devi prasad]

FWIW, I too have concluded that Config.ARGB_8888 is same as RGBA. A
few days ago I had posted a message saying that ARGB_8888 is not
friendly with OpenGL and so on. In the meantime, I have found that
there are the same.

dp

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[Ken Turkowski]

There seems to be two definitions of what something like "RGBA" means:
- R, G, B, A is the ordering of pixel components in a stream
- (R << 24) | (G << 16) | (B << 8) | (A << 0) is the location of components in an int32
The former is machine independent, and the latter is endian-dependent.

On a big-endian machine, the two definitions are equivalent, but on a little-endian machine, the components are reversed.