valid fields after PingImage()?

Hi all,

I'd like to be able to ping an image, allocate a lot of structures to
hold it, then later, only when pixels are actually needed, call the
much more expensive ReadImage().

After pinging an image I can get width and height easily (of course)
but depth and number of channels seem harder to figure out. For
example, with a vanilla RGB JPG image I see:

PingImage("x.jpg"):
GetImageDepth() = 1
image->depth = 8
GetImageType() = 1
IsGrayImage() = 1
IsMonochromeImage() = 1

ReadImage("x.v"):
GetImageDepth() = 8
image->depth = 8
GetImageType() = 6
IsGrayImage() = 0
IsMonochromeImage() = 0

Looking at the code quickly it looks as if the only thing that jpeg.c
sets is ->colorspace and the only way to read this is indirectly with
GetInageType(). However GetImageType() relies on being able to create
a pixel cache, which won't work after PingImage(), only after
ReadImage().

I'm probably missing something!

This is with GM 1.3.12, the standard version on Ubuntu 11.10.

John

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