Joe Lertola wrote:
>
> The file is automatically opened with in fn importImage
> A CGContextRef is then created with fn MyCreateCGBitmapContext
> From what I can tell a CGContextRef is a Core Graphics equivalent of  
> a gworld.

A CGBitmapContext is the equivalent of a GWorld. A CGContextRef refers  
to a general graphics environment, which may or may not be a  
CGBitmapContext.

> To demonstrate direct pixel access I poke values into one vertical  
> column and one horizontal row of the CGContextRef.
> One mystery  here is that if I poke zero into the four values that  
> make up a pixel (RGBA) I expect to get black in the image. Instead I  
> get a medium gray line. If I poke 255 instead I get white as I would  
> expect.

The 4th byte is alpha. If you set it to 0, you see through the image  
to the metal window background.

> Lastly I want to export the image as a new file.

Here's how I save a GWorld image as a file. The format parameter may  
be _"PICT" _"TIFF" _"JPEG" _"GIFf" _"PNTG" etc

local fn WriteGWImageToFile( fileSpec as ^FSSpec, format as OSType, gw  
as CGrafPtr )
dim as GraphicsExportComponent  @ ge
dim as ComponentResult            err, ignore

err = fn OpenADefaultComponent( _"grex", format, @ge )
long if ( err == _noErr )
err = fn GraphicsExportSetInputGWorld( ge, gw )
if ( err == _noErr ) then err = fn GraphicsExportSetOutputFile( ge,  
#fileSpec )
if ( err == _noErr ) then err = fn GraphicsExportDoExport( ge, #0 )
ignore = fn CloseComponent( ge )
end if
end fn = err



You would use GraphicsExportSetInputCGBitmapContext instead of  
GraphicsExportSetInputGWorld. The C declaration is:

extern ComponentResult
GraphicsExportSetInputCGBitmapContext(
   GraphicsExportComponent   ci,
   CGContextRef              bitmapContextRef);

Implementing it will need the function-pointer-and-glue palaver that  
you already use for GraphicsImportCreateCGImage.

Robert P.