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Re: A Tail of Two Bitmaps

Further information...

When using kCGImageAlphaPremultipliedFirst, the image is garbage when it is scaled down.

Mark


On 31-May-05, at 14:03, Mark Morrill wrote:

Hi,

I'm wanting to create a CGImageRef from a CGBitmapContextRef - which is pretty straight forward. However, I'm having a bit of a problem...

I want to use an alpha channel but I'm only getting good results by not using an alpha channel.

Basically I'm doing this:

CGContextRef context = ::CGBitmapContextCreate( ... basic info..., theAlphaInfo );
CGImageRef image = ::CGImageCreate( ... basic info from context ...,
dataProvider, null, true,
kCGRenderingIntentDefault );

The "basic info" is block of data that assumes 32 bits per pixel. The colorspace is CGColorSpaceCreateDeviceRGB

The "basic info from context" are the CGBitmapContextGet... calls. The dataProvider is created with the block of data used to create the context.


When theAlphaInfo is kCGImageAlphaPremultipliedFirst (or last), drawing the image into a context with a size other than the image's size will sometimes produce garbage on the screen, sometimes it will produce results that suggest that the row bytes is wrong.

When theAlphaInfo is kCGImageAlphaNoneSkipLast (32bits per pixel), the drawing lacks the alpha channel but it always draws right.

Thoughts?

I'm still using 10.3

Mark
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 >A Tail of Two Bitmaps (From: Mark Morrill <email@hidden>)



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