Re: Obtaining unpremultiplied bitmap from PNG file
Re: Obtaining unpremultiplied bitmap from PNG file
- Subject: Re: Obtaining unpremultiplied bitmap from PNG file
- From: Chiharu Hirono <email@hidden>
- Date: Tue, 19 Jun 2007 02:11:35 +0900
Thank you for quick responses.
The final goal of may app is to make smaller picture from original
picture without modifying RGBA or gamma or resizing.
I tried this using these step:
1) Make NSImage from original picture file
2) Alloc output NSImage
3) Using compositeToPoint copy an area of original image to output image
4) Get TIFFRepresentation from output image
5) Make NSBitmapImageRep using TIFF data
6) Get encoded data from NSBitmapImageRep using representationUsingType
7) Write data to file
But this method made modified RGB.
My purpose is not get RGBA pixcel values, but get picture with
original RGBA.
Can CGImageRef solve this problem?
On 2007/06/15, at 2:57, David Duncan wrote:
On Jun 13, 2007, at 2:28 PM, Shamyl Zakariya wrote:
As far as I can tell, Cocoa simply won't give you non-
premultiplied bitmap data. The CG constant kCGImageAlphaLast
( IIRC ) is there, but isn't actually implemented.
kCGImageAlphaLast is implemented and fully supported for creating a
CGImageRef. What is not implemented is creating a bitmap context
(via CGBitmapContextCreate) that does not use pre-multiplied alpha.
The distinction is subtle, but practically it means that you can
create an image that does not used pre-multiplied alpha, but in the
current API you cannot obtain the pixel data from that image into
your own buffer without getting it pre-multiplied.
On Jun 14, 2007, at 2:34 AM, Barry wrote:
How do I access the pixel data of a CGImage object?
at
http://developer.apple.com/qa/qa2007/qa1509.html
Is this a solution ???
This Q&A demonstrates obtaining the pixel data via a bitmap
context, which as above requires you to work with pre-multiplied data.
--
David Duncan
Apple DTS Quartz and Printing
email@hidden
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