How do I create an 8-bit PNG with transparency from an NSBitmapImageRep?

Posted by Rob Keniger on Stack Overflow See other posts from Stack Overflow or by Rob Keniger
Published on 2010-03-03T04:11:02Z Indexed on 2010/03/08 1:49 UTC
Read the original article Hit count: 334

Filed under:

png

|

cocoa

|

images

|

nsbitmapimagerep

|

mac

I have a 32-bit NSBitmapImageRep which has an alpha channel with essentially 1-bit values (the pixels are either on or off).

I want to save this bitmap to an 8-bit PNG file with transparency. If I use the -representationUsingType:properties: method of NSBitmapImageRep and pass in NSPNGFileType, a 32-bit PNG is created, which is not what I want.

I know that 8-bit PNGs can be read, they open in Preview with no problems, but is it possible to write this type of PNG file using any built-in Mac OS X APIs? I'm happy to drop down to Core Image or even QuickTime if necessary. A cursory examination of the CGImage docs didn't reveal anything obvious.

EDIT: I've started a bounty on this question, if someone can provide working source code that takes a 32-bit NSBitmapImageRep and writes a 256-color PNG with 1-bit transparency, it's yours.

© Stack Overflow or respective owner

Related posts about png

Bulk convert PNG-24 to PNG-8 files with best quality

as seen on Super User - Search for 'Super User'
Hi, Can anybody recommend a good method of bulk converting a large amount of PNG-24 files to PNG-8 with as little loss of quality as possible and maintaining transparency? I've tried ImageMagick but the resulting images weren't quite as crisp quality as I'd like. Using Paint.NET I was able to achieve… >>> More
Azure Mobile Services: what files does it consist of?

as seen on ASP.net Weblogs - Search for 'ASP.net Weblogs'
Azure Mobile Services is a platform that provides a small set of functionality consisting of authentication, custom data tables, custom API’s, scheduling scripts and push notifications to be used as the back-end of a mobile application or if you want, any application or web site. As described in my… >>> More
how to rename files with count

as seen on Ask Ubuntu - Search for 'Ask Ubuntu'
i have a directory like this: randomized_quad015.png randomized_quad030.png randomized_quad045.png randomized_quad060.png randomized_quad075.png randomized_quad090.png randomized_quad1005.png randomized_quad1020.png randomized_quad1035.png randomized_quad1050.png randomized_quad105.png randomized_quad1065… >>> More
How can I center XHTML content with CSS?

as seen on Stack Overflow - Search for 'Stack Overflow'
so I recently converted a website of mine from a table content format to a div content format. Table format Version: Table version of the website: here. Table version style CSS: body { width: 1020px; margin: 0 auto; background-image: url(images/bg.png); } .logo{ width:301px; … >>> More
Images in the project NOT adding to array - don't know why.

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi there, I have to sets of images. Each set has 16 images. One set is called 0.png through to 15.png the other is a0.png through to a15.png. In my app, it loads each one dependent on a variable (which by logging, I have proved it works) here is the code [MemoryManager sharedMemoryManager]; NSLog(@"THEME:… >>> More

Related posts about cocoa

MVC - Cocoa interface - Cocoa Design pattern book

as seen on Stack Overflow - Search for 'Stack Overflow'
So I started reading this book: http://www.amazon.com/Cocoa-Design-Patterns-Erik-Buck/dp/0321535022 On chapter 2 it explains about the MVC design pattern and gives and example which I need some clarification to. The simple example shows a view with the following fields: hourlyRate, WorkHours, Standarthours… >>> More
Drag String data from My Cocoa App to Third-Party Cocoa App

as seen on Stack Overflow - Search for 'Stack Overflow'
Hello, I want to drag a row from my tableview and drop it into any other NSTextField in Mac OS X 10.6, and have a string of text be dropped. Drag and drop already works within my app (between a NSTableView and an NSBrowser), but I have had no success putting any data on the pasteboard that can… >>> More
Cocoa equivalent of the Carbon method getPtrSize

as seen on Stack Overflow - Search for 'Stack Overflow'
I need to translate the a carbon method into cocoa into and I am having trouble finding any documentation about what the carbon method getPtrSize really does. From the code I am translating it seems that it returns the byte representation of an image but that doesn't really match up with the name… >>> More
String formatting in cocoa

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi I am trying to send some text in an email from my cocoa app (by using Mail.app). Initially I tried using HTML to send properly formatted text. But the mailto: URL does not support html tags (even after setting headers) So I decided to use formatted string (left-aligning of string) This is what… >>> More
Anyone know the state of cocoa#?

as seen on Stack Overflow - Search for 'Stack Overflow'
Having just updated Mono to 2.6.3 (on OS X), I noticed in the installer that cocoa# 0.9.5 is also installed. However using MonoDevelop there are no cocoa# project templates by default, and I was wondering if anyone knew more about creating cocoa# apps. If you goto the cocoa# page on the Mono site… >>> More