Is it possible to make a NSBitmapImageRep with 24 bpp and no alpha channel?

Question

I don't understand really well the premultiplied alpha.

I need a NSBitmapImageRep without alpha channel (I don't need a particular bpp).

My problem is that this code give me errors:

NSSize imageSize = NSMakeSize(200, 200);

//create a non-alpha RGB image rep with the same dimensions as the image
  NSBitmapImageRep* bitmap = [[NSBitmapImageRep alloc]
                         initWithBitmapDataPlanes:NULL
                         pixelsWide:imageSize.width
                         pixelsHigh:imageSize.height
                         bitsPerSample:8
                         samplesPerPixel:3
                         hasAlpha:NO
                         isPlanar:NO
                         bitmapFormat:NSAlphaNonpremultipliedBitmapFormat
                         bytesPerRow:(3 * imageSize.width)
                         bitsPerPixel:24];

//lock focus on the bitmap

   NSGraphicsContext *context = [NSGraphicsContext graphicsContextWithBitmapImageRep:bitmap];
   [NSGraphicsContext saveGraphicsState];
   [NSGraphicsContext setCurrentContext:context];

//draw the image into the bitmap

[prueba drawAtPoint:NSMakePoint(0, 0) withAttributes:[NSDictionary dictionaryWithObjectsAndKeys:[NSFont systemFontOfSize:24], NSFontAttributeName, nil]];

[NSGraphicsContext restoreGraphicsState];

//get the TIFF data
   NSData* tiffData = [bitmap TIFFRepresentation];

//do something with TIFF data
   NSError *error = nil;
   [tiffData writeToFile:@"/Users/Paul/test.tif" options:NSDataWritingAtomic error:&error];

Error: CGBitmapContextCreate: unsupported parameter combination: 8 integer bits/component; 24 bits/pixel; 3-component color space; kCGImageAlphaNone; 640 bytes/row.

Okay, I know this combination it's not supported but I need something like this and I don't find a solution.

Your use of `saveGraphicsState` and `restoreGraphicsState` doesn't make sense: You save in one context, then restore in a different context without having previously saved in that context. Graphics states are per-context, not the other way around. — Peter Hosey, Jan 02 '12 at 20:36

score 3 · Accepted Answer · answered Jan 02 '12 at 20:34

3

Nope. The formats that Quartz supports are listed here, and like the error message you got said, 24-bit without alpha is not one of them. Since AppKit's drawing APIs are built on top of Quartz, that list applies to AppKit, too.

The best you can do is fill your context with a solid color, such as [NSColor blackColor], before you draw whatever you want to draw. You'll still have an alpha channel in the context, but no actual transparency.

answered Jan 02 '12 at 20:34

Peter Hosey

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Okay, I really need a tiff image without alpha channel, this is important because I use a OCR engine "_tesseract_" which need a tiff image without alpha channel. NSImage, NSBitmapImageRep, etc.. all of them use alpha channel. Then, any solution for me please? – PabloLerma Jan 03 '12 at 11:31
@PabloLerma: Use something other than Quartz/AppKit to generate the image, or use an external tool to convert the image to the needed format. – Peter Hosey Jan 03 '12 at 16:35
Any idea? I'm searching about **Magick++** but I don't know others tools – PabloLerma Jan 03 '12 at 20:37

Jim Merkel · Answer 2 · 2012-01-03T00:21:51.270

0

To Clarify, NSBitmapImageRep does supports 24 Bits/pixel. However, NSGraphicsContext does not support this format. Apparently an alpha channel is always required by the Quartz drawing system.

edited Jan 03 '12 at 00:21

answered Jan 02 '12 at 22:03

Jim Merkel

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2
9

But then, if NSBitmapImageRep support 24 Bits/pixel, Why am I getting errors? – PabloLerma Jan 02 '12 at 23:01
I guess NSBitmapImageRep supports 24 Bits/pixel but NSGraphicsContext does not support it (as the error message says). – Jim Merkel Jan 02 '12 at 23:29

Is it possible to make a NSBitmapImageRep with 24 bpp and no alpha channel?

2 Answers2

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