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POtransparency/alpha in CoreGraphics context (iphone, objc)
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Id
3617119
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AcceptedAnswerId
3618190
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1
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CreationDate
2010-09-01T10:32:45.197
FavoriteCount
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LastActivityDate
2014-07-17T20:30:48.343
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LastEditorUserId
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329965
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PostTypeId
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Score
1
ViewCount
2307
LastEditorDisplayName
text
Body
I am currently making a basic image editor for iphone. I take the <code>CGImageRef</code> from a <code>UIImage</code> and create a context for it using the following code <pre><code>origImage = result.CGImage; Iheight = CGImageGetHeight(origImage); Iwidth = CGImageGetWidth(origImage); IcolorSpace = CGColorSpaceCreateDeviceRGB(); IrawData = malloc(Iheight * Iwidth * 4); IbytesPerPixel = 4; IbytesPerRow = IbytesPerPixel * Iwidth; IbitsPerComponent = 8; Icontext = CGBitmapContextCreate(IrawData, Iwidth, Iheight, IbitsPerComponent, IbytesPerRow, IcolorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big ); //[bytesPerRow release]; CGContextSetBlendMode(Icontext, kCGBlendModeCopy); CGContextDrawImage(Icontext, CGRectMake(0,0,Iwidth,Iheight), origImage); </code></pre> I then loop through the pixels <pre><code>for (int x=0; x<Iwidth; x++) { for (int y=0; y<Iheight; y++) { //and set the alpha component to 0 int byteIndex = (y*IbytesPerRow) + x*IbytesPerPixel; IrawData[byteIndex+3] = 0; } } </code></pre> and then create a CGImageRef from the context with <pre><code> CGImageRef imagea = CGBitmapContextCreateImage(Icontext); </code></pre> and add the CGImage to a UIImage and assign to a UIImageView The problem is that the change of alpha isn't effecting the resultant image if I change the colour of the pixels with <pre><code>IrawData[byteIndex+(0/1/2)] </code></pre> the colour changes, but I still can't change the alpha of the pixel Thank you, nonono
Tags
<iphone><objective-c><core-graphics><alpha><cgcontext>
Title
transparency/alpha in CoreGraphics context (iphone, objc)
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1. US123hal321
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