convert jpg UIImage to bitmap UIImage?

Question

I'm trying to let a user pan/zoom through a static image with a selection rectangle on the main image, and a separate UIView for the "magnified" image.

The "magnified" UIView implements drawRect:

// rotate selectionRect if image isn't portrait internally
CGRect tmpRect = selectionRect;
if ((image.imageOrientation == UIImageOrientationLeft || image.imageOrientation == UIImageOrientationLeftMirrored || image.imageOrientation == UIImageOrientationRight || image.imageOrientation == UIImageOrientationRightMirrored)) {
    tmpRect = CGRectMake(selectionRect.origin.y,image.size.width - selectionRect.origin.x - selectionRect.size.width,selectionRect.size.height,selectionRect.size.width);
} 

// crop and draw
CGImageRef imageRef = CGImageCreateWithImageInRect([image CGImage], tmpRect);
[[UIImage imageWithCGImage:imageRef scale:image.scale orientation:image.imageOrientation] drawInRect:rect];
CGImageRelease(imageRef);

The performance on this is atrocious. It spends 92% of its time in [UIImage drawInRect].

Deeper, that's 84.5% in ripc_AcquireImage and 7.5% in ripc_RenderImage.

ripc_AcquireImage is 51% decoding the jpg, 30% upsampling.

So...I guess my question is what's the best way to avoid this? One option is to not take in a jpg to start, and that is a real solution for some things [ala captureStillImageAsynchronouslyFromConnection without JPG intermediary ]. But if I'm getting a UIImage off the camera roll, say...is there a clean way to convert the UIImage-jpg? Is converting it even the right thing (is there a "cacheAsBitmap" flag somewhere that would do that, essentially?)

All UIImages are bitmaps. That is the only internal format they can use. — borrrden, Jul 26 '12 at 01:32
What's setting the UIImage whose source was a jpg to re-decode each drawInRect, then? I mean, your statement makes absolute sense to me, and it was my assumption until profiling this issue...and then switching the still image capture to use raw RGBA instead of JPEG makes that particular slowdown go away.... — Kaolin Fire, Jul 26 '12 at 04:26
I had been using `jpegStillImageNSDataRepresentation` from the video, and have gotten around that by getting the data raw; but still run into the problem with jpgs from the camera roll (`imagePickerController: didFinishPickingMediaWithInfo:`, `UIImage image* = [info objectForKey:UIImagePickerControllerOriginalImage]]` — Kaolin Fire, Jul 26 '12 at 19:11

score 0 · Accepted Answer · answered Jul 27 '12 at 23:53

Apparently this isn't just a JPG issue—it's anything that's not backed by "ideal native representation", I think.

I'm having good success (significant performance improvements) with just the following:

UIGraphicsBeginImageContext(image.size);
[image drawAtPoint:CGPointZero];
image = [UIGraphicsGetImageFromCurrentImageContext() retain];
UIGraphicsEndImageContext();

For both things taken from the camera roll and from the camera.

convert jpg UIImage to bitmap UIImage?

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