Published on February 4, 2014
Advanced Imaging on iOS @rsebbe
Foreword • Don’t make any assumption about imaging on iOS. Why? • Because even if you’re right today, you’ll be wrong next year. • iOS is a moving platform, constantly being optimized versions after versions. • Experiment, and ﬁnd out the best approach for your app.
Understanding • Things to have in mind at all times: execution speed & memory consumption. • How to assess those: Instruments.
APIs Core Image Image IO Core Animation Core Graphics GLKit Core Video AVFoundation Many APIs but a unique reality: ! There’s CPU & there’s GPU Each has pro’s & con’s. ! Use them wisely depending on particular app needs.
Imaging 101 • On iOS, you typically use either PNGs and/or JPEGs. • PNG is lossless, typically less compressed *, CPU decode only, prepared by Xcode if in app bundle. Usecase: UI elements. • JPEG is lossy, typically more compressed *, CPU or GPU decode. Use-case: photo/textures ! • *: for images with single-color areas (UI), PNG can beat JPEG by a factor of 10x !
Imaging 101 • Image on iPhone 5/5s/5c: 3264 x 2448 pixels = 7990272 pixels = ~8MP • Decoded, each pixel is (R,G,B,A), 4 bytes. Whole image is then 32MB in RAM. Original JPEG is ~3MB.
Imaging Purpose • What is your purpose? Load & display (preview thumbnails)? Load, process & display (image editing)? Load, process & save? Process only (augmented reality, page detection)? • Amount of data. Large images or small images? Large input image, small output?
Discrete vs. UMA Discrete GPU (Mac) Decode Uniﬁed Memory Architecture (iOS/Mac) Decode Discrete GPU Uniﬁed Memory Architecture Decode Decode Transfer T Transfer GPU Data go through bus, back & forth is expensive GPU & CPU share same memory. Going back&forth is cheap Display Transfer T Display CPU Process Process Display Display Total speed depends on relative transfer & processing speeds iOS being a UMA gives a lot of ﬂexibility
Comparisons Draw w/ transform Decode Draw w/ transform Decode Pure GPU Decode Process T Display Display T Display CALayer/UIView setTransform CPU Transfer GPU CGContextDrawImage
GPU • Fast, but has constraints. • Low-level APIs: OpenGL ES, GLSL • High-level APIs: GLKit, Sprite Kit, Core Animation, Core Image, Core Video • Max OpenGL texture size is device dependent • 4096x4096: iPad 2/mini/3/Air/mini2, iPhone 4S+ • 2048x2048: iPhone 3GS / 4 / iPad 1 • Has fast hardware decoder for JPEG / Videos • Cannot run if app is in background (exception)
CPU • Slow, but ﬂexible. Like “I’m 15x slower than my GPU friend, OK, but I can be smarter”. • Low-level APIs: C, SIMD. • High-level APIs: Accelerate, Core Graphics, ImageIO. • Has smart JPEG decoder. • Can run in background.
Core Animation • Very efﬁcient, GPU-accelerated 2D/3D transform of image-based content. Foundation for UIKit’s UIView. • CALayer/UIView needs some visuals. How? -drawLayer: or setContents: • -[CALayer drawLayer:] (or -[UIView drawRect:]) uses Core Graphics (CPU) to generate an image (slow), and that image is then made into a GPU-backed texture that can move around (fast). • If not drawing, but instead setting contents -[CALayer setContents:] (or -[UIImageView setImage:]), you get the fast path, that is, GPU image decoding.
Fast Path CPU Pure GPU Decode Decode Display T Display CPU Transfer GPU -[CALayer drawRect:] (or UIView) + CGContextDrawImage (or UIImage draw) CALayer.contents (or UIImageView.image)
Demo 1 The Strong, the Weak, & the Ugly Comparison of CALayer.contents / UIView drawRect: for small images 2MP, 50x Show relative execution speed Show Instruments Time Profiler & OpenGL ES Driver.
Core Graphics / ImageIO • CPU (mostly). • CGImageRef, CGImageSourceRef, CGContextRef • Used with -drawRect: -drawLayer:
Core Graphics / ImageIO • How the load CGImageRef? Either using UIImage (easier) or CGImageSourceRef (more control) • How to create CGImageRef from existing pixel data? CGDataProviderRef • Having a CGImage object does not mean it’s decoded. It’s typically not, and even references mem-mapped data on disk -> no real device memory. • Sometimes, you may want to have it into decoded form (repeated/ high performance drawing, access to pixel values) • How do I do that?
Core Graphics / ImageIO • Need access to pixel values: use CGBitmapContext • Need to draw that image repeatedly? use CGLayer, UIGraphicsBeginImageContext(), or CGImage’s shouldCache property.
Core Graphics / ImageIO • Understanding kCGImageSourceShouldCache • It does *not* cache your image when creating it from the CGImageSourceRef. • Instead it caches it when drawing it the ﬁrst time. • It caches possibly at different sizes simultaneously. If you draw your image at 3 different sizes -> cached 3x. • Check your memory consumption when using caching, and don’t keep that image around when not needed.
Core Graphics / ImageIO • Note on JPEG decoding (CPU) • Image divided in 8x8 blocks of pixels • Encoding: DCT (frequency domain) • Decoding: skip higher frequencies if not needed. • That property can be used to make CPU decoding a lot faster.
Core Graphics / ImageIO • If source image is 3264px wide, • Drawing at 1632px will trigger partial JPEG decoding (4x4 instead of 8x8) -> much faster. Drawing at 1633px triggers full decoding + interpolation (much slower) • Similarly, successive power of 2 dividers have additional speed gain. ÷8 faster than ÷4 faster than ÷2 • If you need to draw a large image to a small size, use Core Graphics API (CPU), not CALayer (GPU). GPU decoding always decodes at full resolution!
I’m CPU, I’m weak but I’m smart! Draw small from large image (GPU) Draw small from large image (CPU) Decode Decode T Display Display CALayer.contents CGContextDrawImage (with small target size) Memory Mem CPU Transfer GPU
Demo 2 The Strong & Idiot vs. the Weak & Smart 11MP, 10x Show GPU is slower. Show GPU version does entire image decoding, while CPU does smarter, reduced drawing. Show Time Profiler function trace Show VMTracker tool, Dirty size. Change code to show influence of draw size on speed (+ function trace)
Core Image • CPU or GPU, ~20x speed difference on recent iPhone • Then, why using CPU? Background rendering (GPU not available) or as OS fallback if image is too large. • API: CIImage, CIFilter, CIContext • CIImage are (immutable) recipes, do not store image data by themselves • CIFilter (mutable) used to transform/combine CIImages • CIContext used to render into destination
Core Image • CIContext targets either EAGLContext or not. If not, it’s meant to create CGImageRef, or render to CPU memory (void*). In both cases, CIContext uses GPU to render, unless kCIContextUseSoftwareRenderer is YES. • Using software rendering is slow. Very slow. Very, very slow. Like 15x slower. Not recommended. • Depending on input image size / output target size, iOS will automatically fallback to software rendering. Query CIContext with inputImageMaximumSize / outputImageMaximumSize
Core Image • -inputImageMaximumSize: 4096 (iPhone 4S+, iPad 2+), 2048 (iPhone 4-, iPad 1) • 4000x3000 image (12MP) ﬁts. Camera sensor is 8MP, OK. • 5000x4000 image (20MP) does not ﬁt. • How do I process images larger than the limit?
Core Image • Answer: Image Tiling. • Large CIImage & -imageByCroppingToRect? NO, CPU fallback, as Core Image sees the original one (> limit). • Do the cropping on the CGImage itself (CGImageCreateImageWithRect), and *then* create a CIImage out of it. • Render tiles as CGImage from the CIContext, and render those tiles in the large, ﬁnal CGContext (> limit). • Art of tiling: Prizmo needs to process scanned images, that can be > 20MP.
Core Image GPU texture size limit Source Image Target Image (result) Perspective Crop Tiling in Prizmo: subdivide until source & target tiles both ﬁt GPU texture size limit
Core Image • Tips & Tricks • Core Image has access to hardware JPEG decoder, just like Core Animation’s CALayer.contents API. • Core Image is not programmable on iOS. But many unavailable functions can be expressed from the builtin CIFilter’s. • Don’t ﬁnd the ﬁlters you need? Give GPUImage a try. • Perfect team mate for OpenGL and Core Video.
CIImage’s Fast Path Pure GPU Decode Process Display CIImage imageWithContentsOfURL (or CGImage) CPU Transfer GPU
Core Image • Live processing or not? Depends. Live Processing Cached Processing OpenGL Layer/View CATiledLayer Atomic Refresh Visible Tiled Rendering Faster computation overall Slower computation Slower interaction Faster interaction
UIKit’s UIImage • Abstraction above CGImage / CIImage • Can’t be both at the same time, either CGImage-backed or CIImage-backed. • Has additional properties such as scale (determines how it’s rendered, Retina display) and imageOrientation • Nice utilities like -resizableImageWithCapInsets:
Core Video • Entry point for media. Both live camera stream & video ﬁles decoding/encoding. • Deﬁnes image types, and image buffer pool concept (reuse). • Native format generally is YUV420v (MP4/JPEG). Luminance plane (full size) + Cr, Cr planes (1:4) • You can ask to get them as GPU-enabled CVPixelBuffer for I/O • As of iOS7, you can render with OpenGL ES using R & RG I/O (resp. 1 & 2 comps for L & Cr/Cr planes) -> no more conversion needed (iPhone 4S+).
OpenGL ES • OpenGL - GLSL - GLKit: low level. You must load image as a texture, create a rectangle geometry, deﬁne a shader that tells how to map texture image to rendered fragments • Image processing is mostly happening in the fragment shader. • GPUImage is an interesting library with so many available ﬁlters. • CeedGL is a thin Obj-C wrapper for OpenGL objects (texture, framebuffer, shader, program, etc.)
OpenGL ES • R / RG planes (GL2.0, iPhone 4s+) • Multiple Render Targets / Framebuffer Fetch (GL3.0, iPhone 5s+) • MRT: before gl_FragColor.rgba = … • MRT: after my_FragColor.rgba = …; my_NormalMap.xy = …, etc. in single shader pass.
GLKit Remark • GLKit does not seem to allow hardware decoding of JPEGs (tested on iOS7, iPhone 5). Could change.
Conclusion • Use CPU / GPU for what it does best. • Don’t do more work than you need. • Overwhelming CPU or GPU is not good. Try to balance efforts to remain ﬂuid at all times.
Cookbook Display thumbnails Have JPEGs ready with target size, use CALayer.contents or UIImageView.image to get faster hardware decoding Compute thumbnails from large image Use CGImageSourceCreateThumbnail (or CGBitmapContext / CGContextDrawImage) Live processing Use CATiledLayer with cached source image (CIImage) at & display of various scales. Or OpenGL rendering if size < 4096 & large images processing can be made as a (fast) shader Offscreen processing of large images if size<=4096: GPU Core Image (or GL). else: GPU Core Image (or GL) with custom tiling + CGContext.
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