compositing - Page 3 - Developer IT

Optimizing drawing on UITableViewCell

- by Brian

I am drawing content to a UITableViewCell and it is working well, but I'm trying to understand if there is a better way of doing this. Each cell has the following components: Thumbnail on the left side - could come from server so it is loaded async Title String - variable length so each cell could be different height Timestamp String Gradient background - the gradient goes from the top of the cell to the bottom and is semi-transparent so that background colors shine through with a gloss It currently works well. The drawing occurs as follows: UITableViewController inits/reuses a cell, sets needed data, and calls [cell setNeedsDisplay] The cell has a CALayer for the thumbnail - thumbnailLayer In the cell's drawRect it draws the gradient background and the two strings The cell's drawRect it then calls setIcon - which gets the thumbnail and sets the image as the contents of the thumbnailLayer. If the image is not found locally, it sets a loading image as the contents of the thumbnailLayer and asynchronously gets the thumbnail. Once the thumbnail is received, it is reset by calling setIcon again & resets the thumbnailLayer.contents This all currently works, but using Instruments I see that the thumbnail is compositing with the gradient. I have tried the following to fix this: setting the cell's backgroundView to a view whose drawRect would draw the gradient so that the cell's drawRect could draw the thumbnail and using setNeedsDisplayInRect would allow me to only redraw the thumbnail after it loaded --- but this resulted in the backgroundView's drawing (gradient) covering the cell's drawing (text). I would just draw the thumbnail in the cell's drawRect, but when setNeedsDisplay is called, drawRect will just overlap another image and the loading image may show through. I would clear the rect, but then I would have to redraw the gradient. I would try to draw the gradient in a CAGradientLayer and store a reference to it, so I can quickly redraw it, but I figured I'd have to redraw the gradient if the cell's height changes. Any ideas? I'm sure I'm missing something so any help would be great.

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Method of transforming 3D vectors with a matrix

- by Drew Noakes

I've been doing some reading on transforming Vector3 with matrices, and am tossing up digging deeper into the math and coding this myself versus using existing code. For whatever reason my school curriculum never included matrices, so I'm filling a gap in my knowledge. Thankfully I only need a few simple things, I think. Context is that I'm programming a robot for the RoboCup 3D league. I'm coding it in C# but it'll have to run on Mono. Ideally I wouldn't use any existing graphics libraries for this (WinForms/WPF/XNA) as all I really need is a neat subset of matrix transformations. Specifically, I need translation and x/y/z rotations, and a way of combining multiple transformations into a single matrix. This will then be applied to my own Vector3 type to produce the transformed Vector3. I've read different advice about this. For example, some model the transformation with a 4x3 matrix, others with a 4x4 matrix. Also, some examples show that you need a forth value for the vector's matrix of 1. What happens to this value when it's included in the output? [1 0 0 0] [x y z 1] * [0 1 0 0] = [a b c d] [0 0 1 0] [2 4 6 1] The parts I'm missing are: What sizes my matrices should be Compositing transformations by multiplying the transformation matrices together Transforming 3D vectors with the resulting matrix As I mostly just want to get this running, any psuedo-code would be great. Information about what matrix values perform what transformations is quite clearly defined on many pages, so need not be discussed here unless you're very keen :)

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Any merit to a lazy-ish juxt function?

- by NielsK

In answering a question about a function that maps over multiple functions with the same arguments (A: juxt), I came up with a function that basically took the same form as juxt, but used map: (defn could-be-lazy-juxt [& funs] (fn [& args] (map #(apply %1 %2) funs (repeat args)))) => ((juxt inc dec str) 1) [2 0 "1"] => ((could-be-lazy-juxt inc dec str) 1) (2 0 "1") => ((juxt * / -) 6 2) [12 3 4] => ((could-be-lazy-juxt * / -) 6 2) (12 3 4) As posted in the original question, I have little clue about the laziness or performance of it, but timing in the REPL does suggest something lazy-ish is going on. => (time (apply (juxt + -) (range 1 100))) "Elapsed time: 0.097198 msecs" [4950 -4948] => (time (apply (could-be-lazy-juxt + -) (range 1 100))) "Elapsed time: 0.074558 msecs" (4950 -4948) => (time (apply (juxt + -) (range 10000000))) "Elapsed time: 1019.317913 msecs" [49999995000000 -49999995000000] => (time (apply (could-be-lazy-juxt + -) (range 10000000))) "Elapsed time: 0.070332 msecs" (49999995000000 -49999995000000) I'm sure this function is not really that quick (the print of the outcome 'feels' about as long in both). Doing a 'take x' on the function only limits the amount of functions evaluated, which probably is limited in it's applicability, and limiting the other parameters by 'take' should be just as lazy in normal juxt. Is this juxt really lazy ? Would a lazy juxt bring anything useful to the table, for instance as a compositing step between other lazy functions ? What are the performance (mem / cpu / object count / compilation) implications ? Is that why the Clojure juxt implementation is done with a reduce and returns a vector ? Edit: Somehow things can always be done simpler in Clojure. (defn could-be-lazy-juxt [& funs] (fn [& args] (map #(apply % args) funs)))

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UITableViewCell with selectable/copyable text that also detects URLs on the iPhone

- by Jasarien

Hi guys, I have a problem. Part of my app requires text to be shown in a table. The text needs to be selectable/copyable (but not editable) and any URLs within the text need to be highlighted and and when tapped allow me to take that URL and open my embedded browser. I have seen a couple of solutions that solve one of either of these problems, but not both. Solution 1: Icon Factory's IFTweetLabel The first solution I tried was to use the IFTweetLabel class made possible by Icon Factory and used in Twitterrific. While this solution allows for links (or anything you can find with a regex) to be detected to be handled on a case by case basis, it doesn't allow for selecting and copying. There is also an issue where if a URL is long enough to be wrapped, the button that the class overlays above the URL to make it interactive cannot wrap and draws off screen, looking very odd. Solution 2: Use IFTweetLabel and handle copy manually The second thing I tried was to keep IFTweetLabel in place to handle the links, but to implement the copying using a long-tap gesture, like how the SMS app handles it. This was just about working, but it doesn't allow for arbitrary selection of text, the whole text is copied, or none is copied at all... Pretty black and white. Solution 3: UITextView My third attempt was to add a UITextView as a subview of the table cell. The only thing that this doesn't solve is the fact that detected URLs cannot be handled by me. The text view uses UIApplication's openURL: method which quits my app and launched Safari. Also, as the table view can get quite large, the number of UITextViews added as subviews cause a noticeable performance drag on scrolling throughout the table, especially on iPhone 3G era devices (because of the creation, layout, compositing whenever a cell is scrolled on screen, etc). So my question to all you knowledgeable folk out there is: What can I do? Would a UIWebView be the best option? Aside from a performance drag, I think a webview would solve all the above issues, and if I remember correctly, back in the 2.0 days, the Apple documentation actually recommended web views where text formatting / hyperlinks were required. Can anyone think of a way to achieve this without a performance drag? Many thanks in advance to everyone who can help.

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Looking for some help working with premultiplied alpha

- by user315142

I am trying to update a source image with the contents of multiple destination images. From what I can tell using premultiplied alpha is the way to go with this, but I think I am doing something wrong (function below). the image I am starting with is initialized with all ARGB values set to 0. When I run the function once the resulting image looks great, but when I start compositing on any others all the pixels that have alpha information get really messed up. Does anyone know if I am doing something glaringly wrong or if there is something extra I need to do to modify the color values? void CompositeImage(unsigned char *src, unsigned char *dest, int srcW, int srcH){ int w = srcW; int h = srcH; int px0; int px1; int px2; int px3; int inverseAlpha; int r; int g; int b; int a; int y; int x; for (y = 0; y < h; y++) { for (x= 0; x< w*4; x+=4) { // pixel number px0 = (y*w*4) + x; px1 = (y*w*4) + (x+1); px2 = (y*w*4) + (x+2); px3 = (y*w*4) + (x+3); inverseAlpha = 1 - src[px3]; // create new values r = src[px0] + inverseAlpha * dest[px0]; g = src[px1] + inverseAlpha * dest[px1]; b = src[px2] + inverseAlpha * dest[px2]; a = src[px3] + inverseAlpha * dest[px3]; // update destination image dest[px0] = r; dest[px1] = g; dest[px2] = b; dest[px3] = a; } } }

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Java: how to do fast copy of a BufferedImage's pixels? (include unit test)

- by WizardOfOdds

I want to do a copy (of a rectangle area) of the ARGB values from a source BufferedImage into a destination BufferedImage. No compositing should be done: if I copy a pixel with an ARGB value of 0x8000BE50 (alpha value at 128), then the destination pixel must be exactly 0x8000BE50, totally overriding the destination pixel. I've got a very precise question and I made a unit test to show what I need. The unit test is fully functional and self-contained and is passing fine and is doing precisely what I want. However, I want a faster and more memory efficient method to replace copySrcIntoDstAt(...). That's the whole point of my question: I'm not after how to "fill" the image in a faster way (what I did is just an example to have a unit test). All I want is to know what would be a fast and memory efficient way to do it (ie fast and not creating needless objects). The proof-of-concept implementation I've made is obviously very memory efficient, but it is slow (doing one getRGB and one setRGB for every pixel). Schematically, I've got this: (where A indicates corresponding pixels from the destination image before the copy) AAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAA And I want to have this: AAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAABBBBAAA AAAAAAAAAAAAABBBBAAA AAAAAAAAAAAAAAAAAAAA where 'B' represents the pixels from the src image. I'm looking for an exact replacement of the method, not for an API link/quote. import org.junit.Test; import java.awt.image.BufferedImage; import static org.junit.Assert.*; public class TestCopy { private static final int COL1 = 0x8000BE50; // alpha at 128 private static final int COL2 = 0x1732FE87; // alpha at 23 @Test public void testPixelsCopy() { final BufferedImage src = new BufferedImage( 5, 5, BufferedImage.TYPE_INT_ARGB ); final BufferedImage dst = new BufferedImage( 20, 20, BufferedImage.TYPE_INT_ARGB ); convenienceFill( src, COL1 ); convenienceFill( dst, COL2 ); copySrcIntoDstAt( src, dst, 3, 4 ); for (int x = 0; x < dst.getWidth(); x++) { for (int y = 0; y < dst.getHeight(); y++) { if ( x >= 3 && x <= 7 && y >= 4 && y <= 8 ) { assertEquals( COL1, dst.getRGB(x,y) ); } else { assertEquals( COL2, dst.getRGB(x,y) ); } } } } // clipping is unnecessary private static void copySrcIntoDstAt( final BufferedImage src, final BufferedImage dst, final int dx, final int dy ) { // TODO: replace this by a much more efficient method for (int x = 0; x < src.getWidth(); x++) { for (int y = 0; y < src.getHeight(); y++) { dst.setRGB( dx + x, dy + y, src.getRGB(x,y) ); } } } // This method is just a convenience method, there's // no point in optimizing this method, this is not what // this question is about private static void convenienceFill( final BufferedImage bi, final int color ) { for (int x = 0; x < bi.getWidth(); x++) { for (int y = 0; y < bi.getHeight(); y++) { bi.setRGB( x, y, color ); } } } }

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