Monday, 30 May 2011


I've started another new project on google code: jjmpeg. It's a simple binding to ffmpeg for java, where possible mirroring the API directly. e.g. compare with avcodec_sample.0.5.0.c.

It uses NIO direct ByteBuffers to allow some of the binding to be done in managed code, and the rest is done with custom callbacks.

I only need it for reading video, so that's all i've implemented; and even then only a tiny subset thereof. I don't know if it will get any further than that.

I did look into using gluegen to do the binding but I couldn't get it to work - for starters the ffmpeg headers are too complex for it's limited parser. Even stripping out the conditionals and the inline code didn't help (using cpp -dD and some editing). So I just manually select which fields to bind and I have some C which works out the offsets (and auto-generates some classes) from some embedded tables (why write a config parser when the c compiler has one?), and hand-code the function calls.

I use 3 classes per 'class' to allow me to get away with binding generated code with hand-written code, as well as supporting 32 or 64 bit (perhaps, eventually - only 64 bit done so far). Base abstract class is auto-generated and sets up the interface for the auto-generated field accessors. The middle abstract class is the public api and includes the hand-written bindings and factory methods (where appropriate). The final concrete class implements the accessors for each processor size.

Up until now i've been using Xuggler ... which I probably will keep using for some software, particularly cross platform stuff. But I don't really like the callback api it uses and for the most part the ffmpeg library api's are fairly simple on their own (despite a bit of bitrot).

Friday, 27 May 2011

Viola & Jones Detector OpenCL

After being a bit distracted yesterday evening by my co-dwellers, I got stuck into a problem I've been wanting to look at for a while - running a Viola & Jones cascade detector on the GPU using OpenCL. I'd just got an integral image calculator done so was eager to use it.

I've had a few goes in the past but always seemed to mess up some of the weighting calculations, so I started with the code from OpenClooVision which is a bit easier to follow than the OpenCV implementation, although could certainly use some work.

So, by about 6am I had a working implementation ... but it was really very slow. Far too slow to even consider for what I need, and worse, it doesn't scale at all well - running it on smaller problem sizes just makes it even less efficient.

Who is that fat bastard?

I went to bed shivering cold but mostly wondering just what I could do to speed it up. I have previously done some work with integral images, and I found they do not work particularly well on GPUs - even calculating them aside (which I managed to solve with an acceptable solution although it took many many dead-ends and grey hairs). Although on paper they look like an efficient solution - a handful of array lookups to calculate an area sum - in use they seem to interact poorly with texture cache.

It was taking in excess of 30 000uS to perform 14 passes on a 640x480 test image in steps of 5 with a scale of 1.25.

The OpenCV and faint implementations both pre-scale the features and feature-weights. I never quite understood why until I had a working implementation, and the OpenClooVision version was calculating them on the fly. So the first stop was to try this. Pre-calculating the weights is extremely cheap, and this lead to around a 50% performance boost.

I still had a problem with the GPU hardly being utilised, particularly at the larger scales (fewer tasks/call) or with smaller images. And because each thread was working on a separate probe, there was very little coherency in processing or data.

However, I noticed that for the cascade I was using (the default one from OpenCV) it was running many feature tests for each stage - 25-200; and that calculating the feature value was unconditional - something ripe for parallelisation.

So I tried launching 64 threads for each probe location, and they work together on the list of features in blocks of 64, and then tally them up at the end using thread 0. This was the biggest improvement and I managed to get it down to around 12 000uS.

I then tried a parallel prefix sum - which got it down to about 11 000uS, although then I tried a sqrt(N) sum (split the summation into 2 passes, first 16 threads produce 16 partial sums, then 1 thread adds those up) and got it down to about 10 500uS. Parallel prefix sum loses again.

And then to cut a long story short I tried a whole bunch of variations, such as storing the regions in integer format - this made it faster, but only because it wasn't calculating the same results. And different work-sizes - 64 worked the best. And different packing formats for the feature descriptors. But no matter what I tried, about 10 500 uS was about the best I could manage with the test image.

I also tried a slightly modified version (no thread dependencies) running on the AMD CPU driver, on the 6 core+HT machine I have. That managed 90mS. So the GPU is only 10x faster, which although nothing to sneeze at is still a bit disappointing.

To me this is still a little on the slow side, but I'm pretty much out of ideas for now. It might just be a problem that defies any particularly efficient mapping to GPU hardware.

Actually one thing I haven't tried is scaling the images instead of the features ... but I think that's something that can wait for another day.

Tuesday, 24 May 2011

OpenCL Kanade Lucas Tomasi feature tracking

I've added a couple of things to socles, the main being an implementation of Kanade Lucas Tomasi feature tracking. It's just a fairly simple translation of the public domain version here (site seems down at time of writing), with a few tweaks for good GPU performance. Ok it looks nothing like it, but all i did was remove the need for 3 temporary images and the loops to create and use them by noting that each element was independent of the other and so they could all be moved inside of a single loop.

I've only implemented the translation-only version of the tracker, with the lighting-sensitive matching algorithm. The affine version shouldn't be terribly difficult - it's mostly just re-arranging the code in a similar fashion (although the 6x6 matrix solver might benefit from more thorough work).

Really awful screenshot of it in action ... (it's very late again)

The 3300 regularly spaced feature points above take about 1.5ms to track in GPU time (480 GTX), including all of the image pyramid and gradient image set-up for the incoming frame. It uses 3 levels of pyramid with each 1/4 the size in each dimension of the one below. Most of the points lose track because there isn't enough detail to meet the quality settings, hence the areas with no green dots.

With a CPU implementation the majority of the time is taken in the convolutions necessary to setup the image pyramid - although eventually with enough features one reaches a break-even point. In this case it's about 500uS for the image setup for 640x480 image (3 convolutions at full res, 2 at each subsampling and one resample), and 1ms for the 3300 tracks (i.e. 0.3 microsecond per additional 16x16 feature tracked). Well that's quick enough to keep me happy anyway. At least for now!

I implemented this using a pattern which I've found after much (much) trial and error to be reliably efficient for mapping many types of rectilinear problems to GPU processes, which i'll just call an NxM execution pattern. For a feature-size of 'NxM', the code assigns 'N' threads to each feature, each which works on a column of 'M' rows of data, with the first thread in the sub-group used to tally results and make decisions. A small amount of local memory is used for the partial sums and to share the decisions amongst the threads. These are mapped to a flat work size which is independent of the local workgroup size, which allows for easy tuning adjustments. So a feature size of 9 would be mapped using 7 features if the local work-size is 64, with 15 threads idle (i.e. choose factors of the worksize for best efficiency, but it can still work well with any size). 100 such features would need 15 work groups (ceil(100/7)). The address arithmetic is a bit messy but it only needs to be calculated once on entry, after that it is simple fixed indexing. And since you can reference any memory/local memory by local id 0 you don't have to worry about memory bank conflicts.

I tried many other variations of mapping the work to the GPU and this was by far the best (and not far off the simplest once the fiddly address calculations are out of the way). For example, using a parallel prefix some across all elements with a 3d work size (i.e. 16x16xnfeatures) was about 1/2 the speed IIRC - the increased parallelism was a hindrance because of both the IPC required and the over-utilisation of resources (more local memory and threads).

Update Ok I found a pretty major problem with this mapping of the jobs - early exit of some sub-workgroups messes up the barrier usage which upsets at least the AMD cpu driver. So all threads need to keep executing until they are all finished and then mask the result appropriately, which will complicate matters but hopefully not by much.

Update 2 I think I managed to solve it, although I had some trouble with the code behaving as I expected, and got misled by bugs in other kernels.

Sunday, 22 May 2011

branchless stackless merge sort

After a previous post I had some other thoughts about implementing a branch-less merge. And further, of removing any stack or recursion as well, at least in certain cases.

This is more of a mental exercise in this case because I only looked at sorting an array - where you're better off using qsort, but it might be worth it on a linked list.

First merge version:
static inline void merge1(void * restrict head1,
int len1,
void *tmpData,
CmpFunc cmp,
size_t itemsize)
void * restrict tail1 = head1 + len1*itemsize;
void * restrict head2 = tail1;
void * restrict tail2 = head2 + len1 * itemsize;

void * restrict h1 = tmpData;
void * restrict t1 = tmpData;
void * restrict tail = head1;

// copy first array to temporary buffer
t1 = tmpData + (tail1-head1);
memcpy(tmpData, head1, tail1-head1+itemsize);

// merge temporary buffer into target buffer
while (1) {
if (cmp(h1, head2) < 0) {
memcpy(tail, h1, itemsize);
tail += itemsize;
if (h1 == t1)
// end of first list - rest is already in place
goto done;
h1 += itemsize;
} else {
memcpy(tail, head2, itemsize);
tail += itemsize;
if (head2 == tail2) {
// end of second list - copy rest of first list back
memcpy(tail, h1, t1-h1+itemsize);
goto done;
head2 += itemsize;
Keeping the loop simple is the best policy in this case.

The branchless merge:
static inline void merge2(void * restrict head1,
int len1,
void * restrict tmp,
CmpFunc cmp,
size_t itemsize,
void *sentinel)
void *h1 = tmp;
void *h2 = head1+len1*itemsize;
void *out = head1;
int len = len1*2;
int i;

// copy first half array to temporary buffer & add sentinal
memcpy(tmp, head1, len1*itemsize);
memcpy(tmp+len1*itemsize, sentinel, itemsize);

// save spot for sentinel in 2nd array
memcpy(tmp+len1*itemsize+itemsize, h2+len1*itemsize, itemsize);
memcpy(h2+len1*itemsize, sentinel, itemsize);

// merge
for (i=0;i<len;i++) {
int c;

c = cmp(h1, h2) < 0;
memcpy(out, c ? h1 : h2, itemsize);
h1 = c ? h1 + itemsize : h1;
h2 = c ? h2 : h2 + itemsize;
out += itemsize;

// restore
memcpy(h2+len1*itemsize, tmp+len1*itemsize+itemsize, itemsize);

It uses a sentinel so there needs to be no testing of loop termination beyond the number of total elements being merged.

And finally the removal of the stack or recursion to find out when merges need to take place. One simple way (breadth-first) is just to iterate over the whole array, at size 2, then 4, 8, and so on, but this has poor locality of references. So this simple logic is used to track when and how large merges are to take place and does them in a depth-first fashion.

void MergeSort(void *data, size_t count, size_t itemsize, CmpFunc cmp, void *sentinal) {
int i, j;
int last = 0;
void *tmp;

tmp = malloc((count+1) * itemsize);

for (i=0;i<count;) {
batchers8(data+i*itemsize, tmp, cmp, itemsize);
i += 8;
for (j=16;j<=count;j<<=1) {
if (j&(i^last)) {
merge2(data+(i-j)*itemsize, j/2, tmp, cmp, itemsize, sentinal);
last = i;

This code also performs the bottom sort in 8 element lots using an unrolled batchers sort. So one might note that this particular sort is limited to sorting arrays whose number of elements is a power of 2 ...

I ran a few timings and this is slightly faster than the natural merge sort that I had previously written in certain cases - that of course is still much faster for the already-sorted case. The version I timed had the merge step manually unrolled 8 times as well.

For random input, the branchless merge is about 10% faster than the original merge, and the original merge is about 10% faster than my original natural merge sort. So the lack of conditional execution ends up a bonus, even considering the extra overheads, and all the extra memory moves. This is on an intel cpu anyway - which is so register starved they've had to make such code run fast, so one might not end up with the same result on other architectures.

There is more possible here too if you're sorting basic types (i.e. not a general interface as above) - all of these algorithms can be vectorised to run on SIMD processors. It could for example independently sort 4 interleaved arrays in the same number of operations, and then merge those in 2-3 passes at the end (or 1, with a 4-way merge).

Sentinels are a pretty nice optimisation trick but they usually require some sort of extra housekeeping that makes their use impractical. For example in the camel-mime-parser code I used a sentinel of \n at the end of the i/o buffer every time i refreshed it so the lowest level loop which identified individual lines was only 3 instructions long - and it only checked for running out of input after the loop had terminated. But I had to write my own i/o buffering to have such control of the memory.

Saturday, 21 May 2011


Damnit, i've done it again. That's what I get for trying to cut and paste some old code that apparently never worked anyway.

I was trying to implement an OpenGL rendering loop for some OpenCL code rather than copying it back to Java2D every time. Except I based the code on some test code i'd written which now I realise never worked.

And mostly it failed because it didn't call putAcquiteGLObject() before writing to the output texture.

A few more hours of my day i'll never get back ... and I think it's the second time i've done it. Maybe i'll remember it this time if I write it down.

Thursday, 19 May 2011

More sort thoughts

Late last night when I should really have already headed to bed I had some more thoughts on the median filter and played a bit with a batchers sort generator.

For a 9 value median, rather than do a full 9 element sort, one could just do an 8 element sort and then attempt to insert the final value into the middle. The insert which only must be valid for the centre value only takes 2 additional cas steps. But an 8 sort takes quite a few fewer steps than a 9 sort, and even then 2 are redundant for finding the median. In short, this allows a 9 element median to be calculated in only 19 cas operations rather than 22.

For larger sorts one can probably repeat the process further.

Wednesday, 18 May 2011

3x3 median filter, or branchless loopless sort

So while thinking of something I could add to socles to make it worth uploading I thought of implementing a median filter.

Median filters are a bit of a pain because one has to sort all of the pixel values (for a greyscale image at least) before getting the result - and sorts generally require loops and other conditional code which make things a bit inefficient when executed on a GPU.

So somehow i'm back to looking into sorts again ... the never-ending quest continues.

Fortunately, this is another case where a batchers sort or other hand-rolled sorting networks come to the rescue. And since each element is being processed independently there are no inter-thread dependencies and all the processing can be unrolled and performed on registers (if you have enough of them).

So I implemented the 9 element sorting network from Knuth Volume 3 section 5.3.4 which sorts 9 elements in 25 compare and swap steps. But since 3 of the comparisons of the 25 aren't involved in the median value I also dropped them and ended up with 22 inline comparison and swaps to find the median value. Fortunately this is a small enough problem to fit entirely into registers too.

This also works pretty well in C - an albeit-inline version - manages to perform 100M median calculations on a 9 integer array (on the same array which is copied over) in 1.26s on my workstation, using the glibc qsort() to do the same task takes 17s. (I didn't verify that the inline version was transferring data to/from memory as it should in the micro-benchmark, but that does sound about right all things considered).

So, given unsorted values s0-s8, and an operator cas(a, b) (compare and swap) which results in a < b, the median can be calculated using the following 22 steps:
        cas(s1, s2);
cas(s4, s5);
cas(s7, s8);

cas(s0, s1);
cas(s3, s4);
cas(s6, s7);

cas(s1, s2);
cas(s4, s5);
cas(s7, s8);

cas(s3, s6);
cas(s4, s7);
cas(s5, s8);
cas(s0, s3);

cas(s1, s4);
cas(s2, s5);
cas(s3, s6);

cas(s4, s7);
cas(s1, s3);

cas(s2, s6);

cas(s2, s3);
cas(s4, s6);

cas(s3, s4);

s4 now contains the median value. cas can be implemented using 1 compare and 2 selects.

But perhaps the nicest thing is the code is directly vectorisable.

Some timings using a 4 channel 32-bit input image 640x480 and producing a 4 channel 32-bit output image: For 1 element it takes 84uS, for 3 elements 175uS and 4 elements 238uS. This is on a NVidia 480GTX.

The current implementation:


I finally started a new project to store some of the simpler OpenCL stuff i've been working on.

I've called it socles - a socle is another word for plynth - the base of a column.

It's a really basic set of operations at the moment, but I hope they are decent implementations thereof at least.

Sunday, 15 May 2011

Convolved out

Bit of a dry post coming up, but when you consider I blew my Sunday on this it might make sense. Curiosity got the better of me today and I spent most of it playing with convolutions with OpenCL on a GPU - I had a pretty fast implementation but wanted to compare it with some other ideas I had.

As it turned out, the implementation I had was the fastest after-all, although I tweaked it a tiny bit during the process.

For kernels at 3x3 (or 7x7 for 4-channel UBYTE images), a simple 2d implementation is very slightly faster than a more complex algorithm.

For non-separable convolution, a complex implementation which uses a rolling buffer is over 3x faster than a naive implementation, at least up to sizes of 31x31.

For separable convolution, my complex implementation is up to 2.5x faster than a naive implementation.

My separable convolution implementation reads 16x16 blocks of image into local memory and then each thread generates all results from the local memory in one pass - e.g. for up to 7x7 convolution it reads 2x16x16 blocks, for up to 15x15 convolution it reads 3x16x16 blocks, and so on i.e. you need the 16x16 data plus 'kernel radius' pixels each size. It uses transpose for the Y convolution case during load and saving of the data but the processing is identical. It also uses the trick of offsetting the odd rows of the data so they avoid local memory contention when they might otherwise - e.g. when the number of bocks being read is even.

FWIW for 640x480 image on a GTX 480 A single channel FLOAT 31x31 separable convolution is about 190uS, or 470uS for naive version. For UBYTE 177uS vs 470uS. For a 4 channel image the timings are 413uS, 916uS, 389uS, and 465uS respectively. So larger (byte size) images gain more - presumably from the reduction in memory reads and lower cache loading.

Actually - yesterday I started working on a JOCL based image processing library that I intend to drop on google code at some point - and this investigation was part of that. More should be forthcoming on that soon although right now I just don't have enough put together to be much use.

Friday, 6 May 2011

Luser error

So it turns out the main problem with the performance issues in my last post wasn't the cache being overloaded after all ... but the compiler running out of registers to unroll a couple of nested loops I asked it to.

It turns out because of a cut and paste job I left in some test code which was limiting the register count for the compiler, and the register spillage was a bit of a disaster and causing the order of magnitude drop. With this fixed things look a lot more promising, although at ~4ms it is still a little slower than I would like and I can't see much being able to be done to improve it.

It's a bit weird that an unconstrained compile for a specific device would choose to use all registers it needs for the code-unrolling, ignoring the device specifics, but I guess this is some particular of the nvidia opencl implementation.

There is still some problems with the texture cache being hit too hard anyway - the integral image has to be stored using the UINT32 data type, and it needs at least 8 lookups per feature tested which is a pretty heavy load to start with. With the other feature tester I could just use 8 bit images which fits better into the cache it seems (contrary to what i'd believed until this point).

Investigations ongoing ...

Thursday, 5 May 2011

Cache only works when there's enough

Update: Seems I was (mostly) wrong - see the follow-up post.

I've been playing with object classifiers in OpenCL - I have one that works, not terribly well but relatively quickly. It's a random tree classifier and just uses pixel intensity comparisons for feature tests. Although I can get some results out of it they just weren't reliable enough.

So I decided to use 2-bit-binary patterns instead, a haar-like feature measure which uses an integral image to accelerate the intensity comparisons (maybe more on the integral image creation another time, that was a bit of a journey in OpenCL as well). Unfortunately the simple modifications required to change the feature detector suddenly blew out the computation time - from under 1ms to over 20ms, making it far too slow. This even though it only requires twice the memory accesses for the same number of tests.

After much experimentation I found the cause - the texture cache was being exhausted, dramatically reducing the apparent memory throughput. I discovered that a single-tree, 3-feature tester is about the limit of the texture cache. That will execute in 150uS. 4-features take 250uS, and if I change the 4-feature system to 2 trees, or 8 features - just twice the amount of work - it blows out to 1500uS.

Well, at least that gives me options for splitting the work into multiple passes and maybe i'll end up with something fast enough over-all. Otherwise I might have to find something else. As it is i'm losing confidence that it is going to be good enough anyway (maybe after running it so many times i've forgotten how poor the previous one was).

Sunday, 1 May 2011

Hot Sauce #2 - Sweet Arson

As it turns out it was a rainy day and I thought the fresh habaneros either needed to be used or frozen so I made another sauce today as well.

The week previously I had excavated a patch of sweet potatoes and ended up with a reasonable haul (the largest in the photo is 2.4kg) ... and I had the idea of using the sweet potato as the base for this sauce. And this time I thought i'd try a warming mix of herbs to compliment it. I'm really quite pleased with this one - it has quite an interesting and pleasant flavour and will leave your lips burning from even quite a small amount.


450gRoasted sweet potato (2-3 average tubers).
12Cayesan Chillies, roasted.
12Habaneros, roasted.
150mlGrapeseed oil.
1Diced onion.
Dry Spices
2 tspCoriander seed.
1 tspCardomon seed.
1 tspBlack peppercorns.
1 tspWhole allspice.
1 tspFennel seed.
2 cupsMalt vinegar.
1 1/2 cupWater.
1 tspCitric Acid.
1 tspSalt
1 tspMSG


Place the sweet potato whole and un-peeled into a roasting tray and roast until done - about an hour. Place the chillies on another tray and roast until they start to collapse - about 20 minutes. Remove the potatoes from the oven and let them cool to a suitable temperature for handling and peel the skins off (perhaps sweating them under foil will help).

Meanwhile, place the oil and onion in a medium sized saucepan and cook slowly on low heat - so they become translucent but do not brown.

Take the dry seeds and toast lightly in a dry pan for a few minutes. Use a medium heat and constantly move them so they do not burn. Transfer the seeds to a mortar and pestle and pound to a fine dust.

Add the dry spices to the onions and cook for a couple of minutes.

Add the roasted chillies and sweet potato and mash them together.

Add all of the liquid before it starts to burn, together with the remaining ingredients and simmer for at least 30 minutes on a very low heat.

Use a stick blender to thoroughly blend everything into as fine a liquid as it's capable of (i'm using a Tiffany hand-me-down and it struggles but works).



This tastes really nice so far. Very subtle, slightly sweet and nutty flavour with a lip-burning kick to follow up which seems to multiply with every subsequent taste.

I didn't think it was going to end up hot enough after adding the liquid I needed to make it thin enough, so I threw in another half-dozen chopped habaneros when it was nearly done, cooked them for a while and re-blended. This might be the last hot sauce I make for a while too, so I wanted to make sure it counted!

I'd probably rate this 7.5 out of 10 for heat.

Update So 6 months on, this is definitely my favourite sauce. I think it could use a bit more heat but the flavour goes with lots of things - i sometimes use it instead of butter on sandwiches, for dipping chips (it gets hot!), or to add a pleasant kick to pasta, steak, or a snag. Please comment if you give it a go ...

Hot Sauce #1 - Toxic Lime

Back with another sauce - this time I thought i'd try something of a Thai theme: lime, kaffier lime, lemongrass, and fresh coriander. I just grabbed a few things from the garden and the pantry that seemed to match and came up with my second sauce, Sauce #1 - Toxic Lime.


350gApples, cored.
60gFresh coriander, including root and stem.
70gFresh lemongrass.
40gFresh ginger.
12gFresh lemon basil leaves.
2Kaffir lime leaf.
1Kaffir lime zest.
2Limes, juice and zest.
200gCayesan chillies (large fleshy chillies)
100gHabaneros (about 15)
1/2 cupMalt vinegar.
30gPalm sugar.
1/2 cupCane Sugar.
1 cupWater.
1 tspCitric Acid.
2 tblMushroom soy sauce (Happy Boy Brand)
100mlOlive oil.


Cut the ginger, lemongrass and lime leaf into fine slices and then put into a food processor and process until finely chopped. Add all of the other fresh ingredients (everything until and including the chillies) and blend thoroughly into a fine paste.

Place into a medium sized saucepan on a low heat and add the liquids and bring to a slow simmer.

Stir in the sugar and simmer everything slowly 'until done' - 30+ minutes.

Use a stick blender to liquefy everything together as much as possible.



This has a slightly strange flavour - despite lime and chilli seeming a good match I've never really found something where it truly 'worked'. Perhaps the kaffier lime zest is a little overpowering, as is the coriander. I will have to leave it for a bit and try it with some suitable food (might go well with pork?).

It definitely has a lime flavour to it though, which is what I was trying to achieve so at this point i'd call it a success. And it smells wonderful. It's probably about a 7 out of 10 for heat as with the previous recipe.