Re: how to achieve fast image 90 deg rotation in C
From: Severian (severian_at_chlamydia-is-not-a-flower.com)
Date: 06/16/04
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Date: Wed, 16 Jun 2004 13:25:42 -0400
On Wed, 16 Jun 2004 09:27:20 -0700, "Richard Zhu" <yzhu@algis.ca>
wrote:
>"Severian" <severian@chlamydia-is-not-a-flower.com> wrote in message
>news:6t2sc0hm8m2a9smih74dqaeb9l8shv94dr@4ax.com...
>> On Mon, 14 Jun 2004 11:11:31 -0700, "Richard Zhu" <yzhu@algis.ca>
>> wrote:
>>
>> >I have the following code to implement a 90 deg rot over an Image in VC,
>> >the speed can't reach what is required,
>> >what can I do to speed up, for width =1024, height =768 color image
>grabbed
>> >from the camera, there is no hardware rotation inside, so I need a real
>> >time rotation, on my 2.4G intel I need less than 10ms , but the following
>> >code gives about 60ms, I wonder if there still the possibilies to get
>there
>> >?
>> >
>> >I have checked, the slowing caused by locating chR1[y0*o_w+x0], ....
>> >
>> >
>> >int x1,y1,x0,y0,i;
>> > int n_w,n_h,o_w,o_h;
>> > int offset;
>> > n_w=img_height;
>> > n_h=img_width;
>> > o_w=img_width;
>> > o_h=img_height;
>> >
>> > for(y1=0;y1<n_h;y1++)
>> > {
>> > offset=y1*n_w;
>> > for(x1=0;x1<n_w;x1++)
>> > {
>> > x0=y1;
>> > y0=o_h-1-x1;
>> > chR_rot[offset+x1]=chR1[y0*o_w+x0];
>> > chG_rot[offset+x1]=chG1[y0*o_w+x0];
>> > chB_rot[offset+x1]=chB1[y0*o_w+x0];
>> > }
>> > }
>> > *new_width=n_w;
>> > *new_height=n_h;
>>
>> I am not a cache expert, but it may be quicker to read rows and write
>> columns. You're reading columns, so you will have a lot more cache
>> misses.
>>
>> My program does a 1024x768 24-bit rotation in around 60-70ms on a P3
>> 800, so that should put you in the ballpark on your faster machine.
>>
>> If you have other image-oriented stuff to do, it might be worthwhile
>> to investigate the Intel performance libraries, which will do things
>> like this with highly-optimized MMX, SSE and SSE2 code.
>>
>> IPL requires 10-20ms to rotate a 1024x768 image in on my P3 800.
>>
>> --
>> Sev
>
>I have tested, that's great !, it's truely improved as you said,
>
> for(y0=0;y0<o_h;y0++)
> {
> offset=y0*o_w;
> for(x0=0;x0<o_w;x0++)
> {
> x1=o_h-1-y0;
> y1=x0;
> chR_side[y1*n_w+x1]=chR1[offset+x0];
> chG_side[y1*n_w+x1]=chG1[offset+x0];
> chB_side[y1*n_w+x1]=chB1[offset+x0];
> }
> }
>
>use offset in read,
>now the time reduced to 40ms from from60-70ms.
>
>I think I still need to try IPL.
You're welcom! Depending on your compiler and cache, you may get more
improvement if you write separate loops for each channel. Also, be
sure you're compiling with optimization.
-- Sev
- Next message: WSPC: "International Journal of Pattern Recognition and Artificial Intelligence - Vol. 18, No. 4"
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