有没有比使用倒置坐标的嵌套循环更快的方法来将大位图旋转90度或270度？
这些位图是8bpp的，通常为2048x2400x8bpp。
目前我只是通过复制参数的倒置来实现，大致如下（伪代码：
for x = 0 to 2048-1
for y = 0 to 2048-1
dest[x][y]=src[y][x];
（实际上我使用指针来完成，以获得更快的速度，但数量级大致相同）
GDI在处理大型图像时非常慢，而纹理的GPU加载/存储时间（GF7显卡）与当前CPU时间的数量级相同。
有什么提示或建议吗？一个原地算法甚至更好，但速度比原地更重要。
目标是Delphi，但这更是一个算法问题。 SSE（2）向量化没有问题，对我来说，这是足够大的问题，可以用汇编语言编写它。
回复 Nils 的答案
- 图像 2048x2700 -> 2700x2048
- 编译器 Turbo Explorer 2006 启用优化。
- Windows：电源计划设置为“始终保持开启”。(重要！！！)
- 机器：Core2 6600 (2.4 GHz)
time with old routine: 32ms (step 1)
time with stepsize 8 : 12ms
time with stepsize 16 : 10ms
time with stepsize 32+ : 9ms
Meanwhile I also tested on a Athlon 64 X2 (5200+ iirc), and the speed up there was slightly more than a factor four (80 to 19 ms).
The speed up is well worth it, thanks. Maybe that during the summer months I'll torture myself with a SSE(2) version. However I already thought about how to tackle that, and I think I'll run out of SSE2 registers for an straight implementation:
for n:=0 to 7 do
begin
load r0, <source+n*rowsize>
shift byte from r0 into r1
shift byte from r0 into r2
..
shift byte from r0 into r8
end;
store r1, <target>
store r2, <target+1*<rowsize>
..
store r8, <target+7*<rowsize>
So 8x8 needs 9 registers, but 32-bits SSE only has 8. Anyway that is something for the summer months :-)
Note that the pointer thing is something that I do out of instinct, but it could be there is actually something to it, if your dimensions are not hardcoded, the compiler can't turn the mul into a shift. While muls an sich are cheap nowadays, they also generate more register pressure afaik.
The code (validated by subtracting result from the "naieve" rotate1 implementation):
const stepsize = 32;
procedure rotatealign(Source: tbw8image; Target:tbw8image);
var stepsx,stepsy,restx,resty : Integer;
RowPitchSource, RowPitchTarget : Integer;
pSource, pTarget,ps1,ps2 : pchar;
x,y,i,j: integer;
rpstep : integer;
begin
RowPitchSource := source.RowPitch; // bytes to jump to next line. Can be negative (includes alignment)
RowPitchTarget := target.RowPitch; rpstep:=RowPitchTarget*stepsize;
stepsx:=source.ImageWidth div stepsize;
stepsy:=source.ImageHeight div stepsize;
// check if mod 16=0 here for both dimensions, if so -> SSE2.
for y := 0 to stepsy - 1 do
begin
psource:=source.GetImagePointer(0,y*stepsize); // gets pointer to pixel x,y
ptarget:=Target.GetImagePointer(target.imagewidth-(y+1)*stepsize,0);
for x := 0 to stepsx - 1 do
begin
for i := 0 to stepsize - 1 do
begin
ps1:=@psource[rowpitchsource*i]; // ( 0,i)
ps2:=@ptarget[stepsize-1-i]; // (maxx-i,0);
for j := 0 to stepsize - 1 do
begin
ps2[0]:=ps1[j];
inc(ps2,RowPitchTarget);
end;
end;
inc(psource,stepsize);
inc(ptarget,rpstep);
end;
end;
// 3 more areas to do, with dimensions
// - stepsy*stepsize * restx // right most column of restx width
// - stepsx*stepsize * resty // bottom row with resty height
// - restx*resty // bottom-right rectangle.
restx:=source.ImageWidth mod stepsize; // typically zero because width is
// typically 1024 or 2048
resty:=source.Imageheight mod stepsize;
if restx>0 then
begin
// one loop less, since we know this fits in one line of "blocks"
psource:=source.GetImagePointer(source.ImageWidth-restx,0); // gets pointer to pixel x,y
ptarget:=Target.GetImagePointer(Target.imagewidth-stepsize,Target.imageheight-restx);
for y := 0 to stepsy - 1 do
begin
for i := 0 to stepsize - 1 do
begin
ps1:=@psource[rowpitchsource*i]; // ( 0,i)
ps2:=@ptarget[stepsize-1-i]; // (maxx-i,0);
for j := 0 to restx - 1 do
begin
ps2[0]:=ps1[j];
inc(ps2,RowPitchTarget);
end;
end;
inc(psource,stepsize*RowPitchSource);
dec(ptarget,stepsize);
end;
end;
if resty>0 then
begin
// one loop less, since we know this fits in one line of "blocks"
psource:=source.GetImagePointer(0,source.ImageHeight-resty); // gets pointer to pixel x,y
ptarget:=Target.GetImagePointer(0,0);
for x := 0 to stepsx - 1 do
begin
for i := 0 to resty- 1 do
begin
ps1:=@psource[rowpitchsource*i]; // ( 0,i)
ps2:=@ptarget[resty-1-i]; // (maxx-i,0);
for j := 0 to stepsize - 1 do
begin
ps2[0]:=ps1[j];
inc(ps2,RowPitchTarget);
end;
end;
inc(psource,stepsize);
inc(ptarget,rpstep);
end;
end;
if (resty>0) and (restx>0) then
begin
// another loop less, since only one block
psource:=source.GetImagePointer(source.ImageWidth-restx,source.ImageHeight-resty); // gets pointer to pixel x,y
ptarget:=Target.GetImagePointer(0,target.ImageHeight-restx);
for i := 0 to resty- 1 do
begin
ps1:=@psource[rowpitchsource*i]; // ( 0,i)
ps2:=@ptarget[resty-1-i]; // (maxx-i,0);
for j := 0 to restx - 1 do
begin
ps2[0]:=ps1[j];
inc(ps2,RowPitchTarget);
end;
end;
end;
end;
Update 2 Generics
I tried to update this code to a generics version in Delphi XE. I failed because of QC 99703, and forum people have already confirmed it also exists in XE2. Please vote for it :-)
Update 3 Generics Works now in XE10
Update 4
In 2017 i did some work on a assembler version for 8x8 cubes of 8bpp images only and related SO question about shuffle bottlenecks where Peter Cordes generously helped me out. This code still has a missed oportunity and still needs another looptiling level again to aggregate multiple 8x8 block iterations into pseudo larger ones like 64x64. Now it is whole lines again and that is wasteful.