var
BitmapImage: TBitmap;
PNGImage: TPngImage;
PngStream: TStream;
begin
// draw on BitmapImage
...
PNGImage := TPngImage.Create;
PNGStream := TMemoryStream.Create;
Try
PNGImage.Assign(BitmapPicture.Bitmap); // Step 1: assign TBitmap to PNG
PNGImage.SaveToStream(PNGStream); // Step 2: save PNG to stream
WS.Shapes.AddPicture(PNGStream,PNGImage.Width,PNGImage.Height); // Step 3: Add PNG from Stream to Excel
finally
PNGImage.Free;
PNGStream.Free;
end;
...
这是通过70000张图片测试的,以下是时间：
第1步：7秒
第2步：93秒
第3步：6秒
为什么保存到Stream这么慢？有什么建议可以优化吗？
使用Delphi XE7
编辑
这是带有简单bmp的示例(MCVE),它将转换为PNG,然后保存到流中.只是为了另一个验证,我添加了SaveToFile,当然需要更长时间,但它保存到磁盘,所以我认为可以接受.
img1.bmp为49.5KB,保存的PNG为661字节.
链接到img1.bmp = http://www.filedropper.com/img1_1
TMemoryStreamAccess = class(TMemoryStream)
end;
procedure TForm1.Button1Click(Sender: TObject);
var BitmapImage:TBitmap;
PNGImage:TPngImage;
PNGStream:TMemoryStream;//TStream;
i,t1,t2,t3,t4,t5,t6: Integer;
vFileName:string;
begin
BitmapImage:=TBitmap.Create;
BitmapImage.LoadFromFile('c:\tmp\img1.bmp');
t1:=0; t2:=0; t3:=0; t4:=0; t5:=0; t6:=0;
for i := 1 to 70000 do
begin
PNGImage:=TPngImage.Create;
PNGStream:=TMemoryStream.Create;
try
t1:=GetTickCount;
PNGImage.Assign(BitmapImage);
t2:=t2+GetTickCount-t1;
t3:=GetTickCount;
TMemoryStreamAccess(PNGStream).Capacity := 1000;
PNGImage.SaveToStream(PNGStream);
// BitmapImage.SaveToStream(PNGStream); <-- very fast!
t4:=t4+GetTickCount-t3;
finally
PNGImage.Free;
PNGstream.Free
end;
end;
showmessage('Assign = '+inttostr(t2)+' - SaveToStream = '+inttostr(t4));
end;
解决方法
This is tested with 70000 images and here are the timings:
Step 1: 7 s
Step 2: 93 s
Step 3: 6 s
Why is saving to Stream so slow?
让我们来处理一些数字：
步骤1：7s = 7000ms. 7000/70000 =每张图像0.1ms
步骤2：93s = 93000ms.每张图像93000/70000 = ~1.33ms
第3步：6s = 6000ms.每张图像6000/70000 = ~0.086ms
你认为每个SaveToStream()的1.33毫秒是慢吗？你只是做了很多,所以他们随着时间的推移加起来,就是这样.
话虽这么说,内存中的PNG数据不会被压缩.保存数据时会压缩它.这是减速的一个原因.此外,保存PNG会对流进行大量写入操作,这会导致流执行多个内存(重新)分配(TPNGImage还会在保存期间执行内部内存分配),这是另一个减速.
Any suggestion to optimize this?
您无法对压缩开销做任何事情,但在调用SaveToStream()之前,您至少可以预先将TMemoryStream.Capacity设置为合理的值,以减少TMemoryStream在写入期间需要执行的内存重新分配.你不需要精确它.如果写入流导致其大小超过其当前容量,则只会相应地增加其容量.由于您已经处理了70000个图像,因此请考虑它们的平均大小并向其添加几个KB,并将其用作初始容量.
type
TMemoryStreamAccess = class(TMemoryStream)
end;
var
BitmapImage: TBitmap;
PNGImage: TPngImage;
PngStream: TMemoryStream;
begin
// draw on BitmapImage
...
PNGImage := TPngImage.Create;
Try
PNGImage.Assign(BitmapPicture.Bitmap); // Step 1: assign TBitmap to PNG
PNGStream := TMemoryStream.Create;
try
TMemoryStreamAccess(PNGStream).Capacity := ...; // some reasonable value
PNGImage.SaveToStream(PNGStream); // Step 2: save PNG to stream
WS.Shapes.AddPicture(PNGStream,PNGImage.Height); // Step 3: Add PNG from Stream to Excel
finally
PNGStream.Free;
end;
finally
PNGImage.Free;
end;
...
如果仍然不够快,请考虑使用线程并行处理多个图像.不要按顺序处理它们.