Re: [Openexr-devel] Reading an arbitrary rectangle from OpenEXR

[Florian Kainz]

Hi Mikhail,

the IlmImf library only allows access to whole scan lines or tiles.
In your example, the bRPixels, bGPixels and bBPixels arrays must be
as wide as the data window in the file you are reading.
The arguments to the readPixels() function specify only the top and
bottom boundaries of the region of interest.  There is no way to
specify left and right boundaries.

We could add a version of readPixels() that would let you specify
minimum and maximum x and y coordinates, but that function would not
necessarily be any faster than user code that reads complete scan
lines and discards the pixels it doesn't need.

Florian


Михаил wrote:
> Is it possible to read an arbitrary rectangle from OpenEXR file?
> I tried to do so but failed, maybe i am doing something wrong?
>
> here is the small test program below that doesnt work for me.
> Kind of errors that appear in debug mode pointing that something wrong  
> occurs in memory accesses.
>
> #define PLATFORM_WIN32
> #define HAVE_IOS_BASE
> #define HAVE_STL_LIMITS
> #define NOMINMAX
> #include <windows.h>
>
> #include <half.h>
> #include <ImfInputFile.h>
> #include <ImfOutputFile.h>
> #include <ImfPixelType.h>
> #include <ImfChannelList.h>
> #include <ImfHeader.h>
> #include <ImfArray.h>
> #include <ImathBox.h>
>
> #include <iostream>
>
> using namespace Imf;
> using namespace Imath;
> using namespace std;
>
> void writeExr (const char fileName[],    const half *rPixels, const 
> half  *gPixels,   
>                              const half *bPixels, int width, int height){
>     Header header (width, height);
>     header.channels().insert ("R", Channel (HALF));
>     header.channels().insert ("G", Channel (HALF));
>     header.channels().insert ("B", Channel (HALF));
>     OutputFile file (fileName, header);
>     FrameBuffer frameBuffer;
>     frameBuffer.insert ("R", Slice (HALF, // type
>         (char *) rPixels, // base
>         sizeof (*rPixels) * 1, // xStride
>         sizeof (*rPixels) * width)); // yStride
>     frameBuffer.insert ("G", Slice (HALF, // type
>         (char *) gPixels, // base
>         sizeof (*gPixels) * 1, // xStride
>         sizeof (*gPixels) * width)); // yStride
>     frameBuffer.insert ("B", Slice (HALF, // type
>         (char *) bPixels, // base
>         sizeof (*bPixels) * 1, // xStride
>         sizeof (*bPixels) * width)); // yStride
>     file.setFrameBuffer (frameBuffer);
>     file.writePixels (height);
> }
>
> int main(){
>
>     cout<<"Generating image 'A'"<<endl;
>
>     int aW = 1024;
>     int aH = 512;
>     Array2D<half>aRPixels(aH, aW);
>     Array2D<half>aGPixels(aH, aW);
>     Array2D<half>aBPixels(aH, aW);
>
>     for( int y=0; y<aH; ++y)
>         for( int x=0; x<aW; ++x){
>             half value = fmod(((float)y/aH+(float)x/aW)*20.0,1);
>             aRPixels[y][x] = value;
>             aGPixels[y][x] = value*0.5;
>             aBPixels[y][x] = value*0.25;
>         }
>
>     cout<<"Writing a.exr..."<<endl;
>
>     writeExr( "a.exr", &aRPixels[0][0], &aGPixels[0][0], 
> &aBPixels[0][0], aW,  aH);
>
> // here is interesting part begins...................................
>
>     cout<<"Reading part of image 'A'"<<endl;
>
>     Array2D<half>bRPixels;
>     Array2D<half>bGPixels;
>     Array2D<half>bBPixels;
>
>     InputFile file ("a.exr");
>
>     Box2i dwA = file.header().dataWindow(); //original data window 
> stored in  a file
>     int wA = dwA.max.x - dwA.min.x + 1;
>     int hA = dwA.max.y - dwA.min.y + 1;
>
>     Box2i dw; //defining a window of our interest
>     dw.min.x = 100;    dw.max.x = 227;
>     dw.min.y = 50;    dw.max.y = 249;
>     int width = dw.max.x - dw.min.x + 1;
>     int height = dw.max.y - dw.min.y + 1;
>
>     bRPixels.resizeErase (height, width);
>     bGPixels.resizeErase (height, width);
>     bBPixels.resizeErase (height, width);
>
>     FrameBuffer frameBuffer;
>     int xstride = sizeof (bRPixels[0][0]) * 1;
>     int ystride = sizeof (bRPixels[0][0]) * width;
>     int dataOffset = dw.min.x + dw.min.y * width;
>     frameBuffer.insert ("R", Slice (HALF, (char *) (&bRPixels[0][0] -  
> dataOffset), xstride, ystride,    1, 1, 0.0));
>     frameBuffer.insert ("G", Slice (HALF, (char *) (&bGPixels[0][0] -  
> dataOffset), xstride, ystride,    1, 1, 0.0));
>     frameBuffer.insert ("B", Slice (HALF, (char *) (&bBPixels[0][0] -  
> dataOffset), xstride, ystride,    1, 1, 0.0));
>     file.setFrameBuffer (frameBuffer);
>     file.readPixels (dw.min.y, dw.max.y);
>
>     writeExr( "b.exr", &bRPixels[0][0], &bGPixels[0][0], 
> &bBPixels[0][0],  width, height);
>
>     return 0;
> }
>
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