Hi,
AFAIK sampling in OpenGL occurs at pixel centers. That would explain
your "quarter" effect.
As for the inconsistency in the center of the output, I can only speculate.
Things I would try:
Use TextureRectangle instead of normal texture (coordinates are then
closer to pixel values)
Try various output sizes to see if it is a sampling/math problem in the
card implementation.
Use a shader to do the sampling to see what is going wrong where.
jp
Guy wrote:
Hello,
I have a problem with GL_LINEAR filtering of a float texture.
I created a 2D float image with the values
100 100
200 200
I stretched this image over 300x300 float buffer.
The results are defiantly not a linear interpolation of these values.
1. The upper and lower quarters of the image are constants. The
constant value is the value that should have been only in the
upper most and lower most rows. It seems the interpolation of a
pixel with 0.001 coordinates for example, takes into consideration
values with negative texture coordinates. (I can't find any other
explanation).
2. Between these quarters, the image SEEMS to be linearly
interpolated, but it is not. I would expect the differences
between two sequential pixels to be constant, but the differences
are not the same.
I had this problem both with OSG1.2 and OSG2.0. I don't believe it has
anything to do with the OSG code, but with the hardware implementation.
I used the following code to create the texture:
image->setImage( width,
height,
1,
GL_RGBA16F_ARB,
GL_RGBA,
GL_FLOAT,
(unsigned char*)(new
float[4*width*height]),
osg::Image::USE_NEW_DELETE);
osg::Texture2D* rtexture = new osg::Texture2D;
//------------------------------------------------------------------------
// make the texture float texture
//------------------------------------------------------------------------
rtexture->setInternalFormat(GL_RGBA16F_ARB);
rtexture->setWrap(osg::Texture2D::WRAP_S,
osg::Texture2D::CLAMP_TO_EDGE);
rtexture->setWrap(osg::Texture2D::WRAP_T,
osg::Texture2D::CLAMP_TO_EDGE);
rtexture->setFilter(osg::Texture2D::MIN_FILTER,osg::Texture2D::LINEAR);
rtexture->setFilter(osg::Texture2D::MAG_FILTER,osg::Texture2D::LINEAR);
rtexture->setSourceFormat(GL_RGBA);
rtexture->setSourceType(GL_FLOAT);
//========================================================================
rtexture->setImage(image);
rtexture->setTextureSize(image->s(), image->t());
rtexture->setResizeNonPowerOfTwoHint(false);
I also tried it with 32 bit float image.
I'm using windows XP and nvidia 8800.
Any ideas how to apply a true linear mapping for textures?
Thanks,
Guy.
------------------------------------------------------------------------
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