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32bit (int) Buffer to Greyscale/Colour-mapped Image in OpenGL, Single Channel 32 bit Texture or TBO?

I have an int buffer of intensity values, I want to display this as a greyscale/colour-mapped image in OpenGL.

What is the best way to achieve this?

Standard Texture?

Can I do it via a standard glTexture, so something like:

gl.TexImage2D(OpenGL.GL_TEXTURE_2D, 0, OpenGL.GL_R32f, width, height, 0, OpenGL.GL_RED_INTEGER, OpenGL.GL_UNSIGNED_INT, pixels);

In the shader I am under the impression I would use it the same as any other texture except I would use usampler2D instead of sampler2D, at which point I would get the true integer value (i.e. not 0-1 range).

TBO?

Or would it be better to achieve with a TBO and do something like:

gl.TexBuffer(OpenGL.GL_TEXTURE_BUFFER, OpenGL.GL_R32F, bufferID);

In terms of the shader I am actually quite confused. I have seen things like g = texelFetch(u_tbo_tex, offset + 1).r.. So I am guessing I would have to translate the texture coordinates into an offset, something like:

int offset = tex_coord.s + (tex_coord.t * imageWidth);

but then texelFetch actually returns a vec4, so presumably I would use:

int intensity = texelFetch( buffer,  offset).r

But then as tex_coord.s & t are in 0-1, that would imply the need to:

int offset = tex_coord.s*imageHeight + ((tex_coord.t * imageWidth) * imageWidth);

Other Buffer

I have very little experience with buffer objects I feel like really all I am doing is using a buffer in GL....so I do feel like I am over complicating it and I am missing the "penny drop".

Important Notes

  • Why Int? : In some cases I do some manipulation on the data before turning into a colour and would prefer to do this at 32 bit precision to avoid potential precision errors. Arguably it might not make a difference as it eventually becomes a screen color...
  • Data update frequency: the intensity data is updated occasionally by user events but certainly not multiple times per frame (so I am presuming STATIC is more appropriate then DYNAMIC in this case?)
  • Use: The data is mainly for GL so _DRAW There is the possibility that the application could make use of GL to compute some values for it but I would probably create a separate READ buffer in this case
  • The highest integer value I have seen so far is "90,000" so I know it goes out of the 16 bit integer range.

Note: I am doing this through SharpGL and I have been unable to test at the moment as it has no definition for GL_R32f, so I shall have to find the gl.h on my windows platform (always fun) and add the correct const number*

Solution

  • You can use a normal texture with integer/unsigned integer format:

     gl.TexImage2D(OpenGL.GL_TEXTURE_2D, 0, OpenGL.GL_R32UI, width, height, 0, OpenGL.GL_RED_INTEGER, OpenGL.GL_UNSIGNED_INT, pixels);

    In the shader you can use a usampler2D, since the texture function has an overload for this you directly get the integer values:

     uniform usampler myUTexture;
 uint value = texture(myUTexture, texCoord).r;

    Edit: Just for completness: texelFetch has also an overload for all types of 2d-sampler. The difference between texture and texelFetch is the coordinate system used ([0,1] for texture and pixel coordinates for texelFetch) and that texelFetch does not take any interpolation/mipmap into account.