**Topic :**Bilinear Filtering 101

**Author :**Paul Nettle

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**Bilinear Filtering 101**

by Paul Nettle

Matti Dahlbom wrote in article

*<34190090.5228@cc.hut.fi>...*

> Has anyone coded a routine performing same as the modern 3D cards, the

> texture smoothing thingy? id like to know how.

> Has anyone coded a routine performing same as the modern 3D cards, the

> texture smoothing thingy? id like to know how.

I believe you're referring to "Bilinear Filtering." The process is simple, but slow. I've got an acceptable frame-rate for KAGE, so I can tell you the trick is in the blending. Here's the basic process.

**BILINEAR FILTERING 101**

First, understand that a scalars I refer to are values that ranges from 0.0f - 1.0f, and all RGB values are scalars of this type.

Normally, what software renderers do is called "point sampling." As they scan through screen-space, they sample points from the texture. When you get up real close to a texture, and the texture density in screen space decreases (i.e. the texels get bigger), the renderer ends up sampling the same texture multiple times. This is where the filtering comes in handy.

As you're sampling these points from your texture, you need to figure out where you are inside that texel (you should have this information already by taking the fractional component of your U/V values). From this, you can determine which are the "closest four texels." This is actually confusing, so I'll explain this in a little more detail.

Consider the "origin" of a texel to be the upper-left corner of it. This point gets the full intensity of the current texel, and none of the remaining three of the "nearest four texels". But as you start sampling points from the texture that are moving right, along the top edge of that texel, you start to incorporate the next texel over, and the two values get blended. That's only half of it, you also need to account for the next texel down, and how close to it you are...

Just so this is clear, get some paper and pencil, and draw this up: You have a 10x10 texture (draw the texture pixels BIG). The sampled point four our example is [3.7, 4.8].

Since both 3.7 and 4.8 would both round UP, the "current texel" is the upper-left of the "nearest four" at [3,4]. The upper-right would be at [4,4], the lower left at [3,5] and the lower right at [4,5].

Put these four texels into the variables texelUL, texelUR, texelLL, texelLR with your fractional U and V values in fracU & fracV. From here this pseudo code snippet *SHOULD* work (I'm pretty sure it's correct, even though I'm typing it in off the top of my head):

// These are the scalars for each texel (ul for texelUL, ur for texelUR, etc.)

float ul = (1.0f - fracU) * (1.0f - fracV);

float ll = (1.0f - fracU) * fracV;

float ur = fracU * (1.0f - fracV);

float lr = fracU * fracV;

// Using the scalars, calculate the new red, green and blue components in scalar format

float red = ul * texelUL.red + ll * texelLL.red + ur * texelUR.red + lr * texelLR.red;

float grn = ul * texelUL.grn + ll * texelLL.grn + ur * texelUR.grn + lr * texelLR.grn;

float blu = ul * texelUL.blu + ll * texelLL.blu + ur * texelUR.blu + lr * texelLR.blu;

// Put it into a 24-bit pixel, THIS is what gets put to the screen (provided RGB ordering)

int newPixel = ((int) (red * 255.0f) << 16) | ((int) (grn * 255.0f) << 8) | (int) (blu * 255.0f);

Garsh, I hope this makes sense. Good luck!

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