Gaussian blur filter and “shimmering”

Question

I have a fairly bog-standard Gaussian blur filter (seperable) I'm using to make a glow effect in my pipeline. At the moment I'm rendering it only to 256x256 texture and then doing a very simple additive combine with the scene to get the effect.

Due to the low resolution of the blur texture, I get quite a bit of "shimmering" of the blur when the camera moves around. Actually I noticed this in the Mass Effect 2 game, where they "glow" the specular highlights and shimmering is visible there.

Does anyone know of any technique for reducing or eliminating shimmering/aliasing that doesn't involve rendering the blur to a higher resolution texture (and so killing performance)? I'm already using GL_LINEAR in my samplers, which helps a bit, but not all that much.

Here's a little YouTube clip of the blur, although it's hard to see after YouTube have downsized and messed around with the clip, unfortunately.

Note that I read this article about real-time glow, and they briefly mention shimmering, but don't go into detail about solutions.

For completeness, I include my blur shader below.

#version 420

// Input uniforms.

uniform vec2 uniform_TexelSize;
uniform float uniform_BlurScale;
uniform float uniform_BlurStrength;
uniform int uniform_Orientation;
uniform int uniform_BlurAmount;

// Shared.

in vec2 attrib_Fragment_Texture;

// Samplers

uniform sampler2D Map_Diffuse;

// Result

out vec4 Out_Colour;

// Local Functions

float Gaussian (float x, float deviation)
{
    return (1.0 / sqrt(2.0 * 3.141592 * deviation)) * exp(-((x * x) / (2.0 * deviation)));  
}

// Main

void main(void)
{
    float halfBlur = float(uniform_BlurAmount) * 0.5;
    float deviation = halfBlur * 0.35;

    deviation *= deviation;

    float strength = 1.0 - uniform_BlurStrength;

    vec4 colour = vec4(0.0), texColour = vec4(0.0);


    // Horizontal or vertical?

    if (uniform_Orientation == 0)
    {        
    // Horizontal.

        for (int i = 0; i < 10; ++i)
        {
            if (i >= uniform_BlurAmount)
                break;

            float offset = float(i) - halfBlur;

            texColour = texture2D(Map_Diffuse, attrib_Fragment_Texture + vec2(offset * uniform_TexelSize.x * uniform_BlurScale, 0.0)) * Gaussian(offset * strength, deviation);

            colour += texColour;
        }
    }
    else
    {
        // Vertical.

        for (int i = 0; i < 10; ++i)
        {
            if (i >= uniform_BlurAmount)
                break;

            float offset = float(i) - halfBlur;

            texColour = texture2D(Map_Diffuse, attrib_Fragment_Texture + vec2(0.0, offset * uniform_TexelSize.y * uniform_BlurScale)) * Gaussian(offset * strength, deviation);

            colour += texColour;
        }
    }

    Out_Colour = colour;    
}

Answer 1

OK, I fixed the problem by implementing a different way of doing it I found here. I've made this a bit quicker by doing the set-up of the incremental Gaussian in the vertex shader and passing that into the pixel shader (no point in doing it in the pixel shader when it's not dependent on anything else).

It still shimmers, but it's no way near as bad as it was before. Not sure why (!). Also it's a bit quicker with the incremental, rather than the full evaluation per-pixel.