I've been trying to achieve bloom using a 8bit render target using rgbm but after I failed several times I've been wondering if it was even possible.

If i'm not mistaken the alpha contain the data about the intensity so when I doing the offset to do my Gaussian blur am I messing up the encode information ?

here's the code I'm using to encode and decode my image

float MaxRange = 6;
float3 DecodeRGBM(float4 rgbm)
{
return rgbm.rgb * (rgbm.a * MaxRange);
}

float4 EncodeRGBM(float3 rgb)
{
float maxRGB = max(rgb.x,max(rgb.g,rgb.b));
float M = maxRGB / MaxRange;
M = ceil(M * 255.0) / 255.0;
return float4(rgb / (M * MaxRange), M);
}


I'm able to encode and decode any image but when I try to blur the image doesn't look right. Highly saturated and lot of crazy color.


And here's my blur function
( taken from matt pettineo)

/ Performs a gaussian blue in one direction
float4 Blur(float2 texcoord, float2 texScale, float sigma)
{
float4 color = 0;
for (int i = -6; i < 6; i++)
{
float weight = CalcGaussianWeight(i, sigma);
float2 texCoord = texcoord;
texCoord += (i / size) * texScale;
float4 sample = tex2D(BloomMap, texCoord);
sample.rgb = DecodeRGBM(sample);
sample.a = 1;
sample*=sample;
color += sample * weight;
}
return color;
}

I also I'm using xna and I'm able to blur the image using hdrblendable but Id prefer to use an 8 bit image so I can use a filter on it. ( xna doesn't support filter anymore for many format).

@ Hodgman thanks for the answer. I'm encoding it back to rgbm but I forgot to put the function in my code.

@ mhagain. I'll try the divisor see if it works.

Since the rgbm is also storing some of the luminance info in the rgb that might be the real problem.

But still I'm wondering how the cryengine guy did their motion blur using rgbm format. I'm getting some pretty harsh line on my bloom when I use the point filtering.