Sign in to follow this  
jeremie009

HDR Bloom with RGBM encoding format

Recommended Posts

jeremie009    696
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).

Share this post


Link to post
Share on other sites
Hodgman    51338
[quote name='jeremie009' timestamp='1348291906' post='4982581']
when I doing the offset to do my Gaussian blur am I messing up the encode information ?
[/quote]You can't bilinearly filter or blend RGBM values.
e.g. 10*10 and 5*20 are the same, but if I blend between them I get 7.5*15 which is different.

So, yeah, if your bloom samples are in-between pixels, then you'll be doing strange things to your colour values. You could try making the BloomMap sampler use nearest/point filtering.

[edit]Also, at the end, instead of [font=courier new,courier,monospace]return color;[/font] you probably need [font=courier new,courier,monospace]return EncodeRGBM(color.rgb);[/font] so that the next blur pass works. Edited by Hodgman

Share this post


Link to post
Share on other sites
jeremie009    696
@ 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.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this