View Full Version : addition of two HDR (RGBE) values?

09-29-2008, 09:01 AM
Pardon me if this question might be more appropriate in the shading language discussion forum.

How do I add two RGBE values together (in a fragment shader)?

I'm sure you all know this better than I do but the true component intensities for an RGBE value is something like
r' = r * 2^e
g' = g * 2^e
b' = b * 2^e

Is there any fast method for adding two such colours together, and get a new RGBE value back? Is there any faster way other than to calculate the true intensities before I calculate the sum and then convert it back to the RGBE components?

09-29-2008, 12:21 PM
I don't know how much the following information is helpful to addition two RGBE.

In this web page (http://www.gamedev.net/reference/articles/article2208.asp) I found that:

Encoding and decoding using the RGBE format is easy. To encode a pixel using the RGBE format the following HLSL pixel shader 2.0 code can be used:

float4 EncodeRGBE8( in float3 rgb )
float4 vEncoded;
float maxComponent = max(max(rgb.r, rgb.g), rgb.b );
float fExp = ceil( log2(maxComponent) );
vEncoded.rgb = rgb / exp2(fExp);
vEncoded.a = (fExp + 128) / 255;
return vEncoded;

To decode a pixel using the RGBE format the following HLSL pixel shader 2.0 code can be used:

float3 DecodeRGBE8( in float4 rgbe )
float3 vDecoded;
float fExp = rgbe.a * 255 - 128;
vDecoded = rgbe.rgb * exp2(fExp);
return vDecoded;

Keep in mind that this is for RGBE8 not RGB9_E5. In this file (http://www.opengl.org/registry/doc/glspec30.20080811.pdf) at section 3.9.1 there is a general formula for encoding RGBE format.

10-01-2008, 05:41 AM
Thanks for the link. It does not really describe how to add two rgbe values effectively, but it has other interesting information of course.

As a quick solution right now I just add together the colour components from the two rgbe values. As for the exponent I simply keep the biggest. It is not right of course and I haven't really tested how it looks either, so I'm still looking for a solution here.

Another question is how I can control various aspects when decoding a colour (from RGBE to RGB). Does anyone have any idea how I can change the decoding so that brighter values are toned down a bit, perhaps linearly?