Christian Bauer; 3/2003

Tweak your hdrs / Synthesised hdrs


It's always the question how good a hdri is and if you can use it in a smart way in your 3D app. At the moment everyone want very high sized hdrs which can be used as a BG image of a rendering too, but IMHO it's better using a smaller hdr which uses less memory and use a high size and easy to create LDR image for BG images. I also think that blurring a hdr is a good way to reduce flickering in animations.

What we all want is a very high dynamic range and this is absolutley independent from the imagesize. The problem is the creation process since no digicams are available at the moment which outputs such hdrs. You have to shoot a series of LDR's and compile it with hdrshop or whatever. This limites us to the range a camera can reproduce with it's exosuretime from ~2sec. to 1/4600 sec. which are not more than 13-fstops and a range from 100:1 - 1000:1 for outdoorimages depending on the lightsituation.

This limited range is the reason why such a hdr gives you a blueish rendering... it's like only the sky is lighten the scene and the sun is somehow only a brighter cloud. A hdr with a bright sun changes that behave, because the sky is not very bright anymore and the sun is a real massive lightsource.

The reason for blueish renderings is often the missing dynamic range

Here are two renderings with the same hdr before and after our tweak:

sample1This is a tweaked hdri rendering


The good thing is that you do not define the characteristic of your hdr while compiling it. You can add the bright sun later with software.



What we need:

For this tutorial we need the trailversion of PhotogeneticsHDR from ...i used an old DL so the userinterface can be maybe different. Photogentics is something like the Photoshop for HDRs. There is a free alternative, Hdrie, which can do at least work with brushes.


What we will do:

We will take one of my hdrs from the library and synthesise it with a sun in two ways:

1. Paint a sun on it with a brush
2. change the levels on bright areas

Both ways are good, but i prefer the first one, because it gives you better controll over the result.
Just to think about another cool way to use that technique in a creative way: get you a normal skypanorama (from 1000skies or similar) with very high size of 15000px and paint a sun on it....and yes..this will be a highsized and highrange HDRimage... it's crazy but..hmmm ..tasty at least ;)



Paint the sun on a hdr image

Here we go. up PG (Photogenetics) and load an hdr image.

You will see the image and an optionbar. The first thing is the control bar where you can change the brush and have controll over your layers.


When you change to the Inspectionbar you can change the exposure of your image. PG will show you the range and not the f-stops, so you have a pretty good measuring tool here.



Now change your exposure to the point where you can see the area of the sun in darkgray and the rest of the image black. This is the exposure where we will add the sun.

decrease the exposure value


Before we start painting you have to change the color of your brush to white ..or something you like.

select the color for painting on the hdr image


Then we have to choose a brush. Go back to the Controlbar and click on the image with the brush. I decide to make a spectaculary lensflare ;)

You can define size, pressure and transparency below.. this is the point where you have to start plaing with it.

select a brush


place the cursor over the sun and paint a nice sun on it. The Painting is stored in a new layer and can be cleared if something goes wrong.

paint the virtual sun on the hdri


Go back to the Inspectionbar and change the exposure and look how high the range is. If it's too low then do the changecolor/paint part again until the range is good enough for you.

Save it and use it in your 3D app and see the difference.

I have upped two lowres hdrs for you to compair: One untouched output from hdrshop and one synthesised with Photogenetics.


One thing to say: this way works good with hdrs where you can clearly see the sun, if a cloud is covering the sun, then you have to change the levels of the bright areas and go the way i will upload next time.






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