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  Tutorials 3D Graphics Tutorials HDRI tutorial

HDRI tutorial

seger Tutorials Oct 12, 2005

Step 1

Create a new, rather large document. I made mine 800x600 pixels. If you plan on printing your card, make the X and Y resolutions at 300dpi. Give it a white background.

image 1

Step 2

On the background layer, we'll apply a gradient. Make your foreground color either a dark red or a dark green and make your background color black. Apply a radial gradient to the background.

image 2

Step 3

Create a new layer and name it "tree reference." Get out the pencil tool. Choose the Circle (01) brush and make the color white. Drag a ruler to the center of the image. With the pencil tool, click once where you want the top of the tree to be, using the ruler as a reference. Now, hold Shift and you should see a line follow the mouse pointer from the point at the top of the tree. Click where you want the base of one side of the tree to be. Do the same for the other side. This will be our reference for our Christmas tree.

Introduction

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In this tutorial, I won't go into the technical and physical backgrounds of High Dynamic Range Image. Instead, I'll present you with a basic instruction about the photography of the images needed to produce HDRI's, the way to go about in Photoshop (which in CS2 has a feature incorporated that will produce a so-called Radiance document with little effort) and how to use the HDRI in 3D (I use Cinema4D but other software will differ only slightly)

Just a little background though: Did you notice that on a sunny day, your camera tells you that you need 1/30 seconds of exposure with a 2.8 lens opening
indoors and when you step outside it just needs 1/1000 seconds at f11.

In fact the difference between complete darkness and complete â??lightnessâ?? is pretty endless. A camera or your eyes can only see a portion of it without adjusting, and once they have, they can't see as well anymore what they saw before the adjustment. When you step back inside the house it takes a second to get used to the darkness. And when you take a picture with your camera still at 1/1000 at f11, the print is black.

In an HDRI, multiple situations are all combined in one image: indoor and outdoor at the same time so to say. You need a slider to make it visible to the eye,
but when you do, it is all there: the sphere of the sun and the inside of a drawer.

In 3D-software maths, this wide boundary or high dynamic range can be inverted to do something special: it can act as an actual light source.

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Ansel Adams (Click to enlarge)

HDRI photography: what do you need?

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If you have all of the items above, you're a rich man: Adobe Photoshop CS2 to convert different camera exposures into a Radiance image, a tripod and a camera that can shoot RAW-images. In this tutorial we'll use them all. But:

If you can't shoot RAW, that's OK. If you don't have Photoshop CS2, visit http://graphicslab.ict.usc.edu/HDRShop/ to get a free copy of HDRshop v1, which does a good job. It's only available to the Wintel platform though. They have a lot of fantastic information about the technical side of HDRI, too.

And if you don't have a tripod, you can ask your sister too stand really still for a while. No, I tripod is handy.

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The item on the top isn't mandatory either. If you don't have a professional probe, you can use a christmas ball. But again here, you can work without a perfectly reflecting sphere.

(You would use it to get 180 degrees of surrounding environment to encapsulate your 3D scene. In the following example I used simple rectangular images and for me that works just fine. If your scene contains smooth, dome-shaped objects, then you start needing a probe of some sort as the seams of a rectangular image would be visible.)

HDRI photography

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Click to enlarge

Open a RAW-image into Photoshop and Photoshop will open its RAW-image dialogue.

This photo of my studio door is pretty much balanced. You can more or less see what is inside and you sort of see what is outside. There is no pixel information in the highlights though. I check that by activating the Highlight-tickbox: image 8

When I do that, Photoshop gives me this feedback:

image 9

Pixels that are pure white are displayed as red. As you can see there's no information in the sky behind the door, nor is there between the blinds. If I want to see that information, I need to bring the exposure down. But when I do that, dark parts in the foto start to become pure black. I can check that by activating the Shadows-tickbox: image 10 (It is difficult to see -overexposure happens much easier than underexposure- but it's the blue areas.)

image 11

You can see, the top image is overexposed but you can see details in the door and the foreground.

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The photo below is underexposed but there is information in the sky. Well, on the particular day I made these shots we had an almost pure grey sky, so still not much to see but the rain drops on the glass become visible:

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Each of the photographs cover another part of the light situation and the range per photo is quite small. Going from the deepest shadows up to the brightest highlights, in this weather, I need to make about 6 to 7 photographs that I import into Photoshop CS2:

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In the dialogue that appears you can either choose multiple images or a folder containing a set of images or images that are already open in Photoshop.

If you didn't take the photos from a tripod and they have slightly different angles you can use this feature. Beware though, it can't make up for big differences and it's time consuming.

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After I browse to my folder with different exposures of my studio door, Photoshop starts to batch process all of them and finally comes up with this:

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Click to enlarge

All images that have a green mark: image 18 next to them will be incorporated in the final HDRI when I click OK. By default, they all have one. When I uncheck one, the preview is updated.

With the Set White Point slider I can view through the whole exposure gamma.

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I want 32 Bit/Channel because I need as much as information as I can into my 3D setup. When I click OK, Photoshop merges all the images into an HDRI that I Save As Radiance. Note that there's nothing really special to see about the HDRI at a glance. That is because your monitor simply can't show all the information it contains. But when you go to Image> Adjustments> Exposure and slide through Exposure, it is like day breaks and night falls..! (Like I said before, I made my HDRI from RAW-im-ages but you could also build it from TIFF's.)

I am going to light my astronaut character with the exact lighting conditions from my studio door.

HDRI in 3D software

Now that we have a Radiance image, we need to set up a simple setup in 3D and make it act as a light source. As I mentioned before, I will use Cinema4D (version 9) to do that. My object is an astronaut. It goed without saying that you can use anything to light with the HDRI output.

Make a new material and load the HDRI in the Luminance channel. Set Mix Mode to Multiply. My HDRI is very bright and if I don't bring down its Mix Strenght it will wash out the scene.

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Put a Sphere in the scene and set its radius to a large value so it will encapsulate everything else. Apply the HDRI to it plus a Compositing Tag.

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In the Attributes of the tag, turn off everything but Seen by Rays and Seen bij GI:

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Throw a Camera in with a Target and a Plane or a Floor object to be able to see the actual forming of shadows. And of course put in your model of choise. The whole thing looks as follows:

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Next, in Render Settings, activate Radiosity and in Options deactivate Auto Light: image 24

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I found the above settings after some trial and error. Parameters for the amount of light you should adjust are Mix Strenght in the Luminance channel of the HDRI or the Radiosity Strenght. It's best to adjust just one or you'll get confused.

Fighting artefacts in your render is done with Stochastic Samples and Min and Max Resolution.

At this stage of the process you need some patience. HDRI-lighting and can give some unexpected results at first and rendering can be slow.

In the reflection of the helmet of my astronaut you can clearly see the door and window of the studio. Notice that the brightest highlight, the one from the door, casts a complemetary shadow.

HDRI tutorial
Click to enlarge

This tutorial cannot cover everything there is to know about HDRI-imaging. There is more to find about the subject on the internet every day. Just Google HDRI and you're presented with a plethora of results. Some of them go into the physics, some of them into rendering, there are manufacturers who sell ready-made HDRIimages and there are many questions answered on the subject in forums. I hope I revealed the basics of making your own and use them yourself. Happy Rendering!

image 3

Step 4

Now we need an image of our Santa Claus. Go find one somewhere on the net, or you can use this one. Place him in the center of the tree. You can use the ruler to be sure he is center. Create a new layer and name it "tree." Get out the Paths Tool. Starting at the top of the tree, create a diagonal path down towards Santa's star.Â

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Step 5

Still using the paths tool, go to the paths dialog and switch from design to edit. Click a path and manuever the little squares to curve the lines, like I have below.

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Step 6

Download these brushes. Because they are a .tar.gz file, you will need a program such as 7zip to extract them unless you are running a Linux platform. Copy them into your Gimp brushes folder.Click the brush tool (don't worry if your path disappeared, we'll make it come back up later) and in the brush dialog, click the little refresh button. Your brushes should appear.

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Select the star brush I have selected above. In the other brush dialog, apply these settings:

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Go ahead and change the brush's size to 0.10 pixels instead of what it is now. Click the Paths tab next to the Layers tab. Double click on the white space on the only path listed. Our path should show up on the document. Click the "Stoke Path" button. Select the "Stroke With a Paint Tool" option and select the paint brush. Enable brush dynamics and click ok. You should have something like this:

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Step 7

Before we go any further, we need to install a couple of filters. Download these filters from the Gimp plugin registry and install them. To do that, simply save the SCM file avaliable for download and paste it into the Scripts folder, in the same archive as the Brush folder. To activate the script, go up to Filters >Script-Fu > Refresh Scripts. A menu should pop up next to the Filters Menu named "Script-Fu".

Open your ecard back up. Select your tree layer. Go to Script-Fu > Layer Effects > Color Overlay. Select a hue similar to either the dark green or dark red you chose at the beginning of the tutorial. Choose the setting Overlay and keep the opacity at 100%. Click ok.

Now go to Script-Fu > Layer Effects > Outer Glow. Keep the default settings, except lower the opacity to 14% and change the blending mode to Screen. Click ok. (Note I have hidden the tree reference layer below.)

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Step 8

Now let's add a star at the top of the tree. Create a new layer and name it "star." Paint a white star at the top of the tree. Also paint a smaller star at the top of the star above Santa's head. Apply outer glows and color overlays like we did in the last step.

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Step 9

Now we'll add in some text. Type in "Christmas," or whatever word you would like. Place it behind the Santa layer. I used size 80 font Arial Bold. The color is the opposite color of my background. (IE red if your background is green, and vice versa). I made the star above Santa's head in the center of the text. Let's give the text some style.

Go to Script-Fu > Layer Effects > Inner Shadow. Keep everything default and make the size 4. Click ok. Go to Filters > Light and Shadow > Drop Shadow. Make the offsets 0, the blur radius 8, and don't allow resizing. Click ok.

Now go to Script-Fu > Layer Effects > Bevel and Emboss. Change the type to Inner Bevel and raise soften to 4. Click ok.

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Step 10

We aren't done with the text yet. Select the text layer again, and go to Script-Fu > Layer Effects> Color Overlay. If your text is red, use #ff0000. If your text is green, use #1fae1c. Set the blending mode to Overlay, and click ok. In the layers dialog, drag the color overlay layer that was just created to above the rest of the layers that make up the text. Lower the opacity to about 63%.

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Step 11

Let's add a reflection to the text. Hide all of the layers that do not make up the text. Edit > Copy Visible. Edit > Paste As > New Layer. You should now have the text all on one layer without any distortion. Go through and delete all of the separate pieces that made up the text to begin with, except for the one we just made. Your picture won't look any different- we just put all of the pieces that make up the text on one layer.

Now duplicated the text layer. Go to Layer > Transform > Flip Vertically. Place it underneath the original text, as if it were a reflection.Right click the reflected layer, and select Add Layer Mask.... Select the first option (full opacity) and click ok. Reset your colors to black and white. Apply the gradient from bottom to top, making a reflection.

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Step 12

Now add some "Merry" text and some more icons like in previous steps. For the "Merry" font. All I did with the text was give it a drop shadow like before and stroked the edges with light green. As far as the icons go, just search for Christmas or holiday icons on Google and you are bound to find some.

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Step 13

Now we'll add some snow. Create a new layer and go to Filters > Render > Clouds > Solid Noise. Raise the X and Y size to 5 and raise the detail to 5. Click ok. Go to Colors > Hue/Saturation. Take the Lightness all the way down and click ok. We still need some more dark spots, so get out the burn tool and burn it in a couple places.

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Filters > Blur > Gaussian Blur. Blur by 30. Now go to Filters > Noise > Hurl. Use 10, 41, and 1. Click ok. Go to Colors > Threshold. Drag the arrow to about the third line. Click ok. Go to Filters > Blur > Motion Blur. Use linear type, length 3, and angle 212. Click ok. Set the layer mode to Overlay and lower the opacity to about 42%.

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I forgot to add the reflections for the Santa, tree, ornament, and gift, so I went ahead and did that as well, the same way the text was done.

Conclusion

Wasn't that easy? Now you can use it as a wallpaper, or email it to your friends and family, even your co-workers, or boss. It's a nice and easy way to get every one into the holiday spirit! Merry Christmas everyone!

Create a Holiday e-Card in Gimp

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