BRYCE 6 Mini Tutorial

How to model
render larger than
4000 pixels.

Horo Wernli
May 2009

HTML Version
by Horo
 

Part of Dream Studio (Horo) >

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Introduction
The maximal width of a picture that can be rendered with Bryce is 4'000 pixels. The height may exceed this limit. A portrait with an aspect ratio of 3:4 can be rendered as 4'000 x 5'333 pixel picture. However, a landscape with an aspect ratio of 4:3 can only reach 4'000 x 3'000 pixels.

Why render larger than 4'000 pixels wide?
SOHO (small office, home office) printers can output at 300 dpi — that is 300 dots per inch or nearly 12 dots per millimetre. That is quite good, even though professional printers reach a density that is six fold. A picture 4'000 pixel wide can be printed on a 300 dpi printer 13.3 inches or about 34 cm wide, if quality is not to be compromised.

What if you got a commission for a 12 ft or 4 m wide artwork? If it is to be printed with a 300 dpi printer, it has to be rendered about 48'000 pixels wide. Even if such a large picture is beheld from a distance and the resolution has not to be very high, with 100 dpi there are still around 16'000 pixels needed. There are cases when a picture must be rendered larger than 4'000 pixels


The Solution: Multiple Renders
Bryce cannot do it in one go. Several renders are needed, which will have to be stitched together. Unfortunately, this cannot be done with an ordinary graphics application.

Picture 1

At left, the example render in one go. It is only 720 pixels wide and here represented in one third of that size.

Below the 9 rendered parts in the same size. As can be seen, the edges do not fit, they are distorted.

Picture 2

Below finally the resulting picture after stitching the nine part renders. The distortions have disappeared and the picture looks perfect.

Picture 3

Of course, this small picture which is about 2'000 pixels wide could have been rendered in one go. The example only shows that the method works nicely. Below, another example will be presented, that was assembled from four part renders with a size of 4'000 x 5'333 pixels each, resulting in a 6'600 x 8'800 pixels picture.


Approach
We cannot simply move the camera to create a full picture from the rendered parts. The camera has a three dimensional scene in front of it and if it is panned, we get perspective errors. We have to go about it the same way as if we would create a panorama with a photographic camera. The camera stays fixed and is rotated around its nodal point.

The Bryce Camera – Field of View and Focal Length: Principally, we work with the free moving camera. It has to be adjusted in such a way, that the scene is shown the way we want it to be shown.

Picture 4

The camera is tilted around its X-axes and rotated around its Y-axes. If the Y-axes gets negative, the camera looks up, if it is positive, down. The camera is rotated to the left if the Y value gets smaller or negative; and right if the value gets larger or positive.

The FOV value determines, how much of the scene the camera «sees». FOV stands for Field Of View and is denoted in degrees (°). An angle of 30° shows less than one of 60°. You can look at it as zoom, but it might be more adequate to think of it as telephoto or wide-angle lens.

It would have been nice if everything were so simple. There is still another field named Scale and the value denoted in percent (%). It is not so that the FOV angle of 30° with a Scale of 100% also shows the scene under an angle of 30°. To make the Bryce camera FOV the same as the field FOV indicates, the Scale value has to be adjusted for every angle. The picture above shows a FOV of 75° at Scale 100%. The Bryce camera actually «sees» the scene with an angle of exactly 60°.

Usually, this is nothing to bother about. You adjust the FOV angle until the scene is shown the way you wish. But if we render several parts of the same scene to assemble them with a stitching program, we have to tell the stitcher the focal length of the lens used. And the focal length can only be calculated if the true FOV is known.

In a nutshell, we can say that we have to know the real AOF (Angle Of View) of the Bryce camera in order to give the stitcher the focal length of the virtual lens used.

These relationships are compiled in a PDF document. The document shows the FOV values with the corresponding Scale values for the true AOF of the camera (FOV=AOF) and the focal length that corresponds to this angle. The values are shown in 5° steps. If angles between 5° are used, the stitching program can usually handle the small error this introduces. Increments of 5° are fine enough for the job. Show the document FOV in a new window. (75 kB). Note: included in the PDF tutorial (last 4 pages).

Stitching Programs
A free program to stitch panoramas is Hugin http://hugin.sourceforge.net/. There is quite some manual work to do. Even though Hugin can generate control points automatically, you may have to correct many of them manually. You have to learn the use of the program. If you need to create large pictures only now and then, Hugin is a very good choice.

Those who have to deliver large pictures often may prefer PTGui. There is a free test version valid for 30 days, otherwise PTGui costs 79 Euro ($ 115) and the professional version PTGuiPro 149 Euro ($ 217). The pro version is only needed if you want to stitch HDRI images — which is not the case with Bryce renders. PTGui http://www.ptgui.com/ usually creates the picture almost fully automatic. Of course, you can alter any variable, if you must.

Here, we will work with PTGui. Both examples could be assembled almost fully automatic.


Example 1 – 9 Render for one Picture
The nine renders shown at the beginning shall be our first example. They were rendered with 720 pixels only to test the concept.

First, you will find the necessary camera positions and note them. At each edge, about one fourth ought to be common — overlap. According to the required size of the final picture, the number of renders is determined. For a 15'000 pixel wide picture, we would use 5 renders with 4'000 pixels each to get enough overlap. A good idea would be to render small at first to test whether the result looks good. Then, the final document size will be set.

Picture 5

When all pictures are rendered, open PTGui and load the 9 pictures.

Picture 6

PTGui does not know, in what order the pictures fit together. Open the Image parameter tab and enter for each picture, how it was «photographed».

Picture 7

Yaw corresponds to the angle the Bryce camera was rotated around the Y-axis; Pitch is the angle the camera was tilted along the X-axis. Attention: Bryce X = -30 is + for PTGui (and Hugin). The signs are inverted. Roll finally is the angle of the Bryce Z-axis. The values shown here are not those entered but the ones the program has calculated after all the optimizing was done.

Next, the values for the final pictures are entered (Panorama Settings). Important at this moment is to select the projection Rectlinear ('flat'). Picture width and height (Field of view) will be adjusted later.

Picture 8

Now for the lens parameters (Lens Settings). Select Rectlinear (normal lens) for the Lens type. If you have not been asked for the Focal length when loading the pictures, enter it here. The Focal length multiplier is 1 because the values in the PDF document are for normal 35 mm SLR cameras. Again, the values shown were filled in by the program after the optimizations.

Picture 9

Now is the moment to go back to the Project Assistant and to click on the button 2. Align Images. This makes the program generating control points. When this has finished, open the Optimizer, select a reference image and optimize. After a few seconds, a window pops up and shows how good the pictures could be stitched.

Picture 10

We have a green very good with the worst error at just under 2 pixels. We could now correct that manually but this would be ridiculous. If there are larger differences between control points, the text will be red. Then, the pictures cannot be properly stitched. Most probably, there was not enough overlap between the individual pictures.

The following step is not mandatory but it shows the camera response and lens vignetting curves. A click on the tab Exposure/HDRI opens the dialog. With a click on the button Optimize Now, the curves are calculated and displayed. That is how good the Bryce camera is…

Picture 11

We are almost done. Click on the tab Create Panorama to open the dialog. You can accept most of the default values. A click on the button Set optimum size… gives to option to select the largest picture size. File format permits to select the file format for the stitched picture and into the field Output file belongs the place where to save and the file name; Browse… opens a dialog where to save. A final click on the button Create Panorama launches the stitcher to create the picture, which may take several minutes.

Picture 12

You ought to scrutinize the final picture very closely for flaws before you close PTGui. If it is good, save the project (File menu). You might have to crop the final picture a bit at the edges.


Example 2 – 4 Render for one Picture
In this example, four pictures with 4'000 x 5'333 pixels were rendered and stitched. It is a render named Conjunction I made in 2004. The four pictures were rendered in this size within one hour. PTGui shows the sizes of the loaded pictures.

Picture 13

The camera FOV was set to 30° with Size at 79.2% this gives an angle of view of 30° and a focal length for the lens of 67 mm. PTGui calculated a different focal length from the diagonal of the picture.

Picture 14

After optimizing, the picture gets a field of view of 48° x 61°.

Picture 15

The Bryce camera was rotated by ±9° left and right; and tilted up and down by ±11°.

Picture 16

Here is the complete Control Points table. It lists all generated and optimized comtrol points. The greatest distance between two identical points in different pictures is lower than 0.9 pixel.
Rather good …

Picture 17

Here the preview of the stitched picture. With the sliders, the format can be adjusted. This governs the Field of view (three pictures up).

Picture 18

If you look closely you will notice small dents on all four sides. The picture ought to have been adjusted a little bit narrower. Now these will have to be cut away in a graphics application.

Picture 19

This is the maximum size for the stitched picture. There are pixel data for this size. There were 4 x 4000 x 5333 = 85'328'000 pixels rendered, the final picture has got 6642 x 8844 = 58'741'848 pixels. This is 69% of the rendered pictures. Or to put it another way: almost a third was rendered for nothing. That part was needed by the stitcher.

The finished picture (online version only), edges cropped in full size as a zipped JPEG with 50% quality is available here. Apart from the low JPEG quality, flaws can be seen on the curtains. These were made with terrains and the flaws are not visible in the original size of 600 x 800 pixel. Download picture (3.9 MB). To display it, about 168 MB are needed.


Recap
Even though Bryce cannot render pictures wider than 4000 pixels it is possible to render parts, similar to photographing a panorama with a real camera. The separately rendered pictures can be assembled with an appropriate stitching program.