to get the best quality when converting
Because the final image appears on a cinema screen it was vital that we didn't lose any video resolution. PAL video gave us a maximum size of 768x576 pixels.
But if we simply generated the image to the correct 35mm film aspect ratio of 1:1.85 (ie wide screen) it would have meant that we only used 768x415 of the PAL screen area, ie. wasting 152 (576 minus 415) valuable lines of pixels.
To solve this problem we used anamorphic images. These are simply widescreen images that have had their vertical axis stretched to fill the full height of the 4:3 screen.
To guarantee the best quality, we found it better to render the images at a bigger size and unsqueezed in their correct aspect ratio of 1024x576, and then scale the horizontal axis down to an anamorphic 768x576.
Note most 3D applications can render scenes out in anamorphic format. But we found the above method was better because the images can be seen in their correct proportion at all stages. This is vital for combining the render scenes with other images such as scanned photos or art work done on the computer.
As always there is a problem:
The devices that convert the anamorphic images into the correct ratio are set up to work for widescreen television and this has a different aspect ratio to widescreen film.
Widescreen PAL Video = 16:9
Widescreen 35mm Film = 1:1.85
To solve this, all images were rendered at the correct film ratio 1024x553 pixels and then a black strip was added to the top and bottom to give television wide screen ratio of 1024x576.
And finally the images need to have their horizontal axis shrunk from 768x576 to 720x576.
This is to do with the difference in shape of pixels found on computer screens (square) and PAL television (oblong). This problem is highlighted if you draw a circle on a computer and then display it on a video monitor, where it will appear as a horizontally-stretched oval.
To solve this problem the horizontal dimension of the picture simply needs to be reduced by 93.75% ie. 768 becomes 720 and the circle will appear round again on the video monitor. Note: Adobe Affect Effects does this automatically if you select "PAL D1/DV 720x576" in the rending size settings.
In practice only one horizontal scaling of 70.31% is required, ie 1024x576 to 720x576.
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