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RKS: Preparing PNG image assets for GDTF

Petr Vanek - Robe

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Rasterized PNG images are used in GDTF in several places - as a device thumbnail or as a MediaFileName in slots for gobo wheels and animation wheels. While the definition of a device thumbnail is quite simple (a png file to provide the rasterized picture with maximum resolution of picture 1024 x 1024), definition for a gobo slot image is more involved:

Gobo images shall be in PNG format with an alpha channel. Indexed, Greyscale and Alpha, 8-bit RGB and Alpha, or 16-bit RGB and Alpha are accepted pixel formats. The Background shall be fully transparent and should be considered to be the equivalent of a gobo holder. The Image region shall be fully opaque aside from anti-aliasing and shall be as large as possible. The Background region, the equivalent of gobo holder, is defined by full transparency (Alpha 0). In the Image region, Pure Black (RGB 0,0,0) is opaque (2), and Pure White (RGB 255,255,255) is transparent (GDTF). Colored gobos (3) shall use an RGB approximation. The RGB approximation shall be calculated on the basis of Pure White being the CCT of the fixture light source and the ICC color profile embedded within the PNG. The default shall be sRGB.


- Maximum resolution of picture: 1 024x1 024;
- Recommended resolution of gobo: 256x256;
- Recommended resolution of animation wheel: 256x256

So for thumbnail images, one can use pretty much any png image with the given dimensions. Ideally, the image should be a square, so the result in any UI is more predictable.

This is the default image provided by the builder:


What we like to do, however, is to ensure that the image is actually trimmed on a transparent background, so it can better fit into any UI (checker board indicating the transparency):


With the transparency:


For wheel slot images, you have ensure that the non-transparent (opaque) and transparent (white, colored) portions are correctly defined and also that transitions between them is also correct.

Here is a 256x256 example. We actually provide gobo images in maximum possible resolution (1024x1024), for the best outcome in visualizers. It is simple to sample down if needed. The only exception we currently do are devices with tenths of gobos, like the MiniMe and  ProMotion, where we scale the images to achieve close to 10MB size of the complete GDTF file.


As i mentioned, critical parts are the transitions. On the edge between alpha and color, you should have no extra color (due to antialiasing), just the alpha (transparency):


On the edge between white and black, you need to have only colors and no transparency:



Same rules apply for the animation wheel:



Hope this helps





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