What are transparent mosaics?

Creating transparent photo mosaics is a unique and unrivalled possibility of Mosaizer Pro. It applies chroma-keying techniques to make a certain colour (range) transparent. This approach is well known in the film industry and for television shows. Although these industries use hardware chroma-keying, we use real-time software chroma-keying algorithms.

Another new feature is to apply a second layer on the original photo mosaic, using the chroma key to leave out any unwanted colour. The results are stunning and allows for a creative and wide variety of a new generation of photo mosaics. In the picture below you can see the effect of the chroma-key (here an example of a green screen picture + multiple backgrounds). Even in Photoshop this is not an easy thing to achieve. Mosaizer Pro does it like making just another photo mosaic.

Transparency management

The picture above shows how to show the controls of the transparency settings. There are three types of controls:

• 'Master settings for transparency'; here you can set the key controls how to make the final photo mosaic.
• 'Transparency settings' to define the chroma-key (actually it's a range of colours in the HSL colour space). On its right the proof picture immediately shows the effect of the chroma-key. You can load your own picture here as well.
• 'Second library for adding transparent pictures' to create a second layer on top of the (original) first mosaic. This library has only a few settings to modify.

Master settings for transparency

• 'Create a transparent mosaic'. Activate the transparency layer of the mosaic. A 'normal' bitmap is not transparent, and to make it transparent an 4th channel has to be activated, next to the RGB channels of the 24bit bitmap. Mosaizer pro applies a 32bit internal data flow to keep track of the 256-scale value for transparency. This way also (256) shades of transparency can be achieved, like shadows and subtle changes from opaque to transparent. Transparency can also be achieved in several manners:
  • use of transparent library pictures, where the current version can (only) read .PNG files (these type of files have a transparency channel)
  • use of the chroma-key to force transparency of the indicated colour range
  • by using masks
• 'Apply the chroma-key for tile transparency'. In case no mask or .PNG files is used or available the only way to create a transparent mosaic is to use the chroma-key. When .PNG library files are used the chroma-key will replace the original transparency information and use the chroma-key instead.
• 'Activate the second layer of (transparent) tiles'. In some cases a second mosaic picture layer is required. The second layer is only useful when applying transparency. It will overwrite the current original mosaic, and in case no transparency is used only the second mosaic will be seen.
• 'Only apply transparency for the second layer' tin case the transparency option might create a 'halo' effect (see also notes on the 'Blur' effect) or otherwise transparency is not required for the base photo mosaic. It is advised to check this box at first, and then decide if also transparency is required for the base layer mosaic.

Combinations of these three options must carefully be considered before making the photo mosaic. For instance: you apply a .PNG picture library without using the chroma-key, but you don't activate the transparency option for the mosaic. In that case the tiles are 'transparent' when positioned on the mosaic canvas, but the end result will have no alpha channel with the transparency data. If also the option 'Random grid' is active, each picture in the mosaic will be seen as transparent, but the final mosaic will not.

Except for the first option ('Create a transparent mosaic') these transparency features are in demo mode for unlicensed users. A library of free (transparent) .png pictures can be downloaded from our website. Here is the link for download: a variety of coloured marbles.

Transparency settings

A number of properties can be set to define the chroma-key for transparency. The proof picture on the right shows which colours will be made transparent. Because the HSL colour definition is a three dimensional space it's almost impossible to show the effect of all three colour components. The application has two internally available proof pictures, one for colour and the other for greyscale tones, starting from white.

• 'Hue'. The colour tone of the chroma-key. This is the basis for all the colour transparency.
• 'Saturation'. The saturation range of the chroma-key colours. It will make all colours transparent that have higher saturations than indicated with this value (but only for the Hue range of course)
• 'Lightness'. The lightness range of the chroma-key colours. It will make all colours transparent that have higher lightness than indicated with this value
• 'Tolerance'. The range of the hue of the colours. Since hue is measured in degrees (0-360) the range can extend beyond 360 or 0. In that case the hue is corrected. The tolerance can vary from 0 (mono colour) to 100/360.
• 'Blur'. When a chroma key is applied the edges between transparency and opague can be abrupt. This blur factor smoothes that edge, allowing for a great professional chroma-key effect. The effect of the blur is also seen in the proof picture. A high blur can create a whitish halo around the tile, so the blur should be carefully kept at the lowest possible value. A value of 2 is usually a good start. See also the remark on blurring in the paragraph on blur for the second library (at the end of this chapter).
• 'Greyscale'. In case pictures are used that need to filter out the white colours, also a greyscale chroma-key can be applied. This filter has some limitations compared with a colour chroma-key. It will only respond to the 'Lightness' value, since saturaion and hue have no meaning in this greyscale. The range of the lightness also changes: where the lightness for a colour chroma-key varies between 0 and 1 (so 0 and 100 as values for lightness), the range for greyscale pictures is 0 to 255. This reflects the true 256 levels from white to black. The effect of the lightness can be seen by using a greyscale proof picture (press any of the < or > buttons). All values lower than the indicated value will become transparent. With the little black-white (toggle) icon you can reverse the greyscale: in that case the black will be made transparent, to a certain level of grey.

The proof picture

The proof picture on the right is a specially prepared picture. The inner circle will show only hue and saturation values in their entire range: the center has saturation 0 and the outer ring 100, with a lightness of 50. The outer ring also only shows hue and lightness values, while the saturation is 100. For that reason you will see an abrupt change in transparency beyond the hue + saturation ring. The second picture (press < or >) is for greyscales. The use and effect is pretty straightforward. Some examples are shown below.

Top left: possible settings for a green screen. Top right: similar for a blue screen.
Bottom left: a great deal of the (white) highlights are made transparent. Bottom right: subtle white removal.

The user can load any picture for proof. By pressing the 'Read proof picture' button a new picture can be added to the viewer. It will be stored, and by pressing the < or > buttons the previous/next proof picture will be shown. There is no limit to the amount of proof pictures. The list of proof pictures will not be saved however.

Second library for adding transparent pictures

Here the second library of pictures can be defined. This library is used for the top-layer of the photo mosaic, writing its tiles on the first mosaic. The first mosaic is made like any other mosaic is made with the settings that apply for a normal photo mosaic. The choice of the second library is important. Some good practice:

• Use .PNG files that have built-in transparency information. Assure that all pictures are indeed transparent.
• Use files that have a white background, avoid files that appear to have '1 pixel' coloured edges. The best pictures are white with the parts of interest positioned in the center, and have a good fill-out of that part of interest.
• Use pictures that have at least the size of the end-tile to avoid pixelation of the chroma-key (not even the blur can filter that out...)

Only a few settings can be applied to this library:

• The 'Tile matching method': sometimes it makes sense to follow a different colour matching method that the first original mosaic.
• The 'Shadow size and depth'. In case a 3D effect is required, this can be a useful setting. For most mosaics this should better not be applied. Us the little checkbox on the left to activate or de-activate this shadow effect.
• The 'Blur'. In case the second layer requires a different blur than the original its value can be set here. It's very important to manage the blur settings: when a square, but not transparent tile is used for the layer the blur effect will create kind of 'halo' at the edges! For that reason blur can only be applied safely when the user is sure that the edges are made transparent. A bit of trial and error may be useful to learn about the effects of blur.
• Not in this control box, but the overall settings for 'Blend' and 'Colourize' apply for the entire mosaic. In case these settings are applied the blend and colourize effects will be applied to both layers.

Two examples of blur. Example one (picture below): the correct use of blur. The top-layer library was composed from green screen pictures, where the chroma key is accurately tuned with the (average) green chroma key of the library. This was done by also reading one or more pictures from the library in the proof image window. The blur was set to 3 for the top-layer, and was set to 0 for the original coloured mosaic. The library for this effect can be downloaded from our website (here is the link).

The following example shows what happens when a chroma key is used for pictures that have scattered colours in the chroma key range. The original picture had a lot of red that was supposed to be chroma-keyed out. Unfortunately the library that was used was not really suited for chroma keying. The yellow arrow shows the scattered transparencies, where also blurring didn't help very much. The red arrow shows the edge effect ('halo') when the edges are not transparent. In this case better colour tuning has to take place with perhaps a larger hue tolerance, while blur should be set to 0. But it may be of no help after all.