3 Animation file size reduction
APNGs have one issue, the file size is pretty large because every frame is standalone PNG image. I've addressed this issue by implementing two optimizations described below.
To allow better PNG compression, anti-aliasing could be turned off. This will result in worst quality of images but interestingly enough the savings are quite large. Figure 1 shows example of animated plant where disabling of anti-aliasing saved around 50% of file size.
By default, anti-aliasing is on but you can turn it off using
set antiAlias = false;.
3.2 Trimming frames & relative positioning
More advanced optimization is taking advantage of the fact, that APNg allows to position individual frames on arbitrary position in the resulting animation frame. Thanks to this feature, background of individual frames can be trimmed and positioned to original position.
Background of each frame is trimmed and the correct offset is set in the Frame Control Chunk (fcTL) of APNG. However this brings a bunch of problems.
L-systems in Malsys are rendered in floating-point space but bitmap (png image) has integer space as well as PNG offset. If every frame would be trimmed in floating-point space (which is the easiest way), frames will be jumping on sub-pixel level because offset is integral. Trimming must be by integer-amount.
The second problem is with background. Some browsers (good old Opera :) do not "turn on" transparency if PNG frames are without transparency channel (24 bpp). This makes background around frames black. To avoid this behavior, we can force 32 bpp rendering by for example this
set bgColor = #FF000000; which will set background color to fully transparent black.
To visualize how trimming optimizes frames we can set background color to some non-fully transparent color (like
set bgColor = #01AAAAAA;) to see individual frames background (Figure 2). This is also a limitation, only transparent background are supported.
By default, trimming is off but you can turn it on using
set autoTrim = true;.
For those who do not see animation, Figure 3 shows all frames as individual PNGs. Note the gray background — every frame has different size.
Figure 4: Comparison of file sizes with different optimization
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Table of contents
- 1 Introduction and prerequsities
- 2 Implementation of APNG renderer
- 3 Animation file size reduction
- 4 Conclusion