Png transparency

[ch4nsuk3]

Fixes #65

Handles reading of the chunk in the png file that handles transparency, the trns chunk. This data is based off of the palette indexes and simply indicates alpha values of 0 or 255. Index 0 in this chunk correlates to index 0 of the color palette and goes on from there.

This chunk can have fewer indexes than the palette, but it is not allowed to contain more. Because of this I implemented a ValueError to check for this, and provides a hopefully helpful message about what is specifically wrong. Without this the user will instead receive errors related to palette errors which will likely be confusing if you dont know whats going on under the hood when you load an image.

For performance checking I measured the average load time of 150 imageload calls on a test image: 120x120px, 256 color palette with transparency enabled for the first and last index as a sort of worst case scenario.

I settled on reading the entirety of the chunk at once into a list, then running a for loop through it looking for the interesting values, then setting the Palette.make_transparent with the correct index number(s). Reading the data into a list, then working on that list proved to be more efficient speed-wise rather than the alternative of reading a single byte at a time then working with that. Read times can add up quickly.

Using a for loop also proved to be faster than doing list comprehension or using enumerate() by a noticeable margin as well. It has a bonus side effect of keeping the code easy to understand.

Added the following to the pylint disable:

• consider-using-enumerate: I did consider it, but it wasnt the best option
• too-many-statements: The added if statements pushes over the limit of 50 statements, to 51. This could be avoided by removing the error check, but I feel it is important to include a useful error message.

[deshipu]

Can we del trns_data once we are done with it?

[ch4nsuk3]

Sure can! Latest commit does so now.

[FoamyGuy]

This looks good to me. I tested it successfully by modifying the existing png example to remove the "manual" call to make_transparent(). My testing was on PyPortal Titano 9.1.1.

Thanks for working on this improvement @ch4nsuk3!