While transforming points using MatrixTransform2x3 we noticed some strange rounding behavior:
It appears the code used to round in the plugin simply adds 0.5
and truncates the result, which does not work correctly for
On a kind of related note: Is there even a reason to round the resulting floats?
While the class comment of MatrixTransform2x3 notes that this behavior is intentional, glancing quickly over its uses we could not find anything taking advantage or benefiting from it. It's also not a limitation of the DisplayTransform interface, since MorphicTransform does produce floating point values. Wouldn't it be much more versatile and easier to leave rounding to users if they actually need it?
On Tue, Jul 28, 2020 at 4:34 AM Stephan Lutz <[hidden email]> wrote:
No. Having a float result means that the primitive would need to allocate two Float objects. Any allocation can fail due to memory exhaustion. So the primitive would have to be made to retry the allocation after running a garbage collection.
Secondly, its results are primarily used to set up a WarpBlt IIRC, for drawing rotated user objects in Etoys. WarpBlt fails if the coords are not integers. The failure code rounds the numbers and retries. Doing the rounding in the matrix primitives ensured a fast path to rendering - that's why it was done that way.
So, there are very good reasons why the plugin returns integers. And there are Squeak VMs where this still is a very reasonable behavior. It also would be a good idea to document the reasoning in the class comment of MatrixTransform2x3.
That being said, there is virtually no reason to use it when running on Cog, much less Sista, especially on 64 bits where we have immediate floats. An interesting thing would be to compare a pure Smalltalk implementation to the performance of the plugin. If you need floating point transform results, just write it in Smalltalk, would be my suggestion.
- Vanessa -
(MatrixTransform2x3 withOffset: -5 @ -10) localPointToGlobal: 0@0. "-4@ -9"
>> On Tue, Jul 28, 2020 at 4:34 AM Stephan Lutz <[hidden email]>
>>It appears the code used to round in the plugin simply adds 0.5 and truncates the result, which does not work correctly for negative numbers.
>>This code can be found in Matrix2x3Plugin >> #roundAndStoreResultPoint: and Matrix2x3Plugin >> #roundAndStoreResultRect:x0:y0:x1:y1: .
Rounding up for negative numbers is wrong.
All the best,
In reply to this post by Vanessa Freudenberg
Ah, yes. I hadn't even thought about that. Probably because I've only ever used Squeak on 64bits with immediate floats ^^.
I followed your suggestion and implemented a Matrix2x3 in pure
Below are some of the measurements I have taken in a 64bit trunk image using squeak.cog.spur_linux64x64:
There are quite a few more benchmarks in the attached file.
On 28.07.20 19:33, Vanessa Freudenberg wrote:
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