[Newbies] [Q] Creating Form from ByteArray
chunsj at gmail.com
Thu Jun 12 10:20:35 UTC 2014
Thank you very much, especially to:do: part. This improves performance of my code(other part) considerably!
Sent from my iPad
> On Jun 12, 2014, at 7:12 PM, Bert Freudenberg <bert at freudenbergs.de> wrote:
> Ah, endianness issue, isn't it fun! BitBlt really operates on 32-bit words, so 4 pixels of your 8-bit form are stuffed into one word. And they are big-endian by default. You can indicate little-endian forms by using a negative depth. So, if you use -8 for your form depth it works.
> You are (probably) on an x86 processor, which is little-endian, so your byte array's first byte ends up in the least significant byte of the first word. Interpreting that as a big-endian word gives the pattern you noticed.
> I should have mentioned that stuffing a byte array into a form is a hack (there is even a method Form>>hackBits:). It is a useful hack, see for example Bitmap>>asByteArray. But by looking at that method you see you need to pay attention to your CPU's endianness:
> Smalltalk isLittleEndian
> ==> true
> Your code could use this:
> Smalltalk isLittleEndian ifTrue: [form swapEndianness].
> (or initialize its depth to 8 or -8 depending on endianness).
> The "proper" way would be to use a "bitPoker", which is independent of endianness, but considerably slower:
> | w h bytes form poker |
> w := 200.
> h := 200.
> bytes := ((1 to: w*h) collect: [:i | 255 ]) asByteArray.
> 1 to: w do: [ :x |
> 1 to: h do: [ :y |
> x = y ifTrue: [ bytes at: (w*(y - 1) + x) put: 0 ]
> form := ColorForm extent: w at h depth: 8.
> form colors: ((0 to: 255) collect: [:i | Color gray: i / 255]).
> poker := BitBlt bitPokerToForm: form.
> 0 to: w-1 do: [ :x |
> 0 to: h-1 do: [ :y |
> poker pixelAt: x at y put: (bytes at: w * y + x + 1).
> form display
> (Btw, don't put parens into "1 to: w do:". It works but defeats an important optimization).
> - Bert -
>> On 12.06.2014, at 02:32, Sungjin Chun <chunsj at castlesoft.co.kr> wrote:
>> Thanks you for your answer, however there is something I missed here.
>> I've tried following code
>> | w h bytes form |
>> w := 200.
>> h := 200.
>> bytes := ((1 to: w*h) collect: [:i | 255 ]) asByteArray.
>> (1 to: w) do: [ :x |
>> (1 to: h) do: [ :y |
>> x = y ifTrue: [ bytes at: (w*(y - 1) + x) put: 0 ]
>> form := ColorForm extent: w at h depth: 8 bits: bytes.
>> form colors: ((0 to: 255) collect: [:i | Color gray: i / 255]).
>> form display
>> What I expected is diagonal line but the result is not. What's wrong with my code?
>> Thank you in advance.
>>> On Wed, Jun 11, 2014 at 7:41 PM, Bert Freudenberg <bert at freudenbergs.de> wrote:
>>> On 11.06.2014, at 00:36, Sungjin Chun <chunsj at castlesoft.co.kr> wrote:
>>> > Hi,
>>> > I want to draw gray scale bitmap using byte array which has pixel by pixel gray
>>> > scale value as byte like this;
>>> > [0 0 0 0 0 0 0 0
>>> > 16 23 255 78 12 12 12 12
>>> > ...
>>> > ...] (8x4 for example)
>>> > I can draw this using Pen class pixel by pixel manner but this is very slow and
>>> > I think there should be faster way of creating a Form using above byte array.
>>> If the width is a multiple of 4, you can use the byte array directly as form bits:
>>> | w h bytes form |
>>> w := 100.
>>> h := 60.
>>> bytes := ((1 to: w*h) collect: [:i | 256 atRandom - 1]) asByteArray.
>>> form := ColorForm extent: w at h depth: 8 bits: bytes.
>>> form colors: ((0 to: 255) collect: [:i | Color gray: i / 255]).
>>> form display
>>> This would be the best and fastest way. If the width is not a multiple of 4 then perhaps you can add the necessary padding when creating the byte array. Otherwise, a very fast way is to use BitBlt to copy whole lines from the byte array to the form.
>>> But even creating a byte array with the right padding just using at:put: would be a lot faster than using a Pen.
>>> - Bert -
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