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Dark GDK / dbGetPixelsPointer to write directly to memory

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Jash
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Posted: 16th Oct 2009 03:34
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In the darkgdk documentation, the description for dbGetPixelsPointer() says we can use this with dbLockPixels() to manipulate pixels directly in memory. I cant find any info about this. dbGetPixelsPointer only returns the first pixel, but what about the rest of them? dbDot() is too slow, so how to I get dbGetPixelsPointer to work?

I make the same kind of games as any Chemisty major would.
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Lilith
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Posted: 16th Oct 2009 05:57
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Did you know that pointers are variable? When you increment the pointer it now points to the next pixel. A pointer to a pixel is not the value of the pixel itself. It's the address of where in memory that pixel resides. You have to dereference the pointer to get the value that it points to.

The best advice I could give you regarding C++ is to learn how to use pointers effectively.

Lilith, Night Butterfly
I'm not a programmer but I play one in the office
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Jash
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Posted: 16th Oct 2009 09:25
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Yes, I know that pointers are variables. I know the basics of pointers - I've used them before when I was playing with large arrays. I see what you mean about incrementing the pointer, so now I understand how to access all the pixels in memory. But, I still have the question of how to change the color values of those pixels.
ty Lilith, I'm one step closer to figuring this out.

I make the same kind of games as any Chemisty major would.
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Lilith
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Posted: 16th Oct 2009 17:02
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*pixelPtr = dbRGB (red, green, blue);

or, to retrieve the color of a pixel

DWORD pixelClr = *pixelPtr;

I don't know if it's any faster but when assigning a color I prefer to use

DWORD color = (255 << 24) | (red << 16) | (green << 8) | blue;

And certainly assign the color to a DWORD ahead of time then use that to set the color instead of using dbRGB() if the color is consistent.

Lilith, Night Butterfly
I'm not a programmer but I play one in the office
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prasoc
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Posted: 16th Oct 2009 18:28
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Isn't it using the deference operator to get a value of a pointer ? ( &pixelPtr rather than *pixelPtr)


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Lilith
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Posted: 16th Oct 2009 18:49
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Nope! the & operator is strictly "address of" in this context. It's never used for dereferncing. It can be used to indicate a logical "and" operation and it can be used to specify that a value being passed as a parameter is the address of the variable it refers to. It can also be used to declare a variable as a reference.

The use of "*" regarding pointers can be a little confusing until you get used to it. You use it to declare a variable as a pointer but you also use it to dereference an address to derive the value stored at that address. Having used it so much over the years it becomes automatic in my thinking. But I used to have to think something along the lines of

*pixelPtr is "that to which pixelPtr points."

Lilith, Night Butterfly
I'm not a programmer but I play one in the office
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prasoc
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Posted: 16th Oct 2009 19:27
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I need to get my head around them some more :S thanks for "addressing" (lol) that problem.


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Jash
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Posted: 17th Oct 2009 07:26
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Ok thanks again Lilith, I've got it working now. Here is the code if anyone needs it:


void pp(int x, int col){

int* pixel = (int*)dbGetPixelsPointer();
*(pixel+x) = col;
}
[/code
the points wrap when they excede the window's pixel limit. you must use dbLockPixels before the function is called and dbUnlockPixels after it.

I make the same kind of games as any Chemisty major would.
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