Easiest & Most Efficient Way to Making a Bitmap Using For Loops

Question

I've been stuck on this for awhile, I eventually gave up but can anyone lead me in the correct direction. Also side note, I need the final result to have alpha.

static std::unique_ptr<unsigned char [ ]> ImageData;

    if ( !ImageData) {
        ImageData = std::make_unique<unsigned char [ ]>( Width* Height);

        for ( int x = 0; i < Width; x++) {
            for ( int y = 0; y < Height; y++ ) {
                float Red = 128, Green = 128, Blue = 255, Alpha = 255;
                // some cool math to determine color based off x/y.
                // . . .
                const unsigned char a[] = { Red, Green, Blue, Alpha };
                *reinterpret_cast<unsigned char*>(ImageData.get() + x + y * Height) = *a;
            };    
        };
    };

The image generated is completely trash and unusable, it's just random corruption everywhere.

The expression `*a` is equal to `a[0]`. That is, you only assign the `Red` value to each byte in the bitmap. — Some programmer dude, Apr 24 '19 at 03:34
You want an array of [`RGBQUAD`](https://learn.microsoft.com/en-us/windows/desktop/api/wingdi/ns-wingdi-tagrgbquad) (if you don't have Windows headers, make your own structure) — Ben Voigt, Apr 24 '19 at 04:16

Spektre · Answer 1 · 2019-04-24T08:11:58.157

You question is unclear as you did not specify pixel format

so what is the pixelformat 8/15/16/24/32 bpp ? which order rgab / bgra ?
why const char ?

that would not change with position !!! and also as Some programmer dude suggested *a will copy just the first BYTE so the rest of the channels are unitialized hence garbage output.
image data is char?

that is OK but then the pointer arithmetics is 8 bit not 32 bit !!!
for(x...) loop has i inside that is most likely a thypo
why float channels?

that only leads to casting problems ...

So if I put all together your code is not working at all as expected. To remedy it and assuming the rest of code (visualization) is OK and the pixel format is 32bpp I would change your code to this:

typedef unsigned char BYTE;
typedef unsigned __int32 DWORD;
static std::unique_ptr<unsigned char [ ]> ImageData;
const int _r=0; // here change the RGB/BGR order
const int _g=1;
const int _b=2;
const int _a=3;
if ( !ImageData)
  {
  ImageData = std::make_unique<unsigned char [ ]>( Width* Height*4);
  int x,y,a;
  BYTE db[4];
  DWORD *dd=(DWORD*)(void*)db;
  DWORD *p=reinterpret_cast<DWORD*>(ImageData.get());
  for (a=0,y=0;y<Height;y++) 
   for (   x=0;x<Width;x++,a++)
     {
     // some cool math to determine color based on x,y.
     db[_r]=x;
     db[_g]=y;
     db[_b]=x+y;
     db[_a]=128;
     // copy pixel
     p[a]=*dd;
     }
  }

Hope I did the pointer cast OK as I do not use the std::unique_ptr. Also I coded it directly in SO/SE editor so there might be hidden minor syntax errors or thypos.

Easiest & Most Efficient Way to Making a Bitmap Using For Loops

1 Answers1