gfx.PutPixel()

[Justcus]

I was trying to make each pixel in the viewport its own color. but for some reason, it fills the whole screen with the same color I know the problem lies in my nested for loops. could someone tell me what I am doing wrong? I've been trying to do this for like 10 hours I have come across some very interesting results from sinewaves to a diagonal line. I also know the colors will be repeated because of the pseudo-randomness of the rand function.

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void Game::ComposeFrame()
{
	int  width = gfx.ScreenWidth - 1, height = gfx.ScreenHeight - 1, x, y;
	int rcolor = rand() % 256;
	int gcolor = rand() % 256;
	int bcolor = rand() % 256;
	for (x = 0; x <= width; x++) {
		for (y = 0; y <= height; y++) {
			gfx.PutPixel(x, y, rcolor, gcolor, bcolor);
		}
	}
}

I am fairly new to C++ but have done some coding in Vb and Java.

Thanks in advance

[albinopapa]

The issue you are having is because you are getting random values before the loops, and then using those same values in each iteration of the loops, try this:

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void Game::ComposeFrame()
{
   const int  width = gfx.ScreenWidth, height = gfx.ScreenHeight;
   for( int y = 0; y < height; ++y ) {
      for( int x = 0; x < width; ++x ) {
      }
         const int rcolor = rand() % 256;
         const int gcolor = rand() % 256;
         const int bcolor = rand() % 256;
         gfx.PutPixel(x, y, rcolor, gcolor, bcolor);
      }
   }
}

[Justcus]

The issue you are having is because you are getting random values before the loops, and then using those same values in each iteration of the loops, try this:

Code: Select all

void Game::ComposeFrame()
{
   const int  width = gfx.ScreenWidth, height = gfx.ScreenHeight;
   for( int y = 0; y < height; ++y ) {
      for( int x = 0; x < width; ++x ) {
      }
         const int rcolor = rand() % 256;
         const int gcolor = rand() % 256;
         const int bcolor = rand() % 256;
         gfx.PutPixel(x, y, rcolor, gcolor, bcolor);
      }
   }
}

Thanks for putting me in the right direction!

When I use what you showed me above I get an undeclared x,y variables it won't compile. for some unknown reason, you have them declared in the for statements.

then when I do an int x,y; above your const int I get this:
for some reason when I put any gfx.PutPixel() in the location where you have the gfx.PutPixel() and the rand() it crashes the program. but when I move it up into the second for it seems to work.

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void Game::ComposeFrame()
{
	
	const int  width = gfx.ScreenWidth, height = gfx.ScreenHeight;
	for (int y = 0; y < height; ++y) {
		for (int x = 0; x < width; ++x) {
			const int rcolor = rand() % 256;
			const int gcolor = rand() % 256;
			const int bcolor = rand() % 256;
			gfx.PutPixel(x, y, rcolor, gcolor, bcolor);
		}

	}
}

I think I get the result I was looking for it sort of looks like Television Snow

I believe that is because of the game loop running the function over and over.

I just noticed you had ++y and ++x is that the same as x++ and y++? if not what is the difference?

[albinopapa]

Hahaha, wow, I messed that up, good job figuring out my own mistake and what I was trying to get across to you.

++variable_name is pre-increment
variable_name++ is post-increment

Difference between the two:

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int increment( int value )
{  
     value += 1;
     return value;
};

int i = 0;
// Preincrement
i = increment( i );
// ... use i ( now i is 1 )

// Post increment
int j = i;
i = increment( i );
// ... use j ( j == 1 and i == 2 )

Here is equivalent code using an increment function in a while loop to simulate pre-increment

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int i = 0;
Enemy enemies[10];
while( i < 10 )
{
     enemies[ i ].SetPositionX( 200 );
     enemies[ i ].SetPositionY( 100 );

     // value of i = 0
     i = increment( i );
}

Here is the same loop, but simulating post increment

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int i = 0;
Enemy enemies[10];
while( i < 10 )
{
     enemies[ i ].SetPositionX( 200 );
     enemies[ i ].SetPositionY( 100 );

     int j = i;
     i = increment( i );
}

Hopefully, you can see that post-increment has an added step. Even though the result is the same in the loop ( because it's the last instruction ), the preincrement operator is slightly ( very slightly ) more efficient. While it doesn't play a huge role here with integers, there are some uses for each.

Let's say you wanted to get the value of something at index 'i' and needed to increment 'i' afterward.

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int pos_x = enemy[ i ].GetPositionX();
i += 1;

You could just increment i in the [ ] brackets using the post increment version

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int pos_x = enemy[ i++ ].GetPositionX();

Here's a use for pre-increment that I've used before. I didn't want to count each element,
so I just increment i as I go.

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int i = -1;
Enemy enemies[] = {
     Position{ 400 + ( 10 * ++i ), 300 + ( 5 * i ) },  // i = 0 ( x = 400 + 0, y = 300 + 0 )
     Position{ 400 + ( 10 * ++i ), 300 + ( 5 * i ) },  // i = 1 ( x = 400 + 10, y = 300 + 5 )
     Position{ 400 + ( 10 * ++i ), 300 + ( 5 * i ) },  // i = 2 ( x = 400 + 20, y = 300 + 10 )
     Position{ 400 + ( 10 * ++i ), 300 + ( 5 * i ) },  // i = 3 ( x = 400 + 30, y = 300 + 15 )
     Position{ 400 + ( 10 * ++i ), 300 + ( 5 * i ) },  // i = 4 ( x = 400 + 40, y = 300 + 20 )
     Position{ 400 + ( 10 * ++i ), 300 + ( 5 * i ) },  // i = 5 ( x = 400 + 50, y = 300 + 25 )
     Position{ 400 + ( 10 * ++i ), 300 + ( 5 * i ) },  // i = 6 ( x = 400 + 60, y = 300 + 30 )
     Position{ 400 + ( 10 * ++i ), 300 + ( 5 * i ) },  // i = 7 ( x = 400 + 70, y = 300 + 35 )
     Position{ 400 + ( 10 * ++i ), 300 + ( 5 * i ) },  // i = 8 ( x = 400 + 80, y = 300 + 40 )
     Position{ 400 + ( 10 * ++i ), 300 + ( 5 * i ) }   // i = 9 ( x = 400 + 90, y = 300 + 45 )
};
// value of i after setting enemy[9] position is 10