Pixel drawing with the miniblix

By Aurélien Brabant

Wanderer, beware!

Hey! If this post is the first you're reading from me about the minilibx, you'd better check out the first post, which explains all the basics you need to know. That said, let's discuss how we are going to handle events using the minilibx!

Screen metrics

Before we actually start, I'd want to make sure we are all on the same page.

A computer screen is basically a 2D object that has x and y coordinates. By convention, it is considered that the top left corner of the screen is the origin (x = 0, y = 0).

Let's consider a common screen resolution, that is 1920x1080.

In this case, that means that there's 1080 rows of pixels on the screen, and that each row has 1920 pixels. The y axis is used to represent the row number, while the x axis is used to represent the column number.

As an exemple, let's say we want to draw a line that has the following endpoints: P1(0, 100) and P2(1920, 100).

The following is what is going to be rendered on the window:

What we get is an horizontal line taking all the width of the screen (we went from x=0 to x=1920), with a top margin of 100 pixels (the two points have the same y coordinate, 100, therefore the line is drawn on the 100th row).

Now that we've the basics in mind, let's move on!

Drawing pixels on the window using mlx_pixel_put

Drawing pixels is the most basic thing a graphical library is used for, and the minilibx provides us simple ways of doing that.

The straightforward way is to use the mlx_pixel_put function. Let's take a look to the prototype:

int	mlx_pixel_put(void *mlx_ptr, void *win_ptr, int x, int y, int color);

We've already explained what mlx_ptr and win_ptr are, and the others parameters are self-explanatory. x and y are the coordinate of the pixel, according to the metric considerations we just discussed.

Finally, we need to tell the minilibx what is going to be the color of the pixel at this coordinates. But how are we going to represent a color ?

Encoding a color, according to the True Color standard

Several ways of representing colors for computer graphics exist. The minilibx is complying to the true color standard. Here's the definition of what the true color standard is, according to techopedia:

With the minilibx, we need to make the color fit into an int datatype. That means that we need the int datatype to be 32 bits on our system.

We need to encode our color into an int by setting the three least significant bits to the amount of red, green and blue, respectively. We can encode our int using two different ways:

Settings the bits directly

We can simply set the bits of the integer directly, using the << (left shift) as shown in the function below:

int	encode_rgb(uint8_t red, uint8_t green, uint8_t blue)
{
	return (red << 16 | green << 8 | blue);
}

This code will simply encode the red, green, and blue values into the returned integer.

Using the hexadecimal notation

Hexadecimal is widely used when using encoded values because it allows us to clearly distiguish the bytes that form an integer. To do so, we need to think about an hexadecimal number as groups of two digits. One group of two digits represents an entire byte.

Be aware tho, that in hexadecimal we have a total of sixteen digits used to represent a number (0123456789 ((abcdef) || (ABCDEF))).

For example, let's say we're assigning the value 0x00FF00FF to an integer. Just by looking at the number, we can tell that red is FF (255), green is 0 and blue is FF (255). Pretty easy to figure out isn't it ?

However, this solution is interesting only if we already know what color we want to use at compile time. If the color is somehow provided by the user or from any external source, we will need to use the encode_rgb function.

Let's put the pixel !

Now that we know how to use the mlx_pixel_put, let's put our first pixel on the window!

#include <stdlib.h>
#include <stdio.h>

#include <X11/X.h>
#include <X11/keysym.h>
#include <mlx.h>

#define WINDOW_WIDTH 600
#define WINDOW_HEIGHT 300

Here, we're making use of the render function, the address of which is passed to the mlx_loop_hook function. In the previous post, we called this function handle_no_event, because it is triggered continuously while the loop is running. Because the render function's code needs to be run at each frame, we can use it to render all the things we want to display on the screen!

Notice the call to mlx_pixel_put. We passed it two coordinates, that are the half of the window's width and height respectively, which gives us the center of the window. The color is specified in the hexadecimal format, as a macro called RED_PIXEL.

Try to run this code. You should notice a small red pixel on the center of the window. That's it, here is our first pixel!

A small check to be safe

Please notice the if statement added before the mlx_pixel_put function. This statement is here to ensure the window still exists, to avoid puting a pixel on a window that is no longer available. Because of how mlx_loop is implemented, this is what would have happened if we didn't add this check. Moreover, we needed to ensure that our win_ptr was set to NULL after the call to mlx_destroy_window to make this check actually work.

Let's draw a rectangle !

Now that we know how to put a pixel on the screen, implementing a function that'll put a rectangle instead is pretty straightforward. Rectangles are really useful, and it is very likely that we're going to use them really much.

Here's one implementation we can use:

#define GREEN_PIXEL 0xFF00

typedef struct s_rect
{
	int	x;
	int	y;
	int width;
	int height;
	int color;
}	t_rect;

We will not discuss the mathematics used here, as it is really basic. For each row we need to draw, we are filling the appropriated number of pixels depending on the rectangle's width.

Note that there's no check performed on the rectangle's member variables. One can implement some checks to ensure the values are correct, but I think that's not necessary here.

In order for us to render the rectangles, we need to modify the render function:

int	render(t_data *data)
{
	render_rect(data, (t_rect){WINDOW_WIDTH - 100, WINDOW_HEIGHT - 100,
			100, 100, GREEN_PIXEL});
	render_rect(data, (t_rect){0, 0, 100, 100, RED_PIXEL});

	return (0);
}

In case you're wondering, (t_rect){} is what is called a compound literal. Since C99, this is a way to initialize structures without having to manually assign each member. I'm directly passing a structure by value here.

These render_rect function calls will display two rectangles: one in the upper left corner of the window (red), and the other in the bottom right corner (green).

Drawbacks of our approach

Were you thinking it would be that easy ?

In this section, I'm going to explain why the approach of using the mlx_pixel_put is bad. I know, I know, that's not really nice. I just teached you something and now I'm saying it's useless! Trust me, what we learned together is far from being pointless. In fact, thanks to that we'll find what the problem is.

At the moment, there's no problem when we're rendering our rectanges on the screen. Well, there's a problem, but we can't really see it in a simple configuration like that.

Let's find out what the problem is

To visualize what is wrong, let's implement a render_background function that will change the background color of the window:

void	render_background(t_data *data, int color)
{
	int	i;
	int	j;

	if (data->win_ptr == NULL)
		return ;
	i = 0;
	while (i < WINDOW_HEIGHT)
	{

This function will set all the pixels of the window to the same color.

Now let's add it to our render function.

int	render(t_data *data)
{
	render_background(data, WHITE_PIXEL);
	render_rect(data, (t_rect){WINDOW_WIDTH - 100, WINDOW_HEIGHT - 100, 100, 100, GREEN_PIXEL});
	render_rect(data, (t_rect){0, 0, 100, 100, RED_PIXEL});

	return (0);
}

Be careful to render the background before the rectangles so that they're not overriden by the background's pixels.

Anyway, let's see what we're getting:

Uh. What an ugly flickering we have now.

Okay. Well, the issue we have here is pretty simple.

The mlx_pixel_put function basically draws the pixel on the window directly, and the person who's looking at the window will see the change instantly. That's bad here because what we actually want to do is waiting for the whole background to be drawn, as well as the rectangles, and then push that on the window. Because everything is done without any delay, this is giving us this dirty flickering effect.

Furthermore, note that this technique is slow. Maybe it is not noticable on your machine, but that's really slow, trust me.

Therefore, we need a solution to these two problems. Well, don't worry, the minilibx provides us a solution. That's a little bit more complicated than pushing a simple pixel on the window, but that's worth it!

Using minilibx images to draw on the screen

One of the prefered way of drawing things on a window is to use images. The goal is to create an image (which is nothing else than a collection of pixels) and edit its pixels directly. When it is done, we will push that image on the window and we should have our graphics properly rendered without any flickering issue.

Well, images is a big and complex topic, so we won't dive into too much details about how it is implemented by the minilibx.

However, it is interesting to notice that the minilibx is making use of the Xshm extension, which allows our program to share images with the X Server through shared memory (/dev/shm) and not through the socket like it is the case when using any Xlib routine.

Remember how we were continously calling mlx_pixel_put to put our pixels on the window. With images shared in memory, we'll be able to change the pixels directly, using a pointer. To be clear, this is way faster, and that's why we want to use it!

Hopefully, the minilibx provides us a really simple way of dealing with images.

Creating an image

The first step is obviously to tell the minilibx we want to create a new image. For that, we need to call mlx_new_image:

void	*mlx_new_image(void *mlx_ptr,int width,int height);

Well, I think there's nothing really complicated with that prototype. We're going to use the dimensions of the window for our image, because the image is supposed to hold the window's pixels.

The first thing we need to do is to create a t_img type that will hold all the stuff we need to work with an mlx image.

typedef struct s_img
{
	void	*mlx_img;
	char	*addr;
	int		bpp; /* bits per pixel */
	int		line_len;
	int		endian;
}	t_img;

The mlx_img member refers to the address mlx_new_image returns. The remaining members are needed, we're going to look at it in a minute. For now, let's create our image. In order to do that, we need to have a place to store it.

This is how our t_data object looks like now:

typedef struct s_data
{
	void	*mlx_ptr;
	void	*win_ptr;
	t_img	img;
}	t_data;

Let's actually create the image:

data.img.mlx_img = mlx_new_image(data.mlx_ptr, WINDOW_WIDTH, WINDOW_HEIGHT);

Now that we've the image, we need to get a bunch of informations about it in order to make the whole thing work.

We'll especially need the address of the image in the shared memory, so that we are able to change the pixels of it directly. We'll also need additional informations to help us with some calculations (bpp, line_len and endian member variables).

To get these informations the minilibx way, we can use the mlx_get_data_addr function.

char	*mlx_get_data_addr(void *img_ptr, int *bits_per_pixel, int *size_line, int *endian);

We need to pass it the img we've got from mlx_new_image. For the three last arguments, we simply need to pass the address of an int variable. The function will set these integers to the correct value. You can see that as a way to "return" multiple values.

Talking about return value, the mlx_get_data_addr function returns the actual address of the image as a simple array of pixels. We're getting a pointer on char, which usually means we're going to naviguate in the array one byte at a time (not one pixel at a time, a pixel usually takes more than one byte as we've seen before).

Accessing one particular pixel of the image

Well, here is the most "complicated" part. To be honest, what is complicated here is to understand what is done, not really the code itself.

We need to retrieve a pixel at some x and y coordinates, but we don't have a two dimensional array here: we're dealing with a one dimensional array. Also remember that we're dealing with bytes here, but one pixel takes more than one byte because we're using the true colors standard. This amount is given by the bpp (in bits) value we've got from mlx_get_data_addr.

However, we don't really know how many bytes an int is, so we can't really perform a cast on the pointer safely.

Finding the pixel's first byte's address

For this example, let's assume we want to get the pixel at coordinates (5, 10). What we want is the 5th pixel of the 10th row.

Window/image dimensions are 600x300.

To begin with, let's find the correct row. The previous mlx_get_data_addr call provided us the line_len value, which is basically the amount of bytes taken by one row of our image. It is equivalent to image_width * (bpp / 8).

In our case, an int is four bytes, so it is 600 * 4 = 2400. Therefore we can say that the first row begins at the index 0, the second one at the index 2400, the third one at the index 4800, and so on. Thus we can find the correct row index by doing 2400 * 10.

To find the correct column, we will need to move in the row by the given number of pixels. In our case, we want to move 5 pixels "right". To do that, we need to multiply 5 by the number of bytes a pixel actually takes (here 4). Thus we will do 5 * 4 = 20.

If we summarize, we can find the correct index with the following computation: index = 2400 * 10 + 5 * 4.

That's it! We just need to generalize the formula using the values mlx_get_data_addr provided us. The following formula is the one we'll use:

index = line_len * y + x * (bpp / 8)

Now that we have the formula, let's implement the img_pix_put function that will put a pixel at (x, y) coordinates of the image. It will act as a replacement for the mlx_pixel_put function.

void	img_pix_put(t_img *img, int x, int y, int color)
{
	char    *pixel;

    pixel = img->addr + (y * img->line_len + x * (img->bpp / 8));
	*(int *)pixel = color;
}

That should work, but we've a problem here. If our bytes per pixel value is not equal to the size of an int on our system, we're not doing well. In most scenarios, they will be equal, and the above implementation will just work, but that's not really the most portable thing in the world.

I've came with what is (in my opinion) a more accurate way of doing it, taking the endianness in account.

(Optional) Alternative implementation of img_pix_put

void	img_pix_put(t_img *img, int x, int y, int color)
{
	char    *pixel;
	int		i;

	i = img->bpp - 8;
    pixel = img->addr + (y * img->line_len + x * (img->bpp / 8));
	while (i >= 0)
	{
		/* big endian, MSB is the leftmost bit */

In this implementation each byte is set manually in a different way, depending on the endianness. If you don't know what the endianness is, I recommend you read that.

Moreover, in this implementation, only the number of bytes specified by bpp is set. I'm not going to explain the bitwise operations in details, as this is out of the scope of this post. However, I think and hope showing you this alternative was interesting.

Let's draw on the screen

Drawing functions refactoring

What we need to do now is to change every drawing function to make it use the t_img object instead of the window directly.

Let's refactor the render_rect function:

int render_rect(t_img *img, t_rect rect)
{
	int	i;
	int j;

	i = rect.y;
	while (i < rect.y + rect.height)
	{
		j = rect.x;
		while (j < rect.x + rect.width)

As well as render_background:

void	render_background(t_img *img, int color)
{
	int	i;
	int	j;

	i = 0;
	while (i < WINDOW_HEIGHT)
	{
		j = 0;
		while (j < WINDOW_WIDTH)

As you can see, the refactor is pretty easy to do, nothing complicated here.

Changing the render function

The most important change we need to do is in the render function:

int	render(t_data *data)
{
	if (data->win_ptr == NULL)
		return (1);
	render_background(&data->img, WHITE_PIXEL);
	render_rect(&data->img, (t_rect){WINDOW_WIDTH - 100, WINDOW_HEIGHT - 100,
		100, 100, GREEN_PIXEL});
	render_rect(&data->img, (t_rect){0, 0, 500, 300, RED_PIXEL});

	mlx_put_image_to_window(data->mlx_ptr, data->win_ptr, data->img.mlx_img, 0, 0);

Now we're performing all our drawing operations on our image instead of directly pushing the pixels on the screen.

We then need to push the updated image on the window, which is done using mlx_put_image_to_window. Coordinates of the image are (0, 0) because it is covering the whole window. The mlx_put_image_to_window will push the image as well as the changes done to it (if any) at each frame.

See what we have now ? Awesome!

Congratulations

If you are reading this, congratulations! It was a really long post, but I found necessary to explain how the things really work.

Now you're mastering the basics of the minilibx, you can do almost what you want with it without compromising efficiency!

You can find the final code here.