31 Dec

Creating a Simple GUI from Scratch


More often than not, the biggest obstacle for a programmer is adding a GUI to a otherwisely ready application. This probably is because you generally can create a program that inputs and outputs text with a few lines of code in any language. However, if you don’t count MessageBox() and other similar helper APIs that are available in most windowing systems and that are as simple to use as gets() and printf(), it’s a unnecessarily big step to change a command line program into a program that outputs the exact same text but with complex command line arguments replaced by a few buttons and a window with the outputted text.

However, if you are developing a program that already draws something on the screen, it’s really easy to add simple mouse interactivity. This is especially true if you are using SDL (even if it’s extremely bare bones in these things!) and already use SDL_rect (your standard rectangle) to draw things. You can simply change the draw routine so that it takes one more argument which would be the SDL_event struct you’re already using to check for a quit message and so on. Then for every object you are drawing on screen, check if the event if a mouse button down event and that the coordinates are inside the rectangle that will be drawn. This eliminates completely the need for having to plan a separate system that interprets the mouse events. It’s sort of piggybacking on the existing code with minimal changes to the existing code.

Now, you might already think that is way too simplistic and lazy but think again. Not many programs need a more complex GUI with multiple movable windows and so on. If you really do need that, then be my guest and create a exhaustive windowing system or take the time to learn an existing system. But it is still overhead and you have to do comparatively a lot of work before any real results. At least I hate that kind of mental overhead. And while this whole idea of combining the drawing and the event processing sounds like a bit of a hack, it really isn’t a “hack” as in patching something you will probably have to replace with a better solution later. It’s just a different way of doing almost the same thing. With less overhead.

I have used this approach in my latest project and from direct experience I know it is possible to create scrollbars, scrolling text fields, text input fields, message boxes and pretty much anything I have needed. And it’s not too much code either even if you have to create absolutely everything from nothing. In other words I do not feel limited or burdened.

How to do it

As explained above, you most likely are drawing to a specific region on the screen for every object you need to check for mouse clicks. It is not necessary to have any kind of array where those regions are. You need to make sure that for every mouse click event runs the draw loop once and that the mouse click event gets passed to the draw routine. Then check if the coordinates are inside the draw region.

This is where the part starts that it admittedly gets a bit hacky: if a button is pressed, the event that the button triggers will be run in the middle of the draw loop (unless you somehow buffer the events). In most cases this doesn’t matter at all but it could be that what is on the screen is not exactly how it really is; you might have two selected items for a duration of one frame and so on. For the most part this doesn’t matter since you are getting results with very little overhead.

Consider this example.

void draw_stuff()
  SDL_Rect position = {10, 10, 40, 40};

  SDL_BlitRect(button_gfx, NULL, screen, &position);

void draw_stuff_and_check_events(SDL_Event *event)
  SDL_Rect position = {10, 10, 40, 40};

  if (event->type == SDL_MOUSEBUTTONDOWN 
   && event->button.x >= position.x
   && event->button.y >= position.y  
   && event->button.x < position.x + position.w 
   && event->button.y < position.y + position.h)

  SDL_BlitRect(button_gfx, NULL, screen, &position);

void event_loop()
  SDL_Event event;

  while (SDL_PollEvent(&event))
    if (event.type == SDL_QUIT) break;


From the above example, the benefits of this approach are obvious. The event checking can simply be injected anywhere in the code as long as you have the event and a region.

Solutions to common problems

One downside in this is that if you draw multiple overlapping regions, the region drawn last will be the one that is visible on the screen but the click is handled by the first region. Our event checking sees the regions from the other side of the screen. In such cases you can first iterate the regions in reverse order checking the event and then draw them in the correct order. An example in the mentioned project is the menu, submenus often overlap the parent menu; I solved that by first going through the menus in the order the user sees them and then drawing them back to front.

Dragging items is easy: simply check for mouse motion events and if the button is held down and the mouse is moved, just adjust the position of the matching region. You do not need any special “drag starts now” and “drag ends now, update objects” phases. However, this simplistic method is subject to the previous issue if you move a region over one that seems to be under it. You can also simply record the clicked object when the mouse button is pressed and make the motion events match only the selected region.


I absolutely need a separate window

This is possible as long as the window can be modal (like a message or open file dialog that takes over from the window behind it). You simply jump to another event loop until the new window is closed, much like it’s done in the Windows API with GetOpenFileName() or the MessageBox() mentioned earlier. Then it’s just a matter of drawing the new window and checking for the events normally.

Is this reusable?

Of course. You could create a small library that has basic functionality and helper functions. A practical example can be seen here.

Even if you can use the event checking straight in the source code, you can still define the regions in an array and link to relevant event handlers.


Combining the drawing and the event processing code can save time in the short term. Many common GUI elements are perfectly possible to replicate. The idea described above should be considered if a project needs mouse interaction and external libraries are not available, the conversion of existing code seems expensive or the learning curve is too steep compared to the future benefits. Nonetheless, in borderline cases, it can be well worth prototyping due to the minimal overhead and developing considerations.

09 Nov

Plenty of Room, Part II

This is the second part of the epic (two three-part) series of articles about tiny intros, the previous part was an essay about 256-byte intros in general.

Ed. note: Since this article seems to take forever to finish, here’s the first half of it. The (hopefully) final part will detail more of the specifics.

rubba As stated earlier, tiny intros written in assembly language fascinate me. I have written a few in x86 assembly language, here’s one of them. I have tried to make the inner workings of the program as accessible — or, at least as thought-provoking — as possible even if assembly wasn’t their weapon of choice.

I’ve included the original DOS intro (you can use DOSBox to run it, it should also work on Windows XP) and a Win32 port of it, written in C while trying to keep the original structure intact. I’ll also try to explain the general idea of the effect in pseudocode where syntax can be an obstacle. The archive rubba_c.zip contains the source code, rubba_b.exe which is the compiled Win32 executable and RUBBA.COM which is the 16-bit MS-DOS executable. To compile the C program, you need the TinyPTC library (like SDL but even more bare-bones).

I won’t go into details about x86 assembly language, I suggest anyone interested first reads an introduction and learns some of the basics. However, I’ll try to make the code look interesting, explain some weird or obfuscated code and probably show some basic size optimization tricks.

The Effect

The intro, called rubba_b, shows one effect: a twisting bar drawn using primitive light-shaded texture mapping. The color palette and the texture are generated run-time. The texturing is done using linear interpolation and no vector math is used even if the bar looks like it is rotated. The lighting is an extremely simple approximation of the light source being exactly where the camera is located. That is, the length of the textured line directly determines the light.

If looked from above, the bar will be a tower of squares. If one of the squares is rotated around the center, the corners will move in a circular pattern. So, the X coordinate will be equal to cos(corner_number*(360 degrees/4)+square_rotation), the Z coordinate (why Z? Because it goes towards the screen) is equal to the sine but it can be discarded because we will not need it for perspective calculation. Remember, we’re short on bytes.

We then modulate the bar rotation for each square in the tower. If the amount of rotation changes when moving up or down the tower, the bar will twist. If the rotation stays the same for each square, the bar will rotate uniformly and look uninteresting.

The textured horizontal line is drawn from each corner to the next corner, from left to right. If the line would be drawn from right to left, we know it isn’t facing towards the camera, another line facing the camera will be drawn over it and we simply skip the line. The color value fetched from the texture is multiplied by the line length which makes short lines darker.

Still with me?

The Code


First things first. We need to set the video mode before we continue. In the Win32 version we simply open a TinyPTC window, in the original version we need to tell BIOS to go in a nice 320x200x8 video mode (the de facto video mode back in the day).

C asm
mov al,13h
int 10h

In the above code, the Win32 part is self-explanatory. The assembly bit probably needs some clarification: we put the number 13h (hex) in the 8-bit register al and we request for interrupt 10h. This is the BIOS video interrupt and since register ax (which consists of al – “low” – and ah – “high”) equals to 0013h (when the program starts, ax is always zeroed), BIOS will call the routine for changing the video mode (ah=00h) to 13h.

If above sounds complicated, don’t worry. It’s just a matter of memorization – similar to how you would memorize the function name for changing the video mode.

The next thing we need is some space for the texture, the sine table and the double buffer. In the Win32 version this is obvious, we just add a few arrays (although since TinyPTC only supports 32-bit video modes, we will also have an array for the palette). Again, in the assembly version we won’t use any fancy way to reserve memory to save some precious bytes: we simply decide certain areas of the memory will be used for whatever we want. The benefits of no memory protection and single tasking. ;)

C asm
short int sinetab[SINETAB];
unsigned char palette[256*4];
unsigned char texture[256*256];
unsigned char screen[320*200];
mov dh,80h
mov gs,dx           
mov dh,70h
mov es,dx
mov dh,60h
mov fs,dx

The assembly version basically sets the register dx to 60xxh-80xxh (we set only the high byte, i.e. dh to save bytes, thus the low byte of dx is undefined – it won’t matter) and puts the value into various segment registers (es-gs).

This makes it so that if we use the different segment registers, we can access each of the three 64 kilobyte segments as a 64 kilobyte array. E.g. the sine is in gs, thus mov ax,[gs:2] would move the second word in the sine table in ax. In C, the equivalent would be short int value=sinetab[1] (note how the C compiler knows the fact that a word is 2 bytes but in assembly you have to take care of that yourself – all memory accesses are always by the exact location, not the array element!).

All this is because in 16-bit memory accessing you can see only 64 kilobytes at one time. You can’t have a 128 KB array, nor can you have two 64 K arrays in the same segment. It’s something comparable to having a flashlight and a big sheet of paper in a dark room; you can move the light to show different areas but you will never see more than what there is in the spotlight.

The next two parts calculate the sine table (back in the day you simply could not do trigonometric stuff real-time, even in hardware — although in the intro it’s there just for show) and set the palette. This is pretty straight-forward stuff across the two versions. The only difference is that in the Windows version we have to remember the palette has 8-bit color components and the original has 6-bit components (0..255 ~ 0..63). And of course, the Windows version simply puts the values in a palette look-up table (because 32-bit video mode doesn’t use a palette) and the original actually sets the video mode colors.

I won’t reiterate the source code for the sine table and palette change here, I think you should be able to figure it out by comparing the source code. But in theory, here’s how to change a color: first, write the color index in port 3C8h, then write the three color components in port 3C9h (hint: dx first contains 3C8h and it’s incremented to 3C9h to save bytes).

The sine routine increases the angle (st0 the topmost register on the FPU) by 2*PI/32768 (a full circle is 2*PI, the sine table has 32768 elements). You probably should check some FPU tutorial, arithmetic on the 8087 is pretty interesting due to the stack-based architecture. For example, you first push in two numbers and then do an add, which (in most cases) pops out the two values and pushes in the result.

The texture generation bit is interesting. It also was annoying to port to C thanks to the fact you have to emulate how some instructions work – there are no accurate analogies in the C standard. A big thanks goes to baze whose code I originally cannibalized for this (I think). To be honest the conversion doesn’t work 100 % accurately but does still produce a nice texture.

The algorithm uses addition, bitwise operations and other simple things to achieve complexity thanks to how processors do arithmetics. Mainly, the results from an earlier calculation is carried over to the next calculation — when an addition or a subtraction overflows, i.e. the result is too large or too small to fit in a register, the processor lets the result wrap over but sets the carry flag.

This is quite similar to how you would carry numbers when calculating with a pen and a paper. The flag affects the results unpredictably because it’s used across quite different operations; usually you would just use to to add big numbers together as in the pen and paper example.

The Main Loop

Here is the meat of the code. The C version has many variables that are named after registers in order to see the connection with the original code. Sometimes, as with the 8-bit registers, some code doesn’t work exactly as in the original because you can’t do things similarly in C. E.g. you can’t have two variables that also are a part of one big register similarly how al and ah form ax (well, you can with pointers or unions but that is besides the point, kind of).

Self Modifying Code

I use self modifying code (SMC) in a few places because it produces faster and also much simpler code. For example, if you have a variable that is changed in a few places but used by one instruction only (and the instruction performs arithmetic or otherwise accepts a constant parameter), it’s much faster to store the variable where the constant for the instruction would be. That way you don’t have to read the variable in a register and then use the register to do something.

E.g. Let’s multiply cx by the variable foo:

Original SMC
  push ax ; save ax
  mov ax,[foo] ; move variable foo in ax
  imul cx,ax ; multiply cx by ax
  pop ax  ; restore ax
  mov ax,123   ; set foo ...
  mov [foo],ax ; ... to 123
foo: dw 0
  imul cx,123
foo equ $-2
  mov ax,123   ; set foo ...
  mov [foo],ax ; ... to 123

We can exploit the fact imul (signed multiplication) accepts constant multipliers. If you looked at the code with a hex editor, you’d see 123 next to the opcode. You can change the constant run-time and you do that exactly like you would change a variable: if you just define foo as the address where the constant is (the above code defines it as the last two bytes (i.e. word) of the previous instruction: in NASM, $ is the current location and $-2 equals the address of the previous word).

To be concluded…

22 Sep

Introducing My Latest Projects

… Or, How to Procrastinate Productively.


I decided to make one of my current projects open source and post them on Google Code just for fun. The project is a tool chain that I’m using to remake Thrust. In reality, I decided to divide the project into two separate projects: the actual game engine (called klystron) and related tools, and a music editor that uses the engine.

Here are two videos I made a while ago that demonstrate the engine. The first is the music editor (called klystrack) — it’s much less ugly at the moment but the sound synthesis is the same, and that’s what matters:

The sound engine (“Cyd”) is basically a very simple software synthesizer with capabilities comparable to the SID or any 8-bit machine from the 80s. The editor is a fairly standard tracker, much like GoatTracker.

The graphics half of the engine is basically a wrapper around a quite fast collision detection system (pixel-accurate, or it wouldn’t be much good for a thrustlike) built on SDL. It also does background collisions and drawing as well. As you may have guessed, the whole point is to provide a limited but still helpful set of routines that are useful for creating 2D games not unlike what video games were in 1991.

And, here’s a proof I’m actually working on the actual game (the sound effects are created in real time by the sound engine):

A note on Google Code: it’s rather nice. It provides the standard open source development stuff like source control an such but I really like how clean and hassle-free it is. Adding a project takes a minute and after that it’s simply coding and some quick documentation on the project wiki. The project wiki is good example of how simple but elegant the system is: the wiki pages actually exists inside the source control as files, just like your source code.

Go check Google Code out and while you’re at it, contribute on my projects. :)

08 Sep

You Can Stop Programming Now

The above is puls, a 256-byte intro by ?r?ola. It’s basically a raytracer with screen space ambient occlusion (which makes it so much realistic and cooler). While tube — which I think was the best 256-byte intro until now (when design and code are judged together) — also did raytracing of a cylinders, and after that many other intros did similar tracing of more complex surfaces, puls simply crushes all of them with objects that are formed by multiple plane surfaces (e.g. a cube would be a combination of six intersecting planes), a very nice color palette and that delicious ambient occlusion.

Thinking it out in C and sketching it out in asm took about a week, byte crunching took another one… that’s like forty hours of full focus and eighty of playing.

It’s also really, really slow which is the only minus especially because you can’t run 16-bit executables on Windows 7, so you have to use DOSBox to watch it (or, use a boot floppy to run it or something). There’s now a Windows port including a screensaver, see the Pouet.net page for more. A big thank you to nordak5 who was kind enough to upload a video on Youtube.

?r?ola has also included source code with the binary that you can find over here. That said, I’ll be deleting all my own source code since perfection has finally been achieved and there is no need for programmers anymore.

02 Sep

Retargeting Images Using Parallax

I came up with a neat way to retarget images using a mesh that is transformed by rotating and doing an ortographic (non-perspective) projection. This is generally quite interesting since it can be done using a mesh and simple transformations and so can be done almost completely on the GPU. Even using a mesh can be avoided if one uses a height map à la parallax mapping to alter the texture coordinates so just one quad needs to be drawn (with a suitable fragment shader, of course).

The idea is simply to have areas of images at a slope depending of how much the areas should be resized when retargeting. The slope angle depends of from what angle the source image is viewed to get the retargeting effect since the idea is to eliminate the viewing angle using the slope.

Here’s a more detailed explanation:

  1. Create an energy map of the source image, areas of interest have high energy

  2. Traverse the energy map horizontally accumulating the energy value of the current pixel and the accumulated sum from the previous pixel

  3. Repeat the previous step vertically using the accumulated map from the previous step. The accumulated energy map now “grows” from the upper left corner to the lower right corner. You may need a lot of precision for the map

  4. Create a mesh with the x and y coordinates of each vertex encoding the coordinates of the source image (and thus also the texture coordinates) and the z coordinate encoding the accumulated energy. The idea is to have all areas of interest at a steep slope and other areas with little or no slope

  5. Draw the mesh with ortographic projection, using depth testing and textured with the source image

  6. Rotate the mesh around the Y axis to retarget image horizontally and around the X axis to retarget image vertically

Here is a one-dimensional example (sorry for the awful images):

Source image

Source image

The red dots represent areas of interest, such as sharp edges that we don’t want to resize as much as we want to resize the areas between the details. We then elevate our line for every red dot:


Elevated mesh

Imagine the above example as something you would do for every row and column of a two-dimensional image. Now, when the viewer views the mesh (which is drawn without perspective) he or she sees the original image:


Viewing the mesh from zero angle

However, if the viewing angle is changed, the red dots don’t move in relation to each other as much as the areas that are not elevated when they are projected on the view plane. Consider the below example:


Viewing the mesh from an angle (gray line is the projected mesh)

Note how the unelevated line segments will seem shorter from the viewer’s perspective while the distance between the red dots is closer to the original distance. The blue dots in the above image show how areas that have little energy and so are not on a slope, thus will be move more compared to the red dots.