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A gaming and technology blog by TWHL admins Penguinboy and Ant. A music blog by TWHL users Ant and Hugh.

Real-Time "TRON 2.0" Glow For Low-Spec Hardware

By Francis 'DeathWish' Woodhouse

I won't take long to introduce the effect this article demonstrates, mainly because I introduced it before in the previous article. It received a good response, but the main complaint was that the method used (vertex and pixel shaders) made it unsuitable for anything but fairly high-end hardware.

This article aims to rectify that. Using the same basic algorithm as before, I will describe how you can implement this glow effect without the use of any vertex or pixel shaders. All the hardware requires is the GL_NV_texture_rectangle OpenGL extension (or better), which is supported on even fairly low-end cards. As before, this article will walk you through the steps necessary to implement this in your modification.

Here's an image of a scene with no effect applied and this article's effect. More examples of how the effect looks can be found in the previous article.

Attached to this article is a pre-compiled version of the effect, bundled into a mini-mod. Extract it into your Half-Life directory, then load up one of the HL maps to see the effect in action.

As before, this effect only works under OpenGL, which is no great loss as the majority of people use OpenGL in preference to Direct3D with Half-Life.

Let's get moving. First things first, go to your project's settings (Project->Settings in MSVC++) and add 'opengl32.lib' as a library to link with. This enables us to use OpenGL commands as required.

Next, open up tri.cpp. At the top, add these two include directives:

#include <windows.h>
#include <gl/gl.h>

Notice that we're not using gl/glext.h, despite the fact that we'll be using an OpenGL extension. This is because all we need is one constant, which we can add in ourselves. Below #define DLLEXPORT, add this:

#define GL_TEXTURE_RECTANGLE_NV 0x84F5

Then, after the extern "C" block, add this block of code:

#include "r_studioint.h"

extern engine_studio_api_t IEngineStudio;

We need IEngineStudio to retrieve which renderer is being used - software, OpenGL or Direct3D. After this, add in this block of definitions:

// TEXTURES
unsigned int g_uiScreenTex = 0;
unsigned int g_uiGlowTex = 0;

// FUNCTIONS
bool InitScreenGlow(void);
void RenderScreenGlow(void);
void DrawQuad(int width, int height, int ofsX = 0, int ofsY = 0);

Now that we've added all the definitions that we require, we can move on to creating the functions. I'll go through them step-by-step, explaining as we go along.

After the definitions, add this:

bool InitScreenGlow(void)
{
     // register the CVARs
     gEngfuncs.pfnRegisterVariable("glow_blur_steps", "4", 0);
     gEngfuncs.pfnRegisterVariable("glow_darken_steps", "3", 0);
     gEngfuncs.pfnRegisterVariable("glow_strength", "2", 0);

These three CVARs will be used to control the glow. The first one controls how much the screen is blurred, the second one controls how much darker colours are decreased by before blurring, and the third one controls how strongly the glow is applied to the scene.

     // create a load of blank pixels to create textures with
     unsigned char* pBlankTex = new unsigned char[ScreenWidth*ScreenHeight*3];
     memset(pBlankTex, 0, ScreenWidth*ScreenHeight*3);

We create a block of memory with which the textures we create will be initialised.

     // Create the SCREEN-HOLDING TEXTURE
     glGenTextures(1, &g_uiScreenTex);
     glBindTexture(GL_TEXTURE_RECTANGLE_NV, g_uiScreenTex);
     glTexParameteri(GL_TEXTURE_RECTANGLE_NV, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
     glTexParameteri(GL_TEXTURE_RECTANGLE_NV, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
     glTexImage2D(GL_TEXTURE_RECTANGLE_NV, 0, GL_RGB8, ScreenWidth, ScreenHeight, 0, GL_RGB8, GL_UNSIGNED_BYTE, pBlankTex);

This is the texture we use to grab the screen and store it so that we can re-combine it with the glow later on. Notice that we're using rectangular textures, as the screen is never a size that is a power of two in both dimensions.

     // Create the BLURRED TEXTURE
     glGenTextures(1, &g_uiGlowTex);
     glBindTexture(GL_TEXTURE_RECTANGLE_NV, g_uiGlowTex);
     glTexParameteri(GL_TEXTURE_RECTANGLE_NV, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
     glTexParameteri(GL_TEXTURE_RECTANGLE_NV, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
     glTexImage2D(GL_TEXTURE_RECTANGLE_NV, 0, GL_RGB8, ScreenWidth/2, ScreenHeight/2, 0, GL_RGB8, GL_UNSIGNED_BYTE, pBlankTex);

This texture is used to store the blurred and glowing version of the scene. It's half the size of the screen to conserve fillrate.

     // free the memory
     delete[] pBlankTex;

     return true;
}

All that's left is to delete the memory we allocated.

Next is a small utility function to make the code easier to read:

void DrawQuad(int width, int height, int ofsX, int ofsY)
{
     glTexCoord2f(ofsX,ofsY);
     glVertex3f(0, 1, -1);
     glTexCoord2f(ofsX,height+ofsY);
     glVertex3f(0, 0, -1);
     glTexCoord2f(width+ofsX,height+ofsY);
     glVertex3f(1, 0, -1);
     glTexCoord2f(width+ofsX,ofsY);
     glVertex3f(1, 1, -1);
}

This function is fairly self-evident. All it does is draw a quad with dimensions 1x1, and applies the specified texture coordinates with offsets if required. (Remember that the offsets are given a default value of 0 in the prototype for this function.)

The last function we need to create is the one that does all the work, and which will be called every frame to do the actual blurring:

void RenderScreenGlow(void)
{
     // check to see if (a) we can render it, and (b) we're meant to render it

     if (IEngineStudio.IsHardware() != 1)
          return;

     if ((int)gEngfuncs.pfnGetCvarFloat("glow_blur_steps") == 0 || (int)gEngfuncs.pfnGetCvarFloat("glow_strength") == 0)
          return;

     // enable some OpenGL stuff
     glEnable(GL_TEXTURE_RECTANGLE_NV);     
     glColor3f(1,1,1);
     glDisable(GL_DEPTH_TEST);

We first check to see whether the user's running OpenGL mode - if not, then we just bail and leave the screen as-is. We also check to see if the effect should be rendered at all, by looking at the CVAR settings. Finally, some OpenGL commands are issued that are required for the upcoming stages.

     // STEP 1: Grab the screen and put it into a texture

     glBindTexture(GL_TEXTURE_RECTANGLE_NV, g_uiScreenTex);
     glCopyTexImage2D(GL_TEXTURE_RECTANGLE_NV, 0, GL_RGB, 0, 0, ScreenWidth, ScreenHeight, 0);

We need to store the original scene's image so that we can re-combine it with the glowing version later on. If we didn't do this, we'd have nothing to combine the blur with, since the on-screen image gets mangled when we do the blurring.

     // STEP 2: Set up an orthogonal projection

     glMatrixMode(GL_MODELVIEW);
     glPushMatrix();
     glLoadIdentity();
     
     glMatrixMode(GL_PROJECTION);
     glPushMatrix();
     glLoadIdentity();
     glOrtho(0, 1, 1, 0, 0.1, 100);

The orthogonal projection makes it much easier to render quads exactly across the whole screen. We map the screen from (0,0) to (1,1), meaning the DrawQuad function draws across the whole screen.

     // STEP 3: Render the current scene to a new, lower-res texture, darkening non-bright areas of the scene
     // by multiplying it with itself a few times.

     glViewport(0, 0, ScreenWidth/2, ScreenHeight/2);

     glBindTexture(GL_TEXTURE_RECTANGLE_NV, g_uiScreenTex);

     glBlendFunc(GL_DST_COLOR, GL_ZERO);
     
     glDisable(GL_BLEND);

     glBegin(GL_QUADS);
     DrawQuad(ScreenWidth, ScreenHeight);
     glEnd();

     glEnable(GL_BLEND);

     glBegin(GL_QUADS);
     for (int i = 0; i < (int)gEngfuncs.pfnGetCvarFloat("glow_darken_steps"); i++)
          DrawQuad(ScreenWidth, ScreenHeight);
     glEnd();

     glBindTexture(GL_TEXTURE_RECTANGLE_NV, g_uiGlowTex);
     glCopyTexImage2D(GL_TEXTURE_RECTANGLE_NV, 0, GL_RGB, 0, 0, ScreenWidth/2, ScreenHeight/2, 0);

To make darker colours have much less of a glow, we multiply the scene by itself a few times (as determined by the glow_darken_steps CVAR) to make darker colours get even darker, leaving brighter colours still relatively bright (0.2*0.2 = 0.04, which is a 0.16 difference, but 0.9*0.9 = 0.89, which is a 0.01 difference). The result is then stored in the glow texture.

     // STEP 4: Blur the now darkened scene in the horizontal direction.

     float blurAlpha = 1 / (gEngfuncs.pfnGetCvarFloat("glow_blur_steps")*2 + 1);

     glColor4f(1,1,1,blurAlpha);

     glBlendFunc(GL_SRC_ALPHA, GL_ZERO);

     glBegin(GL_QUADS);
     DrawQuad(ScreenWidth/2, ScreenHeight/2);
     glEnd();

     glBlendFunc(GL_SRC_ALPHA, GL_ONE);

     glBegin(GL_QUADS);
     for (i = 1; i <= (int)gEngfuncs.pfnGetCvarFloat("glow_blur_steps"); i++) {
          DrawQuad(ScreenWidth/2, ScreenHeight/2, -i, 0);
          DrawQuad(ScreenWidth/2, ScreenHeight/2, i, 0);
     }
     glEnd();
     
     glCopyTexImage2D(GL_TEXTURE_RECTANGLE_NV, 0, GL_RGB, 0, 0, ScreenWidth/2, ScreenHeight/2, 0);

Now that we've corrected the colours, the next step is to blur the image. We first blur it in the horizontal direction by averaging the pixels around each other. The number of pixels sampled is controlled by the glow_blur_steps CVAR.

     // STEP 5: Blur the horizontally blurred image in the vertical direction.

     glBlendFunc(GL_SRC_ALPHA, GL_ZERO);

     glBegin(GL_QUADS);
     DrawQuad(ScreenWidth/2, ScreenHeight/2);
     glEnd();

     glBlendFunc(GL_SRC_ALPHA, GL_ONE);

     glBegin(GL_QUADS);
     for (i = 1; i <= (int)gEngfuncs.pfnGetCvarFloat("glow_blur_steps"); i++) {
          DrawQuad(ScreenWidth/2, ScreenHeight/2, 0, -i);
          DrawQuad(ScreenWidth/2, ScreenHeight/2, 0, i);
     }
     glEnd();
     
     glCopyTexImage2D(GL_TEXTURE_RECTANGLE_NV, 0, GL_RGB, 0, 0, ScreenWidth/2, ScreenHeight/2, 0);

To complete the blur, we take the horizontally blurred image and blur it in the vertical direction. This gives a Gaussian blur-like appearance, with the number of pixels sampled again controlled by glow_blur_steps.

     // STEP 6: Combine the blur with the original image.

     glViewport(0, 0, ScreenWidth, ScreenHeight);

     glDisable(GL_BLEND);

     glBegin(GL_QUADS);
     DrawQuad(ScreenWidth/2, ScreenHeight/2);
     glEnd();

     glEnable(GL_BLEND);
     glBlendFunc(GL_ONE, GL_ONE);

     glBegin(GL_QUADS);
     for (i = 1; i < (int)gEngfuncs.pfnGetCvarFloat("glow_strength"); i++) {
          DrawQuad(ScreenWidth/2, ScreenHeight/2);
     }
     glEnd();

     glBindTexture(GL_TEXTURE_RECTANGLE_NV, g_uiScreenTex);

     glBegin(GL_QUADS);
     DrawQuad(ScreenWidth, ScreenHeight);
     glEnd();

All that's left is to combine the blurred version with the original to give the finished glow effect. The glow_strength CVAR controls the number of times that the blurred version is added to the original (try setting it to 50 to get some weird psychadelic effects!).

     // STEP 7: Restore the original projection and modelview matrices and disable rectangular textures.

     glMatrixMode(GL_PROJECTION);
     glPopMatrix();

     glMatrixMode(GL_MODELVIEW);
     glPopMatrix();

     glDisable(GL_TEXTURE_RECTANGLE_NV);
     glEnable(GL_DEPTH_TEST);
     glDisable(GL_BLEND);
}

Finally, we clear up after ourselvs by resetting the projection matrix back to what it was and enabling or disabling what we disabled or enabled, respectively.

All that's left now is to add in the code to make it actually apply the blur. Open up cdll_int.cpp and before the second extern "C" block, add this:

// SCREEN GLOW
extern bool InitScreenGlow();
extern void RenderScreenGlow();

Then, go to HUD_VidInit (around line 170) and after VGui_Startup() is called, add this:

     // SCREEN GLOW
     if (!InitScreenGlow())
          gEngfuncs.Con_Printf("ERROR: Could not initialise screen glow!");

Last but definately not least, go to HUD_Redraw (around line 210) and add this before gHUD.Redraw() is called:

     RenderScreenGlow(); // SCREEN GLOW

That's it. You now have the "TRON 2.0"-esque glow effect without any of the vertex or pixel shaders involved in the previous article. All that remains is to compile it, run it, and bask in the warm glow!
attached files
  
  • tronglow.zip (760.75KB)