Sign in to follow this  

OpenGL Transparent Texture

Recommended Posts

Solano    185
I'm setting an ortho projection and am drawing a texture plane. Now I want the texture for that plane to be transparent.
ILuint image = 0;
ilGenImages(1, &image);

int w = ilGetInteger(IL_IMAGE_WIDTH);
int h = ilGetInteger(IL_IMAGE_HEIGHT);
BYTE *bits = new BYTE[w * h * 4];
ilCopyPixels(0, 0, 0, w, h, 1, IL_RGBA, IL_UNSIGNED_BYTE, bits);

GLuint textureID[1];

glGenTextures(1, &textureID[0]);//generate texture
glBindTexture(GL_TEXTURE_2D, textureID[0]);//work with this texture

GL_LINEAR );//set the min filter
GL_LINEAR );//set the mag filter
glTexImage2D (GL_TEXTURE_2D, 0, GL_RGBA, w, h, 0, GL_RGBA, GL_UNSIGNED_BYTE, (GLvoid*)bits);
I use DeviL to load the image. The image in question is this:

Following an article I found on opengl's site
[b]15.080 [/b][b]How can I make part of my texture maps transparent or translucent?[/b] It depends on the effect you're trying to achieve.

If you want blending to occur after the texture has been applied, then use the OpenGL blending feature. Try this:

[indent] glEnable (GL_BLEND); glBlendFunc (GL_ONE, GL_ONE); [/indent] You might want to use the alpha values that result from texture mapping in the blend function. If so, (GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA) is always a good function to start with.

[b]However, if you want blending to occur when the primitive is texture mapped (i.e., you want parts of the texture map to allow the underlying color of the primitive to show through), then don't use OpenGL blending. Instead, you'd use glTexEnv(), and set the texture environment mode to GL_BLEND. In this case, you'd want to leave the texture environment color to its default value of (0,0,0,0).[/b]

So thats what im doing.Blending is disabled (glDisable(GL_BLEND). I set up the texture environment like it says in the article.
int w, h;
glBindTexture(GL_TEXTURE_2D, (int)textid);
glGetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_WIDTH, &w);
glGetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_HEIGHT, &h);
glTexCoord2i(0, 1);glVertex2f(x, y);
glTexCoord2i(0, 0);glVertex2f(x, y + h);
glTexCoord2i(1, 0);glVertex2f(x + w, y + h);
glTexCoord2i(1, 1);glVertex2f(x + w, y);

This is the code I use to draw my plane. with that the result i get is :


This is what I want to get:
Can anyone help me? The block color is how it should be. And the alpha is being loaded correctly. Its values from 0 to 255.

Share this post

Link to post
Share on other sites
Solano    185
Except for a very big fps drop the alphatest did nothing
//Enable Z buffering
//Enable alpha testing
glAlphaFunc(GL_LEQUAL, 1);//glAlphaFunc(GL_LEQUAL, 255) doesnt work either.
These are all the things im enabling when starting opengl
The result is still the same.

And the fps drop is from 670 fps to 233 fps. Why such a huge fps drop. All I'm doing is drawing vertex buffered blocks (36 blocks 8 verts GL_QUADS non textured)

Share this post

Link to post
Share on other sites
Solano    185
Not only does it not work
but now fps is down to 183

Intel Core 2 Duo E6300 1.8GHz 2mb L2 Cache
Ati Radeon HD4670 1764 mb vram

Share this post

Link to post
Share on other sites
Hodgman    51334
Get rid of "[font="Lucida Console"]glTexEnvf( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_BLEND );[/font]" -- that's doing weird things that you don't want it to do. The documentation you've quoted is describing how to configure the fixed function shader to blend the texture over the top of other primitive colors ([i]e.g. from [font="Courier New"]glColor[/font][/i]), not how to make your primitives transparent.

If that doesn't help, it sounds like your texture doesn't actually have an alpha channel. Inspect your [font="Courier New"]bits[/font] array and make sure the data is valid.

Turning on alpha-blending ([i]translucency[/i]) or alpha-testing ([i]transparency[/i]) is expected to hurt performance somewhat - the fragment shader/[url=""]ROP[/url] have to do extra work. Are you only enabling it for your transparent overlay quad, or are you leaving it enabled for the entire scene?

BTW, an FPS drop from 670 to 233 is an increase of 2.8ms. Never use FPS as a measure of performance because it's very misleading - always convert to milliseconds.
[font="arial, verdana, tahoma, sans-serif"][size="2"][quote name='Bow_vernon' timestamp='1302350009' post='4796294']
[code]glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);[/code]
I think it should be:
[code]glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);[/code][/quote]The former is pre-multiplied alpha blending, the latter is traditional alpha blending. Either of them should induce [i]some[/i] form of translucency assuming the fragment shading stage is outputting useful alpha values.[/size][/font][quote name='Solano' timestamp='1302344617' post='4796268'][size="2"][font="arial, verdana, tahoma, sans-serif"]All I'm doing is drawing vertex buffered blocks (36 blocks 8 verts GL_QUADS non textured)[/quote]Never use [/font][font="Lucida Console"]GL_QUADS[/font] - use some form of triangles instead.
[/size][font="arial, verdana, tahoma, sans-serif"][size="2"]GPUs don't support quads, so your graphics driver has to use the CPU to convert them into triangles on the fly every time you submit a draw-call.[/size][/font]
[size="2"]Also, in high performance code, you shouldn't use the immediate mode renderer either (i.e. [font="Lucida Console"]glBegin[/font]/[font="Lucida Console"]glEnd[/font]) because it involves making the graphics driver to a large amount of work on the CPU.[/size]

Share this post

Link to post
Share on other sites
NicoG    172
+1 for getting rid of immediate Mode. That is deprecated since roughly 10 Years. :)
No offense though.

Use OpenGL 2.x with VBO or use at least the "old" Vertex Arrays.
Also, alpha testing could also be done with GLSL:

#version 330

uniform sampler2D NL_TextureID;

smooth in vec4 theColor;
smooth in vec2 theTextureCoord;

out vec4 outputColor;

void main()
vec4 value = texture2D(NL_TextureID, theTextureCoord);

if ( value.a <= 0.01 ){

outputColor = value;

Sample from one of my Shaders.
This simply discards all fragments which have a lower Alpha Value than 0.01.
Shaders can make your life really easy :).

Image with a Shader that has no Alpha Testing:

Same Image with above Code:

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Similar Content

    • By pseudomarvin
      I assumed that if a shader is computationally expensive then the execution is just slower. But running the following GLSL FS instead just crashes
      void main() { float x = 0; float y = 0; int sum = 0; for (float x = 0; x < 10; x += 0.00005) { for (float y = 0; y < 10; y += 0.00005) { sum++; } } fragColor = vec4(1, 1, 1 , 1.0); } with unhandled exception in nvoglv32.dll. Are there any hard limits on the number of steps/time that a shader can take before it is shut down? I was thinking about implementing some time intensive computation in shaders where it would take on the order of seconds to compute a frame, is that possible? Thanks.
    • By Arulbabu Donbosco
      There are studios selling applications which is just copying any 3Dgraphic content and regenerating into another new window. especially for CAVE Virtual reality experience. so that the user opens REvite or CAD or any other 3D applications and opens a model. then when the user selects the rendered window the VR application copies the 3D model information from the OpenGL window. 
      I got the clue that the VR application replaces the windows opengl32.dll file. how this is possible ... how can we copy the 3d content from the current OpenGL window.
      anyone, please help me .. how to go further... to create an application like VR CAVE. 
    • By cebugdev
      hi all,

      i am trying to build an OpenGL 2D GUI system, (yeah yeah, i know i should not be re inventing the wheel, but this is for educational and some other purpose only),
      i have built GUI system before using 2D systems such as that of HTML/JS canvas, but in 2D system, i can directly match a mouse coordinates to the actual graphic coordinates with additional computation for screen size/ratio/scale ofcourse.
      now i want to port it to OpenGL, i know that to render a 2D object in OpenGL we specify coordiantes in Clip space or use the orthographic projection, now heres what i need help about.
      1. what is the right way of rendering the GUI? is it thru drawing in clip space or switching to ortho projection?
      2. from screen coordinates (top left is 0,0 nd bottom right is width height), how can i map the mouse coordinates to OpenGL 2D so that mouse events such as button click works? In consideration ofcourse to the current screen/size dimension.
      3. when let say if the screen size/dimension is different, how to handle this? in my previous javascript 2D engine using canvas, i just have my working coordinates and then just perform the bitblk or copying my working canvas to screen canvas and scale the mouse coordinates from there, in OpenGL how to work on a multiple screen sizes (more like an OpenGL ES question).
      lastly, if you guys know any books, resources, links or tutorials that handle or discuss this, i found one with marekknows opengl game engine website but its not free,
      Just let me know. Did not have any luck finding resource in google for writing our own OpenGL GUI framework.
      IF there are no any available online, just let me know, what things do i need to look into for OpenGL and i will study them one by one to make it work.
      thank you, and looking forward to positive replies.
    • By fllwr0491
      I have a few beginner questions about tesselation that I really have no clue.
      The opengl wiki doesn't seem to talk anything about the details.
      What is the relationship between TCS layout out and TES layout in?
      How does the tesselator know how control points are organized?
          e.g. If TES input requests triangles, but TCS can output N vertices.
             What happens in this case?
      In this article,
      the isoline example TCS out=4, but TES in=isoline.
      And gl_TessCoord is only a single one.
      So which ones are the control points?
      How are tesselator building primitives?
    • By Orella
      I've been developing a 2D Engine using SFML + ImGui.
      Here you can see an image
      The editor is rendered using ImGui and the scene window is a sf::RenderTexture where I draw the GameObjects and then is converted to ImGui::Image to render it in the editor.
      Now I need to create a 3D Engine during this year in my Bachelor Degree but using SDL2 + ImGui and I want to recreate what I did with the 2D Engine. 
      I've managed to render the editor like I did in the 2D Engine using this example that comes with ImGui. 
      3D Editor preview
      But I don't know how to create an equivalent of sf::RenderTexture in SDL2, so I can draw the 3D scene there and convert it to ImGui::Image to show it in the editor.
      If you can provide code will be better. And if you want me to provide any specific code tell me.
  • Popular Now