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graveyard filla

OpenGL how to do modulation to simulate lighting effects ???

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hi, im working on a 2D RPG w / C++ , SDL, and OpenGL. anyway, i was talking to a friend of mine whos making an RPG w / d3d. in it, he has some really cool lighting effects. basically, each tile, sprite, etc, has a darkness value. there is no real "light", he just blends everything with a dark color to make things that arent as dark appear lit. basically, he makes his character / tile quad blended with a dark color to make it appear darker then it really is. i dont think this will work by just using glColor4f(), since once i bind a texture to a quad, the color no longer matters (except the alpha). i was thinking i could draw a black quad over everything and blend it, but this seems very expensive and too much work to do. surely there must be a way to blend a color with a textured quad? thanks for any help!!

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Ajare    174
Have you actually tried using it? I see no reason why it shouldn't work. I may be missing your point, but it seems simple enough.

glBindTexture (GL_TEXTURE_2D, textureId);
glBegin (GL_QUADS);
glColor3f (0.5f, 0.5f, 0.5f);
glTexCoord2f (...
glVertex3f (...

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zedzeek    529
>>i dont think this will work by just using glColor4f(), since once i bind a texture to a quad, the color no longer matters (except the alpha).<<

yes it does, if lighting is disabled and texenv is set to get MODULATE

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stormrunner    720
i think he means -
void glTexEnvi( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE )
glTexEnvi specifies how your textures are combined with colors in the frame buffer. GL_MODULATE allows your texture colors to be multiplied by the existing pixel color, and not replace them.
hope that helps.

<edit> the other two modes are
GL_BLEND - the texture color is multiplied by the pixel color and is combined with a constant color
GL_DECAL - the texture color replaces the pixel color

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thanks stormrunner.

the one thing is, i couldnt find in google how to disable it once i set it to MODULATE. do i just set it to DECAL to go back to normal? im assuming DECAL is the defualt, since ive never called this function before.

thanks again!

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stormrunner    720
*sheepish grin* i would be more help if i read the next sentence -
Quote:

taken from Opengl Game Programming
The default value for glTexEnvi() is GL_MODULATE

i'm pretty sure you have to call the function in order for that to be true, though. someone correct me if i'm wrong.

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Quote:
Original post by stormrunner
i think he means -
void glTexEnvi( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE )
glTexEnvi specifies how your textures are combined with colors in the frame buffer. GL_MODULATE allows your texture colors to be multiplied by the existing pixel color, and not replace them.
hope that helps.

<edit> the other two modes are
GL_BLEND - the texture color is multiplied by the pixel color and is combined with a constant color
GL_DECAL - the texture color replaces the pixel color


It's not true that glTexEnvi specifies how your textures are combined with colors in the frame buffer - it would be blending and glBlendFunc and glEnable/Disable(GL_BLEND). Tex env specifies how during particular pass color's from different sources(textures,diffuse,specular etc) are mixed withing texture units - that's all. Only the output value is blended into frame buffer

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stormrunner    720
Quote:
It's not true that glTexEnvi specifies how your textures are combined with colors in the frame buffer

my only defense is that that is specifically what "the book" said - word for word.
Quote:

taken from Opengl Game Programming - pg. 242 section Texture Functions
The target parameter must be equal to GL_TEXTURE_ENV. You must also set the pname parameter equal to GL_TEXTURE_ENV_MODE, which tells OpenGL that you will be specifying how textures are going to be combined with colors in the frame buffer.

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Quote:
Original post by stormrunner
Quote:
It's not true that glTexEnvi specifies how your textures are combined with colors in the frame buffer

my only defense is that that is specifically what "the book" said - word for word.
Quote:

taken from Opengl Game Programming - pg. 242 section Texture Functions
The target parameter must be equal to GL_TEXTURE_ENV. You must also set the pname parameter equal to GL_TEXTURE_ENV_MODE, which tells OpenGL that you will be specifying how textures are going to be combined with colors in the frame buffer.


Quote:
from MSDN
glTexEnvf, glTexEnvi, glTexEnvfv, glTexEnviv
These functions set texture environment parameters.

void glTexEnvf(
GLenum target,
GLenum pname,
GLfloat param
);

void glTexEnvi(
GLenum target,
GLenum pname,
GLint param
);
Parameters
target
A texture environment. Must be GL_TEXTURE_ENV.
pname
The symbolic name of a single valued–texture environment parameter. Must be GL_TEXTURE_ENV_MODE.
param
A single symbolic constant, one of GL_MODULATE, GL_DECAL, GL_BLEND, or GL_REPLACE.
void glTexEnvfv(
GLenum target,
GLenum pname,
const GLfloat *params
);

void glTexEnviv(
GLenum target,
GLenum pname,
const GLint *params
);
Parameters
target
A texture environment. Must be GL_TEXTURE_ENV.
pname
The symbolic name of a texture environment parameter. Accepted values are GL_TEXTURE_ENV_MODE and GL_TEXTURE_ENV_COLOR.
params
A pointer to an array of parameters: either a single symbolic constant or an RGBA color.
Remarks
A texture environment specifies how texture values are interpreted when a fragment is textured. The target parameter must be GL_TEXTURE_ENV. The pname parameter can be either GL_TEXTURE_ENV_MODE or GL_TEXTURE_ENV_COLOR.

If pname is GL_TEXTURE_ENV_MODE, then params is (or points to) the symbolic name of a texture function. Three texture functions are defined: GL_MODULATE, GL_DECAL, and GL_BLEND.

A texture function acts on the fragment to be textured using the texture image value that applies to the fragment (see glTexParameter) and produces an RGBA color for that fragment. The following table shows how the RGBA color is produced for each of the three texture functions that can be chosen. C is a triple of color values (RGB) and A is the associated alpha value. RGBA values extracted from a texture image are in the range [0,1]. The subscript f refers to the incoming fragment, the subscript t to the texture image, the subscript c to the texture environment color, and subscript v indicates a value produced by the texture function.

A texture image can have up to four components per texture element (see glTexImage1D and glTexImage2D). In a one-component image, Lt indicates that single component. A two-component image uses Lt and At. A three-component image has only a color value, Ct. A four-component image has both a color value Ct and an alpha value At.


Besides, I'm using it;)

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stormrunner    720
cheers - you win the doc war. but if your only point is that that one assumption is wrong, perhaps it'd be beneficial to the OP to explain how in conjunction with glTexEnvi (or without ?)
Quote:
It's not true that glTexEnvi specifies how your textures are combined with colors in the frame buffer - it would be blending and glBlendFunc and glEnable/Disable(GL_BLEND).

is relevant and would be used.

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I didn't meant to take part in a war:) - only to correct one thing that event wasn't important in this thread but might be confusing for graveyard filla in the future. You was right, execpt this one little thing...

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ok, im a little confused now. what was the dispute about? and what do i have to do to get the effect i want?


"It's not true that glTexEnvi specifies how your textures are combined with colors in the frame buffer - it would be blending and glBlendFunc and glEnable/Disable(GL_BLEND). Tex env specifies how during particular pass color's from different sources(textures,diffuse,specular etc) are mixed withing texture units - that's all. Only the output value is blended into frame buffer"

im just a little confused... so glTexEnvi is what i want to use to get the modulation effect (IE blend the actual color of the quad with the texture).... so then what does blendFunc have to do with it then? and was i correct about how to disable it? just send the function DECAL to go back to normal? thanks again for your guys help!

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stormrunner    720
*sheepish grin* don't worry, we were just being anal about the details. i paid no attention to detail and made a general statement that ignored those details/got them completely wrong.
tomek_zielinski's point that
Quote:
Tex env specifies how during particular pass color's from different sources(textures,diffuse,specular etc) are mixed withing texture units

was correct.
you can use glTexEnvi without blending being enabled provided you aren't trying to blend two textures over each other (multi-texturing). neither tomek_zielinski's msdn doc nor my book mentioned needing GL_BLEND for regular tasks, so i'm assuming you don't. someone correct me if thats wrong.
Quote:
just send the function DECAL to go back to normal?

Quote:
taken from Opengl Game Programming
The default value for glTexEnvi() is GL_MODULATE

normal = default. the "normal" value is GL_MODULATE.
just for reference, you can specify a different glTexEnvi() for each texture.

[Edited by - stormrunner on August 4, 2004 7:37:19 PM]

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hmm.. im just a little confused still.. sorry if you think im stupid.

so its not a state then? i would specify it for each texture? cuz the docs that ive found show no where to specify the texture. im guessing once you call the function, any primitives drawn after that will follow the rules the function called, untill i call the function again with different parameters??

also, if the default is MODULATION, how come when i draw a quad with a texture and color, i only see the texture?

thanks again for all your help!

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stormrunner    720
Quote:
im guessing once you call the function, any primitives drawn after that will follow the rules the function called, untill i call the function again with different parameters

precisely, its just like most other opengl functions. its effects (as far as i can tell) are activated once you call it, but the glTexEnvi call is only valid between the glEnable(GL_TEXTURE_2D) and the subsequent glDisable call, just like the functions beneath blending and lighting. sorry if that wasn't clear. opengl doesn't presume to know what you want - so even though the default is GL_MODULATE, you have to let it know to activate that, or change it to something else, otherwise it won't happen. i suppose your original supposition was "technically" correct - that the "global" opengl default is GL_DECAL, until you call glTexEnvi or use a blending function.
Quote:
cuz the docs that ive found show no where to specify the texture

examples of using glTexEnvi can be found here for multi-texturing (don't know if that helps) here, and here. check them out - each shows a different way( in application) to use the function (i think, i didn't read them through), although the last one is probably the clearest use of it.

<edit> deleted redundant link
<edit> "using" the function as in loading textures with it

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