Re: Draw multiple textures with additive alpha

Subject: Re: Draw multiple textures with additive alpha

From: Richard Salvatierra <email@hidden>

Date: Fri, 16 May 2008 16:17:14 -0400

Delivered-to: email@hidden

Is glBlendMode() the same as glBlendFunc()

This was not working. Continues to show the background (semi opaque)

[vid_A bindCurrentTexture];

glBlendFunc(GL_ALPHA, GL_ZERO);

[vid_A compositeChannelInRect:theRect];

[vid_B bindCurrentTexture];

glBlendFunc(GL_ALPHA, GL_ONE);

[vid_B compositeChannelInRect:theRect];

-Rich

On May 16, 2008, at 3:44 PM, Kenneth Dyke wrote:

On May 16, 2008, at 12:37 PM, Richard Salvatierra wrote:

I am drawing multiple textures during a transition from one image to another (slide show)

// FIRST image with opacity = .4

glBlendMode(GL_SRC_ALPHA, GL_ZERO);

glColor4f(1.0f, 1.0f, 1.0f, opacity);
glBegin(GL_QUADS);
glTexCoord2f(...);
glVertex2f (...); etc.
...

glBlendMode(GL_SRC_ALPHA, GL_ONE);

// SECOND image with opacity = .6
glColor4f(1.0f, 1.0f, 1.0f, opacity);
glBegin(GL_QUADS);
glTexCoord2f(...);
glVertex2f (...); etc.

Try adding those modes. The first will ensure that the background is effectively removed by the fist pass, leaving only .4 of the first image. The second pass will then add .6 of the second plus the .4 from the first image, leaving you with an opaque blend of the two without any background.

-Ken

Kenneth Dyke, email@hidden (personal), email@hidden (work)
Sr Engineer, Graphics and Imaging Team, Apple Computer, Inc.
Java: The simple elegance of C++ combined with the blazing speed of SmallTalk.

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References:
	>Draw multiple textures with additive alpha (From: Richard Salvatierra <email@hidden>)
	>Re: Draw multiple textures with additive alpha (From: Kenneth Dyke <email@hidden>)