context free grammar - Page 238

Drawing a texture with an alpha channel doesn't work -- draws black

- by DevDevDev

I am modifying GLPaint to use a different background, so in this case it is white. Anyway the existing stamp they are using assumes the background is black, so I made a new background with an alpha channel. When I draw on the canvas it is still black, what gives? When I actually draw, I just bind the texture and it works. Something is wrong in this initialization. Here is the photo - (id)initWithCoder:(NSCoder*)coder { CGImageRef brushImage; CGContextRef brushContext; GLubyte *brushData; size_t width, height; if (self = [super initWithCoder:coder]) { CAEAGLLayer *eaglLayer = (CAEAGLLayer *)self.layer; eaglLayer.opaque = YES; // In this application, we want to retain the EAGLDrawable contents after a call to presentRenderbuffer. eaglLayer.drawableProperties = [NSDictionary dictionaryWithObjectsAndKeys: [NSNumber numberWithBool:YES], kEAGLDrawablePropertyRetainedBacking, kEAGLColorFormatRGBA8, kEAGLDrawablePropertyColorFormat, nil]; context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES1]; if (!context || ![EAGLContext setCurrentContext:context]) { [self release]; return nil; } // Create a texture from an image // First create a UIImage object from the data in a image file, and then extract the Core Graphics image brushImage = [UIImage imageNamed:@"test.png"].CGImage; // Get the width and height of the image width = CGImageGetWidth(brushImage); height = CGImageGetHeight(brushImage); // Texture dimensions must be a power of 2. If you write an application that allows users to supply an image, // you'll want to add code that checks the dimensions and takes appropriate action if they are not a power of 2. // Make sure the image exists if(brushImage) { brushData = (GLubyte *) calloc(width * height * 4, sizeof(GLubyte)); brushContext = CGBitmapContextCreate(brushData, width, width, 8, width * 4, CGImageGetColorSpace(brushImage), kCGImageAlphaPremultipliedLast); CGContextDrawImage(brushContext, CGRectMake(0.0, 0.0, (CGFloat)width, (CGFloat)height), brushImage); CGContextRelease(brushContext); glGenTextures(1, &brushTexture); glBindTexture(GL_TEXTURE_2D, brushTexture); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, brushData); free(brushData); } //Set up OpenGL states glMatrixMode(GL_PROJECTION); CGRect frame = self.bounds; glOrthof(0, frame.size.width, 0, frame.size.height, -1, 1); glViewport(0, 0, frame.size.width, frame.size.height); glMatrixMode(GL_MODELVIEW); glDisable(GL_DITHER); glEnable(GL_TEXTURE_2D); glEnable(GL_BLEND); glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_DST_ALPHA); glEnable(GL_POINT_SPRITE_OES); glTexEnvf(GL_POINT_SPRITE_OES, GL_COORD_REPLACE_OES, GL_TRUE); glPointSize(width / kBrushScale); } return self; }

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