sprite sheet - Page 22 - Developer IT

Weird y offset when using custom frag shader (Cocos2d-x)

- by Mister Guacamole

I'm trying to mask a sprite so I wrote a simple fragment shader that renders only the pixels that are not hidden under another texture (the mask). The problem is that it seems my texture has its y-coordinate offset after passing through the shader. This is the init method of the sprite (GroundZone) I want to mask: bool GroundZone::initWithSize(Size size) { // [...] // Setup the mask of the sprite m_mask = RenderTexture::create(textureWidth, textureHeight); m_mask->retain(); m_mask->setKeepMatrix(true); Texture2D *maskTexture = m_mask->getSprite()->getTexture(); maskTexture->setAliasTexParameters(); // Disable linear interpolation on the mask // Load the custom frag shader with a default vert shader as the sprite’s program FileUtils *fileUtils = FileUtils::getInstance(); string vertexSource = ccPositionTextureA8Color_vert; string fragmentSource = fileUtils->getStringFromFile( fileUtils->fullPathForFilename("CustomShader_AlphaMask_frag.fsh")); GLProgram *shader = new GLProgram; shader->initWithByteArrays(vertexSource.c_str(), fragmentSource.c_str()); shader->bindAttribLocation(GLProgram::ATTRIBUTE_NAME_POSITION, GLProgram::VERTEX_ATTRIB_POSITION); shader->bindAttribLocation(GLProgram::ATTRIBUTE_NAME_TEX_COORD, GLProgram::VERTEX_ATTRIB_TEX_COORDS); shader->link(); CHECK_GL_ERROR_DEBUG(); shader->updateUniforms(); CHECK_GL_ERROR_DEBUG(); int maskTexUniformLoc = shader->getUniformLocationForName("u_alphaMaskTexture"); shader->setUniformLocationWith1i(maskTexUniformLoc, 1); this->setShaderProgram(shader); shader->release(); // [...] } These are the custom drawing methods for actually drawing the mask over the sprite: You need to know that m_mask is modified externally by another class, the onDraw() method only render it. void GroundZone::draw(Renderer *renderer, const kmMat4 &transform, bool transformUpdated) { m_renderCommand.init(_globalZOrder); m_renderCommand.func = CC_CALLBACK_0(GroundZone::onDraw, this, transform, transformUpdated); renderer->addCommand(&m_renderCommand); Sprite::draw(renderer, transform, transformUpdated); } void GroundZone::onDraw(const kmMat4 &transform, bool transformUpdated) { GLProgram *shader = this->getShaderProgram(); shader->use(); glActiveTexture(GL_TEXTURE1); glBindTexture(GL_TEXTURE_2D, m_mask->getSprite()->getTexture()->getName()); glActiveTexture(GL_TEXTURE0); } Below is the method (located in another class, GroundLayer) that modify the mask by drawing a line from point start to point end. Both points are in Cocos2d coordinates (Point (0,0) is down-left). void GroundLayer::drawTunnel(Point start, Point end) { // To dig a line, we need first to get the texture of the zone we will be digging into. Then we get the // relative position of the start and end point in the zone's node space. Finally we use the custom shader to // draw a mask over the existing texture. for (auto it = _children.begin(); it != _children.end(); it++) { GroundZone *zone = static_cast<GroundZone *>(*it); Point nodeStart = zone->convertToNodeSpace(start); Point nodeEnd = zone->convertToNodeSpace(end); // Now that we have our two points converted to node space, it's easy to draw a mask that contains a line // going from the start point to the end point and that is then applied over the current texture. Size groundZoneSize = zone->getContentSize(); RenderTexture *rt = zone->getMask(); rt->begin(); { // Draw a line going from start and going to end in the texture, the line will act as a mask over the // existing texture DrawNode *line = DrawNode::create(); line->retain(); line->drawSegment(nodeStart, nodeEnd, 20, Color4F::RED); line->visit(); } rt->end(); } } Finally, here's the custom shader I wrote. #ifdef GL_ES precision mediump float; #endif varying vec2 v_texCoord; uniform sampler2D u_texture; uniform sampler2D u_alphaMaskTexture; void main() { float maskAlpha = texture2D(u_alphaMaskTexture, v_texCoord).a; float texAlpha = texture2D(u_texture, v_texCoord).a; float blendAlpha = (1.0 - maskAlpha) * texAlpha; // Show only where mask is invisible vec3 texColor = texture2D(u_texture, v_texCoord).rgb; gl_FragColor = vec4(texColor, blendAlpha); return; } I got a problem with the y coordinates. Indeed, it seems that once it has passed through my custom shader, the sprite's texture is not at the right place: Without custom shader (the sprite is the brown thing): With custom shader: What's going on here? Thanks :) EDIT It looks like after passing through the shader when I set the position of the sprite I set it in points, with (0,0) being in the top-right. Indeed, when I do sprite->setPosition(320, 480), the sprite is perfectly placed at the top of the screen.

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