Mask Preprocessing Method (Java)

Updated: 01/18/2024

Some class names have been changed in Cubism 5 SDK or later.

See CHANGELOG for more information.

In order to maintain drawing speed on smartphones and other devices, the Live2D Cubism SDK for Java uses the “preprocessing method” in which all mask shapes are drawn in a single mask buffer at the beginning of the model drawing process.

In the general drawing method, the mask shape is drawn each time a Drawable that requires a mask is drawn (see figure).
This method would result in a relatively expensive process of switching render targets, clearing buffers, etc. each time the Drawable needs a mask.
This may cause slow rendering speeds on smartphones and other devices.

However, simply preparing masks in advance requires multiple mask buffers, which can overwhelm memory.
To solve this problem, the following processing can be performed on a single mask buffer to minimize memory usage while treating it as if multiple mask buffers were used.

Mask Integration

Since all masks are generated in advance, Drawables with the same mask specification can use the same mask image to reduce the number of masks to be generated.

This is done, in the CubismRendererAndroid.initialize function call, by the ACubismClippingManager.initialize function.

    public void initialize(
        CubismRenderer.RendererType type,
        CubismModel model,
        int maskBufferCount
    ) {
        renderTextureCount = maskBufferCount;

        // Initialization of an array of clear flags for render textures
        clearedMaskBufferFlags = new boolean[renderTextureCount];

        final int drawableCount = model.getDrawableCount();     // Number of drawable objects
        final int[][] drawableMasks = model.getDrawableMasks();     // List of drawable object indices to mask drawable objects
        final int[] drawableMaskCounts = model.getDrawableMaskCounts();     // Number of drawable objects to mask drawable objects

        // Register all drawable objects that use clipping masks.
        // Clipping masks should normally be used only for a few pieces.
        for (int i = 0; i < drawableCount; i++) {
            if (drawableMaskCounts[i] <= 0) {
                // ArtMeshes without clipping masks (often not used)

            // Check if it is the same as an already existing ClipContext.
            T_ClippingContext cc = findSameClip(drawableMasks[i], drawableMaskCounts[i]);
            if (cc == null) {
                // Generate if the same mask does not exist.
                cc = (T_ClippingContext) ACubismClippingContext.createClippingContext(



Use of Multiple Mask Textures

In Cubism SDK for Java R1 beta1 or later, you can optionally use multiple textures for masks.
Therefore, even if a model is set with more masks than the maximum number of masks (36) that existed up to alpha1, the model will be displayed correctly in the SDK.

However, if two or more mask textures are used, the maximum number of masks generated for each mask texture is 32.
(The maximum number of masks is 36 when only one mask is used. Details are described below.)
If two mask textures are used, the maximum number of masks that can be used is 32 × 2, or 64.

The settings for using multiple textures for masks are as follows.

int maskBufferCount = 2;

CubismRenderer renderer = CubismRendererAndroid.create();
renderer.initialize(cubismModel, maskBufferCount);

If nothing is passed to the second argument of CubismRendererAndroid.initialize(), only one texture for the mask will be generated and used.


Separation by Color Information

The mask buffer is an RGBA video array, just like a normal texture buffer, etc.
The normal mask process uses only this A channel to apply the mask, but not the RGB channels.
Therefore, by having separate mask data for R, G, B, and A, one mask buffer can be treated as four mask buffers.

Separation by Dividing

When 4 mask images are not enough, the number of masks can be increased by handling the mask buffer in 2, 4, or 9 divisions.
Combined with the separation by color information, up to 36 (4 x 9) different masks can be held.

Also, to prevent the mask image from being crushed, the mask is drawn on all Drawable rectangles to which the mask is applied.
This requires range generation as well as matrix generation for the mask generation and the use of masks.

When multiple mask textures are used, the division method is the same as when only one mask texture is used.
However, if multiple mask textures are used, the quality of the drawing of the same model will be improved because the allocation of masks to each mask texture will be equally divided as much as possible. (More textures for masks will cost more.)
For example, if a model has 32 masks, normally one mask texture is used to try to draw 32 masks, but if two mask textures are used, the mask allocation is “16 masks per texture.”

Checking rectangles

In the first step of mask generation, for each mask, check the rectangle that fits all the mask application destinations.

    public void calcClippedDrawTotalBounds(CubismModel model, T_ClippingContext clippingContext) {
        // The entire rectangle of the clipped mask (drawable object to be masked)
        float clippedDrawTotalMinX = Float.MAX_VALUE;
        float clippedDrawTotalMinY = Float.MAX_VALUE;
        float clippedDrawTotalMaxX = -Float.MAX_VALUE;
        float clippedDrawTotalMaxY = -Float.MAX_VALUE;

        // Determine if this mask is actually needed.
        // If even one “Drawable Object” that uses this clipping is available, a mask must be generated.
        final int clippedDrawCount = clippingContext.clippedDrawableIndexList.size();
        for (int clippedDrawableIndex = 0; clippedDrawableIndex < clippedDrawCount; clippedDrawableIndex++) {
            // Find the rectangle to be drawn for the drawable object that uses the mask.
            final int drawableIndex = clippingContext.clippedDrawableIndexList.get(clippedDrawableIndex);

            final int drawableVertexCount = model.getDrawableVertexCount(drawableIndex);
            float[] drawableVertices = model.getDrawableVertices(drawableIndex);

            float minX = Float.MAX_VALUE;
            float minY = Float.MAX_VALUE;
            float maxX = -Float.MAX_VALUE;
            float maxY = -Float.MAX_VALUE;

            int loop = drawableVertexCount * VERTEX_STEP;
            for (int pi = VERTEX_OFFSET; pi < loop; pi += VERTEX_STEP) {
                float x = drawableVertices[pi];
                float y = drawableVertices[pi + 1];
                if (x < minX) minX = x;
                if (x > maxX) maxX = x;
                if (y < minY) minY = y;
                if (y > maxY) maxY = y;

            if (minX == Float.MAX_VALUE) {
                continue; // If a single valid point was not obtained, skip it.

            // Reflected in the entire rectangle
            if (minX < clippedDrawTotalMinX) clippedDrawTotalMinX = minX;
            if (maxX > clippedDrawTotalMaxX) clippedDrawTotalMaxX = maxX;
            if (minY < clippedDrawTotalMinY) clippedDrawTotalMinY = minY;
            if (maxY > clippedDrawTotalMaxY) clippedDrawTotalMaxY = maxY;

        if (clippedDrawTotalMinX == Float.MAX_VALUE) {
            clippingContext.isUsing = false;

            csmRectF clippedDrawRect = clippingContext.allClippedDrawRect;
        } else {
            clippingContext.isUsing = true;
            float w = clippedDrawTotalMaxX - clippedDrawTotalMinX;
            float h = clippedDrawTotalMaxY - clippedDrawTotalMinY;

            csmRectF clippedDrawRect = clippingContext.allClippedDrawRect;

Layout settings with color separation and divisional separation

Defines the color channel and divisional position of the mask buffer that each mask belongs to.

    public void setupLayoutBounds(int usingClipCount) {
        final int useClippingMaskMaxCount = renderTextureCount <= 1

        if (usingClipCount <= 0 || usingClipCount > useClippingMaskMaxCount) {
            if (usingClipCount > useClippingMaskMaxCount) {
                // Warn about the mask count limit.
                int count = usingClipCount - useClippingMaskMaxCount;
                    "not supported mask count : %d\n[Details] render texture count: %d\n, mask count : %d",
            // In this case, one mask target is cleared and used each time.
            for (int index = 0; index < clippingContextListForMask.size(); index++) {
                T_ClippingContext cc = clippingContextListForMask.get(index);

                cc.layoutChannelIndex = 0;   // It’s fine to fix, since it will be erased every time anyway.
                cc.bufferIndex = 0;

        // If there is one render texture, divide it into 9 pieces (max 36 pieces).
        final int layoutCountMaxValue = renderTextureCount <= 1 ? 9 : 8;

        // Layout masks using a single RenderTexture that is as full as possible.
        // If the number of mask groups is 4 or less, 1 mask is placed on each RGBA channel; if the number is between 5 and 6, 2 masks are in each RG channel and 1 mask is in each BA channel.
        // NOTE: The decimal point is rounded up because the number of mask divisions to be assigned to one piece is to be increased.
        final int countPerSheetDiv = (usingClipCount + renderTextureCount - 1) / renderTextureCount;     // Indicates how many masks to allocate per render texture.
        final int reduceLayoutTextureCount = usingClipCount % renderTextureCount;     // Number of render textures to reduce the number of layouts by one (for this number of render textures)

        // Use RGBA in order.
        final int divCount = countPerSheetDiv / COLOR_CHANNEL_COUNT; // Basic number of masks to be placed on one channel.
        final int modCount = countPerSheetDiv % COLOR_CHANNEL_COUNT;     // Remainder. Allocate one by one to this number of channels (not index).

        // Provide a channel for each RGBA (0: R, 1: G, 2: B, 3: A)
        int curClipIndex = 0; // Set in order.

        for (int renderTextureIndex = 0; renderTextureIndex < renderTextureCount; renderTextureIndex++) {
            for (int channelIndex = 0; channelIndex < COLOR_CHANNEL_COUNT; channelIndex++) {
                // Number of layouts for this channel
                // NOTE: Add one if the number of layouts in the channel is the number of basic masks to be placed in one channel plus the number of remaining masks.
                int layoutCount = divCount + (channelIndex < modCount ? 1 : 0);

                // Determine the channel to be used when reducing the number of layouts by one.
                // Adjust div to normal index range when div is 0.
                final int checkChannelIndex = modCount + (divCount < 1 ? -1 : 0);

                // If this is the target channel and there is a render texture that reduces the number of layouts by one.
                if (channelIndex == checkChannelIndex && reduceLayoutTextureCount > 0) {
                    // Reduce the number of layouts by one if the current render texture is the target render texture.
                    layoutCount -= ! (renderTextureIndex < reduceLayoutTextureCount) ? 1 : 0;

                // Determine the separation method.
                if (layoutCount == 0) {
                    // Do nothing.
                } else if (layoutCount == 1) {
                    // Use everything as is.
                    T_ClippingContext cc = clippingContextListForMask.get(curClipIndex++);
                    cc.layoutChannelIndex = channelIndex;
                    csmRectF bounds = cc.layoutBounds;


                    cc.bufferIndex = renderTextureIndex;
                } else if (layoutCount == 2) {
                    for (int i = 0; i < layoutCount; i++) {
                        final int xpos = i % 2;

                        T_ClippingContext cc = clippingContextListForMask.get(curClipIndex++);
                        cc.layoutChannelIndex = channelIndex;
                        csmRectF bounds = cc.layoutBounds;

                        // Divide UV into 2 and use
                        bounds.setX(xpos * 0.5f);

                        cc.bufferIndex = renderTextureIndex;
                } else if (layoutCount <= 4) {
                    // Divide into 4 and use
                    for (int i = 0; i < layoutCount; i++) {
                        final int xpos = i % 2;
                        final int ypos = i / 2;

                        T_ClippingContext cc = clippingContextListForMask.get(curClipIndex++);
                        cc.layoutChannelIndex = channelIndex;
                        csmRectF bounds = cc.layoutBounds;

                        bounds.setX(xpos * 0.5f);
                        bounds.setY(ypos * 0.5f);

                        cc.bufferIndex = renderTextureIndex;
                } else if (layoutCount <= layoutCountMaxValue) {
                    // Divide into 9 and use
                    for (int i = 0; i < layoutCount; i++) {
                        final int xpos = i % 3;
                        final int ypos = i / 3;

                        T_ClippingContext cc = clippingContextListForMask.get(curClipIndex++);
                        cc.layoutChannelIndex = channelIndex;
                        csmRectF bounds = cc.layoutBounds;

                        bounds.setX(xpos / 3.0f);
                        bounds.setY(ypos / 3.0f);
                        bounds.setWidth(1.0f / 3.0f);
                        bounds.setHeight(1.0f / 3.0f);

                        cc.bufferIndex = renderTextureIndex;
                // Processing when the number of masks exceeds the limit
                else {
                    int count = usingClipCount - useClippingMaskMaxCount;
                        "not supported mask count : %d\n[Details] render texture count: %d\n, mask count : %d",

                    // Stop if in development mode.
                    assert false;

                    // If you continue to run the program, SetupShaderProgram will cause over-access, so you have no choice but to put it in the appropriate place.
                    // As expected, the drawing results are not satisfactory.
                    for (int i = 0; i < layoutCount; i++) {
                        T_ClippingContext cc = clippingContextListForMask.get(curClipIndex++);
                        cc.layoutChannelIndex = 0;

                        csmRectF bounds = cc.layoutBounds;

                        cc.bufferIndex = 0;

Matrix generation for drawing and using masks

Prepare transformation matrices for mask generation and mask use based on the area and the location of the rectangle examined before drawing.

/* Omitted */
            // ---- Actually draw one mask ----
            T_ClippingContext clipContext = clippingContextListForMask.get(clipIndex);
            csmRectF allClippedDrawRect = clipContext.allClippedDrawRect; // Enclose the logical coordinates of all drawable objects that use this mask in a rectangle.
            csmRectF layoutBoundsOnTex01 = clipContext.layoutBounds;    // Fit this mask.

            final float margin = 0.05f;
            float scaleX, scaleY;
            final float ppu = model.getPixelPerUnit();
            final float maskPixelWidth = clipContext.getClippingManager().getClippingMaskBufferSize().x;
            final float maskPixelHeight = clipContext.getClippingManager().getClippingMaskBufferSize().y;
            final float physicalMaskWidth = layoutBoundsOnTex01.getWidth() * maskPixelWidth;
            final float physicalMaskHeight = layoutBoundsOnTex01.getHeight() * maskPixelHeight;


            if (tmpBoundsOnModel.getWidth() * ppu > physicalMaskWidth) {
                tmpBoundsOnModel.expand(allClippedDrawRect.getWidth() * margin, 0.0f);
                scaleX = layoutBoundsOnTex01.getWidth() / tmpBoundsOnModel.getWidth();
            } else {
                scaleX = ppu / physicalMaskWidth;

            if (tmpBoundsOnModel.getHeight() * ppu > physicalMaskHeight) {
                tmpBoundsOnModel.expand(0.0f, allClippedDrawRect.getHeight() * margin);
                scaleY = layoutBoundsOnTex01.getHeight() / tmpBoundsOnModel.getHeight();
            } else {
                scaleY = ppu / physicalMaskHeight;

            // Obtain the matrix to be used when generating the mask.
            createMatrixForMask(isRightHanded, layoutBoundsOnTex01, scaleX, scaleY);

/* Omitted */

Dynamic Resizing of Mask Buffers

The OpenGL ES 2.0 renderer provides an API to resize the mask buffer at runtime.
Currently, the mask buffer size is initially set to 256*256 (pixels), but if the mask generation area is to be cut into 9 pieces, the mask shape drawn in an 85*85 (pixels) rectangle area will be further enlarged and used as the clipping area.
As a result, the edges of the clipping result are blurred or blotchy.
As a solution to this problem, an API is provided to change the size of the mask buffer at program execution time.

For example, if the mask buffer size is 256*256 => 1024*1024 and the mask generation area is cut into 9 pieces, the mask shape can be drawn in a rectangle area of 341*341, so when enlarged and used as a clipping area, it eliminates edge blurring or blotches.

Note: Increase the size of the mask buffer => The more pixels to be processed, the slower the speed, but the cleaner the drawing result.
Note: Reduce the size of the mask buffer => The fewer pixels to be processed, the faster the speed, but the drawing result will be less clean.

    public void setClippingMaskBufferSize(final float width, final float height) {
        if (clippingManager == null) {

        // Save the number of render textures before destroying the instance.
        final int renderTextureCount = this.clippingManager.getRenderTextureCount();

        // Destroy and recreate instances to change the size of MaskBuffer
        clippingManager = new CubismClippingManagerAndroid();
        clippingManager.setClippingMaskBufferSize(width, height);

        CubismModel model = getModel();

Why Preprocessing Methods Can Improve Performance

Specifically for mobile devices, the processing cost of the Clear instruction and rendering target switching instruction to the GPU may be higher than other instructions.
When drawing using the general method, these instructions with high processing costs are executed as many times as the number of Drawables that require masks.
However, with the preprocessing method, the number of times these instructions are executed can be reduced, resulting in improved performance in smartphones and other devices.

We are currently conducting experiments to ascertain its effectiveness.
See “Why pre-processing methods can improve performance” in “Mask preprocessing methods (Native)” for the measurement method and results of this experiment.

Switching to a high-definition method for processing masks

The method of generating a mask each time a drawing is made will affect performance on low-end devices.

However, this method is more suitable when screen quality is more important than performance at runtime, as in the case of final output as video.

In the Cubism SDK for Java, the mask process can be switched to a high-definition method.
To switch to the high-definition method, pass true to the following API.

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